This post describes a thermistor calibration achievable by people who don’t have access to lab equipment with an accuracy better than ±0.15°C. This method is particularly suitable for the 10k NTC on our 2-module data logger handling them in a way that is easy to standardize for batch processing (ie: at the classroom scale). We use brackets to keep the loggers completely submerged because the thermal conductivity of the water around the housing is required or the two sensors would diverge. The target range of 0° to 40°C used here covers moderate environments including the underwater and underground locations we typically deploy into. This method is unique in that we use a freezing process rather than melting ice for the 0°C data point.
Use stainless steel washers in your hold-downs to avoid contamination of the distilled water and provide nucleation points to limit super-cooling. Before creating this bracket we simply used zip-ties to hold the washer weights.
Reading a thermistor with digital pins uses less power, and gives you the resistance of the NTC directly from the ratio of two Interrupt Capture Unit times. Resolution is not set by the bit depth of your ADC, but by the size of the reservoir capacitor: a small ceramic 0.1µF [104] delivers about 0.01°C with jitter in the main system clock imposing a second limit on resolution at nearly the same point. Large reservoir capacitors increase resolution and reduce noise but take more time and use more power. The calibration procedure described in this post will work no matter what method you use to read your NTC thermistor.
The I2C reference sensor is connected temporarily during the calibration via Dupont headers. Always give your reference sensors serial numbers so that you can normalize them before doing the thermistor calibrations.
Off-the-shelf sensors can be used as ‘good enough’ reference thermometers provided you keep in mind that most accuracy specifications follow a U-shaped curve around a sweet spot that’s been chosen for a particular application. The Si7051 used here has been optimized for the medical market, so it has ±0.1° accuracy from 35.8 to 41° Celsius, but that falls to ±0.13° at room temperatures and only ±0.25° at the ice point. If you use some other reference sensor (like the MAX30205 or the TSYS01) make sure it’s datasheet specifies how the accuracy changes over the temperature range you are targeting with the calibration.
The shortened Steinhart–Hart equation used here is not considered sufficiently accurate for bench-top instruments which often use a four or five term polynomial. However in ‘The Guide on Secondary Thermometry‘ by White et. al. (2014) the three-term equation is expected to produced interpolation errors of about 0.0025°C over a range from 0 to 50°C, and that is acceptable for most monitoring. To calculate the three equation constants you need to collect three temperature & resistance data pairs which can be entered into the online calculator at SRS or processed with a spreadsheet.
While these technical sources of error limit the accuracy you can achieve with this method, issues like thermal lag in the physical system and your overall technique are more important. In general, you want each step of the calibration process to occur as slowly as possible. If the data from a run doesn’t look the way you were expecting – then do the procedure over again until those curves are well behaved and smooth. Make sure the loggers stay dry during the calibration – switching to spare dry housing tubes between the baths: Moisture is the greatest cause of failure in sensors and humidity/water always lowers the resistance of thermistors. If in doubt, let everything dry for 24 hours before re-doing a calibration.
Data Point #1: The freezing point of water
The most common method of obtaining a 0°C reference is to place the sensor into an insulated bucket of stirred ice slurry that plateaus as the ice melts. This is fine for waterproof sensors on the end of a cable but it is not easily done with sensors mounted directly on a PCB. So we immerse the loggers in collapsible 1200ml silicone food containersfilled withdistilled water. This is placed inside of a well insulated lunch box and the combined assembly is left in the freezer overnight, reading every 30 seconds.
Weighted holders keep each logger completely immersed. Soft-walled silicone containers expand to accommodate any volume change as the water freezes. This prevents the centrifuge tube housings from being subjected to pressure as the ice forms. Position the loggers so that they are NOT in direct contact with the sides or the lid of the silicone container.
The outer box provides insulation to slow down the freezing process. After testing several brands it was found that the Land’s End EZ wipe (9″x8″x4″) and Pottery Barn Kids Mackenzie Classic lunch boxes provided the best thermal insulation because they have no seams on the solid molded foam interior which also doesn’t absorb water spilled while moving the containers around.
For the purpose of this calibration (at ambient pressure) we can treat the freezing point of pure water as a physical constant. So no reference sensor is needed on the logger while you collect the 0°C data. Leave the lunch box in the freezer just long enough for a rind of ice to form around the outer edgeswhile the main volume of water surrounding the loggers remains liquid. I left the set in this photo a bit too long as that outer ice rind is much thicker than it needed to be for the data collection. Do not let the water freeze completelysolid (!) as this will subject the loggers to stress that may crack the tubes and let water in to ruin your loggers.
The larger bubbles in this photo were not present during the freeze, but were created by moving the container around afterward for the photo.
The trick is recognizing which data represents the true freezing point of water. Distilled water super-cools by several degrees, and then rises to 0°C for a brief period after ice nucleation because the phase change releases 80 calories per gram while the specific heat capacity of water is only one calorie, per degree, per gram. So freezing at the outer edges warms the rest of the liquid – but this process is inherently self-limiting which gives you a plateau at exactly 0°C after the rise:
NTC (ohms) gathered during the freeze/thaw process graphed with the y axis is inverted because of the negative coefficient. The warm temperature data has been removed from the graphs above to display only the relevant cold-temperature data. Only the 10-20 minutes of data immediately after the rise from a super cooled state is relevant to the calibration. Cooling the insulated chamber from its room temperature starting point to the supercooling spike shown above took 7-8 hours.
Depending on the strength of your freezer, and the quality of the outer insulating container, the ice-point may only last a few minutes before temperatures start to fall again. An average of the NTC readings from that SHORT plateau immediately after the supercooling ends is your 0°C calibration point. This is usually around 33000 ohms for a 10k 3950 thermistor. Only the data immediately after super cooling ends is relevant and the box can be removed from the freezer any time after that event. I left the example shown abovein the freezer too long but you have a reasonable window of time to avoid this. Once the freeze process initiates, it usually takes about 8 hours for the entire volume to freeze solid – after which you can see the compressor cycling as the now solid block cools below 0°C. You want to pull the sensors out of the freezer before that solid stair-step phase (at 8:00 above) if possible.
If the supercooling spike is not obvious in your data then change your physical configuration to slow the cooling process until it appears. You want the inner surface of your silicone container to have smooth edges, as sharp corners may nucleate the ice at 0°C, preventing the supercooling spike from happening. Use as much distilled water as the container will safely hold -the loggers should be surrounded by water on all sides.
In this image a freezer compressor cycle happened during post supercooling rise making it hard to see where the plateau occurred. This run was re-done to get better data.
Most refrigerators cycle based on how often the door is opened and those cycles can overprint your data making it hard to interpret. If you put a room-temperature box of water in the freezer between 6-7pm, it usually reaches the supercooling point around 2am, reducing the chances that someone will open the refrigerator/freezer door at the critical time. Even then, unexpected thermal excursions may happen if the freezer goes into a defrost cycle or an automatic ice-maker kicks in during the run. The time to reach that supercooling event can be reduced by pre-cooling the distilled water to ~5°C in the refrigerator before the freezer run. If any of the points on your curves are ambiguous, then do that run again, making sure the water is completely ice free at the start.
As a technical aside, the energy released (or absorbed) during the phase change of water is so much larger than its typical thermal content that water based heat pumps can multiply their output significantly by making slushies.
Data Point #2: Near 40°C
We have used the boiling point of water for calibration in the past, but the centrifuge tube housings would soften considerably at those temperatures. Ideally you want to bracket your data with equally spaced calibration points and 100°C is too far from the environmental conditions we are targeting. Heated water baths can be found on eBay for about $50, but my initial tests with a Fisher Scientific IsoTemp revealed thermal cycling that was far too aggressive to use for calibration – even with a circulation pump and many layers of added insulation. So we created an inexpensive DIY version made with an Arctic Zone Zipperless Coldloc hard-shell lunch box and a 4×6 inch reptile heating mat (8 watt). Unlike the ice point which must be done with distilled water, ordinary tap water can be used to collect the two warm temperature data pairs.
These hard-sided Arctic Zone lunch boxes can often be obtained for a few dollars at local charity shops or on eBay.
Place the 8-watt heating pad under the hard shell of the lunch box. At 100% power this tiny heater takes ~24 hours to bring the bath up to ~38°C. The bath temp is relatively stable since the heater does not cycle, but it does experience a slow drift based on losses to the environment. These heating pads sell for less than $15 on Amazon.
To record the temperature inside each logger, an Si7051 breakout module (from Closed Cube) is attached to the logger. A hold down of some kind must keep the logger completely submerged for the duration of the calibration. If a logger floats to the surface then air within the housing can thermally stratify and the two sensors will diverge. That data is not usable for calibration so the run must be done again with that logger.
The reference sensor needs to be as close to the NTC sensor as possible within the housing – preferably with the chip directly over top and facing the NTC thermistor.
Data Point #3: Room Temperature
The loggers stay in the heated bath for a minimum of 4 hours, but preferably 8 -12 hours. The idea is you want the whole assembly to have enough time to equilibrate. Then they are transferred to an unheated water-filled container (in this case a second Arctic Zone lunch box) where they run at ambient temperatures for another 8 -12 hours. This provides the final reference data pair:
Si7051 temperature readings inside a logger at a 30 second sampling interval. The logger was transferred between the two baths at 8am. Both baths are affected by the temperature changes in the external environment.
Detail: Warm temp. NTC ohms (y-axis inverted)
Detail: Room temp. NTC ohms (y-axis inverted)
As the environment around the box changes, losses through the insulation create gentle crests or troughs where the lag difference between the sensors will change sign. So averaging several readings across those inflection pointscancels out any lag error between the reference sensor and the NTC. Take care that you average exactly the same set of readings from both the Si7051 and from the NTC. At this point you should have three Temperature / Resistance data pairs that can be entered into the SRS online calculator to calculate the equation constants ->
I generally use six digits from the reference pairs, which is one more than I’d trust in the temperature output later. I also record the Beta constants for live screen output because that low accuracy calculation takes less time on limited processors like the 328p.
The final step is to use those constants to calculate the temperature from the NTC data with: Temperature °C = 1/(A+(B*LN(ohms))+(C*(LN(ohms))^3))-273.15
Then graph the calculated temperatures from the NTC calibration readings over top of the reference sensor temperatures. Provided the loggers were completely immersed in the water bath, flatter areas of the two curves should overlapone another precisely. However, the two plots will diverge when the temperature is changing rapidly because the NTC exhibits more thermal lag than the Si7051. This is because the NTC is located near the thermal mass of the ProMini circuit board.
Si reference & NTC calculated temperatures: If your calibration has gone well, the curves should be nearly identical as shown above. With exceptions only in areas where the temperature was changing rapidly and the two sensors got out of sync because of different thermal lags.
Also note that the hot and warm bath data points can be collected with separate runs. In fact, you could recapture any individual data pair and recalculate the equation constants with two older ones any time you suspect a run did not go smoothly. Add the constants to all of the data column headers, and record them in a google doc with the three reference pairs and the date of the calibration.
Validation
You should always do a final test to validate your calibrations, because even when the data is good it’s easy to make a typo somewhere in the process. Here, a set of nine calibrated NTC loggers are run together for a few days in a gently circulatingwater bath at ambient temperature –>
(Click to enlarge)
Two from this set are a bit high and could be recalibrated, but all of the NTC temperature readings now fall within a 0.1°C band. This is a decent result from a method you can do without laboratory grade equipment, and the sensors could be brought even closer together by using this validation data to normalize the set.
Comments
The method described above uses equipment small enough to be portable, allowing easy classroom setup/takedown. More importantly this also enablesthe re-calibration of loggers in the field if you have access to a freezer. This makes it possible to re-run the calibrations and then apply compensation techniques to correct for sensor drift. Validating calibration before and after each deployment is particularly important with DIY equipment to address questions about data quality at publication. Glass encapsulated NTC thermistors drift up to 0.02 °C per year near room temperatures, while epoxy coated sensors can drift up to 10x that.
At the ice-point, our resolution is ~0.0025°C but our time-based readings vary by ±0.0075°C. This is due to timing jitter in the ProMini oscillator and in the interrupt handling by a 328p. So with a [104] reservoir capacitor in the timing circuit, our precision at 0°C is 0.015°C.
Having a physical constant in the calibration data is important because most of the affordable reference sensors in the Arduino landscape were designed for applications like healthcare, hvac, etc. So they are usually designed minimize error in warmer target ranges, while getting progressively worse as you approach 0°C. But accuracy at those lower temperatures is important for environmental monitoring in temperate climates. The method described in this post could also be used to calibrate commercial temperature sensors if they are waterproof.
Calibrating the onboard thermistor a good idea even if you plan to add a dedicated temperature sensor because you always have to do some kind of burn-in testing on a newly built logger – so you might as well do something productive with that time. I generally record as much data as possible during the calibration to fill more memory and flag potentially bad areas in the EEprom. (Note: Our code on GitHub allows only 1,2,4,8, or 16 bytes per record to align with page boundaries) . And always look at the battery record during the calibration as it’s often your first clue that a DIY logger might not be performing as expected. It’s also worth mentioning that if you also save the RTC temperatures as you gather the NTC calibration data, this procedure gives you enough information to calibrate that register as well. The resolution is only 0.25°C, but it does give you a way to check if your ‘good’ temperature sensors are drifting because the DS3231 tends to be quite stable.
While the timing jitter does not change, non-linearity of the NTC resistance reduces the resolution to 0.005°C. Precision at 35°C also suffers, falling to 0.02°C. Using a 10x larger [105] reservoir cap would get us back to resolution we had at 0°C, as would oversampling which actually requires this kind of noise for the method to work. Either of those changes would draw proportionally more power from the coincell for each read so its a tradeoff that might not be worth making when you consider sensor lag.
For any sensor calibration the reference points should span the range you hope to collect later in the field. To extend this procedure for colder climates you could replace the ice point with the freezing point of Galinstan (-20°C) although a domestic freezer will struggle to reach that. If you need a high point above 40°C, you can use a stronger heat source. Using two of those 8 watt pads in one hard sided lunch box requires some non-optimal bending at the sides, but it does boost the bath temp to about 50°C. 3D printed PLA hold-downs will start to soften at higher temps so you may need to alter the design to prevent the loggers from popping out during the run.
If your NTC data is so noisy you can’t see where to draw an average, check the stability of your regulator because any noise on the rail will affect the Schmitt trigger thresholds used by our ICU/timer method. This isn’t an issue running from a battery, but even bench supplies can give you noise related grief if you’ve ended up with some kind of ground loop. You could also try oversampling, or a leaky integrator to smooth the data – but be careful to apply those techniques to both the reference and the NTC in exactly the same way because they introduce significant lag. Temperature maximums are underestimated and temperature minimums are overestimated by any factor that introduces lag into the system. In general, you want to do as little processing to raw sensor readings as possible at capture time because code-based techniques usually require some prior knowledge of the data range & variation before they can be used safely. Also note that our digital pin ICU based method for reading resistors does not work well with temperature compensated system oscillators because that compensation circuitry could kick in between the reference resistor and NTC readings.
And finally, the procedure described here is not ‘normalization’, which people sometimes confuse with calibration. In fact, it’s a good idea to huddle-test your sensors in a circulating water bath after calibration to bring a set closer together even though that may not improve accuracy. Creating post-calibration y=Mx+B correction constants is especially useful for sensors deployed along a transect, or when monitoring systems that are driven by relative deltas rather than by absolute temperatures. Other types of sensors like pressure or humidity have so much variation from the factory that they almost always need to be normalized before deployment – even on commercial loggers.
2023 is the ten-year anniversary of the Cave Pearl Project, with hundreds of loggers built from various parts in the Arduino ecosystem and deployed for Dr. Beddows research. During that time her EARTH 360 – Instrumentation course evolved from using commercial equipment to having students assemble a logging platform for labs on environmental monitoring. The experience of those many first-time builders has been essential to refining our educational logger design to achieve maximum utility from a minimum number of components. So, in recognition of their ongoing and spirited enthusiasm, we call this new model the e360.
A standard 50mL centrifuge tube forms the housing, which is waterproof to about 8 meters depth. For better moisture resistance use tubes with an O-ring integrated into the cap (made of silicone or ethylene propylene) which gets compressed when the threads are tightened.
A bracket for the logger which can be installed horizontally or vertically. Zip ties pass through the central support to wrap around the 50ml centrifuge tube. This prints without any generated supports and the STL can be downloaded for printing from the Github repository for this logger.
Many parallel trends have advanced the open-source hardware movement over the last decade, including progress towards inexpensive and (mostly) reliable 3D printing. In keeping with the project’s ethos of accessibility, we use an Ender 3 for the rails and you can download that printable stl file directly from Tinkercad. Tinkercad is such a beginner-friendly tool that students are asked to create their own logger mounting brackets from scratch as as an exercise in the Lux/LDR calibration lab. This directly parallels our increasing use of 3D prints for installation brackets & sensor housings on the research side of the project.
One of the things that distinguishes this project from others in the open science hardware movement is that instead of constantly adding features like IOT connectivity, we have been iterating towards simplicity. Cheap, flexible, stand-alone loggers enable many teaching and research opportunities that expensive, complicated tools can not. However there are a few trade-offs with this minimalist 2-module design: Supporting only Analog & I2C sensors makes the course more manageable but loosing the DS18b20, which has served us so well over the years, does bring a tear to the eye. Removing the SD card used on previous EDU models means that memory becomes the primary constraint on run-time. The RTC’s one second alarm means this logger is not suitable for higher frequency sampling and UV exposure makes the 50ml tubes brittle after 3-4 months in full sun. Coin cell chemistry limits operation to environments that don’t go far below freezing – although it’s easy enough to run the logger on two lithium AAA‘s in series and we’ve tested those down to -15°C.
The basic logger kit costs about $10 depending on where you get the DS3231 RTC & 3.3v/8MHz ProMini modules. Pre-assembly of the UART cable, NTC cluster & LED can be done to shorten lab time. CP2102 6pin UARTs are cheap, and have good driver support, but you have to make that Dupont crossover cable because the pins don’t align with the ProMini headers.
Sensor modules for lab activities: TTP233 touch, BMP280 pressure, BH1750 lux, AM312 PIR, 1k &10k pots and a sheet of metal foil for the capacitive sensing lab. Other useful additions are a Piezo Buzzer, a 0.49″ OLED and 32k AT24c256 EEproms. The screen is $4, but the other parts should should cost about $1 each.
You can find all the parts shown here on eBay and Amazon – except for the rail which needs to be printed but these days it’s relatively easy to send 3D models out to an printing service if someone at your school doesn’t already have a printer. Expect 15% of the parts from cheap suppliers like eBay or Amazon to be high drain, or simply DOA. Weorder three complete lab kits per student to cover defects, infant mortality, and replacement of parts damaged during the course. This is usually their first time soldering and some things will inevitably get trashed in the learning process – but that’s OK at this price point. We also order each part from three different vendors, in case one of them is selling rejects from a bad production run. The extra parts allow students to build a second or third logger later on in the course which is often needed for their final project.
I’ve used short jumpers here to make the connections clear, but it’s better use longer wires from a 20cm, F-F Dupont ribbon to make these cables. Only the 3.3v output from the Cp2102 gets connected to the logger.
Cp2102 UART
ProMini
DTR
->
DTR
RXD
->
TXO
TXD
->
RXI
GND
->
GND
3V3
->
VCC
Macintosh USBc to USBa adapters are smart devices with chips that will shut-down if you unplug from the computer with a battery powered logger is still connected. The coin cell back-feeds enough voltage to put the dongle into an error state. Always disconnect the UART to logger connection FIRST instead of simply pulling the whole string of still-connected devices out of the computer.
After installing OS drivers for your UART, you need to select the IDE menu options: [1] TOOLS > Board: Arduino Pro or Pro Mini [2] TOOLS > Processor: ATmega328 (3.3v, 8mhz) [3] TOOLS > Port: Match the COM# or /dev that appears when you connect the UART
On this UART the 5v connection had to be cut with a knife before soldering the 3.3v pads together to set the output voltage.
For many years we used FT232s, but the current windows drivers will block operation if you get one of the many counterfeit chips on the market. If you do end up with one of those fakes only OLD drivers from 2015 / 2016 will get that UART working with the IDE. To avoid that whole mess, we now use Cp2102’s or Ch340’s. Some UARTs require you to cut or bridge solder pads on the back side to set the 3.3v that an 8MHz ProMini runs on. Many I2C sensor modules on the market also require this lower voltage. Avoid Pro Mini’s with the much smaller 328P-MU variant processors. They may be compatible inside the chip but the smaller solder pad separation makes the overall logger noticeably more susceptible to moisture related problems later.
Assembling the logger:
This e360 model is based on the 2-Module logger we released in 2022, with changes to the LED & NTC connections to facilitate various lab activities in the course. That post has many technical details about the logger that have been omitted here for brevity, so it’s a good idea to read through that extensive background material when you have time.
Prepare the RTC module:
Clipping the Vcc supply leg (2nd leg in from the corner) puts the DS3231 into a low-power mode powered by backup battery, and also disables the 32k output.
Disconnect the indicator LED by removing its limit resistor.
Remove the 200Ω charging resistor, and bridge the Vcc via to the battery power trace at the black end of diode.
Cutting the VCC input leg forces the clock to run on VBAT which reduces the DS3231 chip constant current to less than 1µA, but that can spike as high as 550µA when the TCXO temperature reading occurs (every 64 seconds). The temp. conversions and the DS3231 battery standby current averages out to about 3µA so the RTC is responsible for most of the power used by this logger over time. If the time reads 2165/165/165 instead of the normal startup default of 2000/01/01 then the registers are bad and the RTC will not function. Bridging Vcc to Vbat means a 3.3V UART will drive some harmless reverse current through older coin cells while connected. DS3231-SN RTCs drift up to 61 seconds/year while -M chips drift up to 153 sec/year. If the RTCs temperature readings are off by more than the ±3°C spec. then the clocks will drift more than that.
It’s a good idea to do a breadboard test of those RTC modules (with the logger base-code) before assembling your logger.
Modify & Test the Pro Mini:
A Pro Mini style board continues as the heart of the logger, because they are still the cheapest low-power option for projects that don’t require heavy calculations.
Carefully clip the 2-leg side of the regulator with sharp side-snips and wobble it back and forth till it breaks the 3 legs on the other side.
Remove the limit resistor for the power indicator LED with a hot soldering iron tip.
Clip away the reset switch. This logger can only be started with serial commands via a UART connection.
Add 90° UART header pins and vertical pins on D2 to D6. Also add at least one analog input (here shown on A3). Students make fewer soldering errors when there are different headers on the two sides of the board for orientation.
Bend these pins inward at 45° and tin them for wire attachments later. D4 & D5 are used for the capacitive sensor lab.
Do not progresswith the builduntilyou have confirmed the ProMini has a working bootloader by loading the blink sketch onto it from the IDE.
Add the NTC/LDR Sensors & LED indictor
These components are optional, but provide opportunities for pulse width modulation and sensor calibration activities.
Join a 10k 3950 NTC thermistor, a 5528 LDR, a 330Ω resistor and a 0.1µF [104] ceramic capacitor. Then heat shrink the common soldered connection.
Thread these through D6=LDR, D7=NTC, D8=300Ω, and the cap connects to groundat the end of the Pro Mini module. Note that the D6/D7 connections could be any resistive sensors up to a maximum value of 65k ohms.
Solder the sensor cluster from the bottom side of the Pro Mini board and clip the tails flush with the board. Clean excess flux with alcohol & a cotton swab.
The way we read resistive sensors using digital pins is described in this post from 2019 although to reduce part count & soldering time in this e360 model we use the 30k internal pullup resistor on D8 as the reference value that the NTC and LDR get compared to. We have another post describing how to calibrate NTC thermistors in a classroom setting. Noise/Variation in the NTC temperature readings is ±0.015°C, so the line on a graph of rapid 1second readings is usually about 0.03°C thick. Range switching with two NTCs could also be done if the max / min resistance values of one thermistor can’t deliver the resolution you need.
Add a 1/8 watt 1kΩ limit resistor to the ground leg of a 5mm common cathodeRGB led. Extend the red channel leg with ~5 cm of flexible jumper wire.
Insert Blue=D10, Green = D11, GND = D12. Solder these from the under side of the Pro Mini and clip the excess length flush to the board.
Bring the red channel wire over and solder it through D9. Note that if the RGB is not added, the default red LED on D13 can still be used as an indicator.
You can test which leg of an LED is which color with a cr2032 coin cell using the negative side of the battery on the ground leg. The LED color channels are soldered to ProMini pins R9-B10-G11 and a 1k limit resistor is added to the D12-GND connection to allow multi-color output via the PWM commands that those pins support.
Join the Two Modules via the I2C Bus:
Use legs of a scrap resistor to add jumpers to the I2C bus connections on A4 (SDA) and A5 (SCL). Trim any long tails from these wires left poking out from the top side of the ProMini.
Cover the wires with small diameter heat-shrink and bend them so theycross over each other. The most common build error is forgetting the to cross these wires.
Use another scrap resistor to extend the Vcc and GND lines vertically from the tails of the UART headers. This is the most challenging solder joint of the whole build.
Add a strip of double-sided foam tape across the chips on the RTC module and remove the protective backing.
Carefully thread the I2C jumpers though the RTC module.
Press the two modules together and check that the two boards are aligned.
Check that the two I2C jumpers are not accidentally contacting the header pins below, then solder all four wires into place on the RTC module.
Bend the GND wire to the outer edge of the module, and trim the excess from the SDA and SCL jumpers. Adding a capacitor to the power wires helps the coin cell handle brief loads:
Optional: Solder a 440µF [477A] to 1000µF [108J] tantalum capacitor to the VCC and GND wires. Clip away the excess wire.
Tin the four I2C headers on the RTC module and the SQW alarm output pin.
Join the RTC’s SQW output to the header pin on D2 with a short length of flexible jumper wire. At this point the logger core is complete and could operate as a stand-alone unit.
Bend the four I2C header pins up to 45 degrees.
As soon as you have the two modules together: connect the logger to a UART and run an I2C bus scanning program to make sure you have joined them properly. This should report the DS3231 at address 0x68, and the 4K EEprom at 0x57.
Add Rails & Breadboard Jumpers:
Clip the bottom posts away from two 25 tie-point mini breadboards.
Insert the breadboards in the rails. Depending on the tolerance of your 3D print, this may require more force and/or a deburring tool to make the hole larger.
Mount the breadboards flush with the upper surface of the rails. If the they are too loose in your print they can be secured quickly with a drop of hot glue or cyanoacrylate super-glue sprinkled with a little baking soda to act as an accelerant.
The 3D printed rails have a pocket cutout for the logger stack. The RTC module board should sit flush with the upper surface of the rail. Hot glue can be applied from the underside through the holes near the corners to hold the logger to the rails…
or thin zip ties or twisted wire can hold the logger stack in place. The legs of a scrap resistor can be used if the holes on your RTC module are too small for zips. (see 1:06 in the build video)
Check that the RTC pcb is flush in the pocket at opposite diagonal corners.
Cut two 14 cm lengths of 22AWG solid core wire. Insert stripped ends into the breadboards as shown, then route though the holes in the rail.
Secure the wires from the underside with a zip tie. Note: the ‘extra’ holes in the rail are used to secure small desiccant packs during deployment.
Route the solid core wires along the side of the breadboard and back out through the two inner holes near the logger stack.
The green wire should exit on the analog side of the Pro Mini and the blue wire should be on the digital side.
Route and trim the green wire to length for the A3 header.
Strip, tin and solder the wire to the A3 pin.
Repeat the process for the blue wire, connecting it to D3.
Extend the four I2C headers on the RTC module with 3cm solid core jumpers. Here, white is SDAta and yellow is SCLock.
Bend the jumpers into the breadboard contacts. Bmp280 and Bh1750 sensor modules usually require this crossover configuration.
A video covering the whole assembly process:
NOTE: For people with previous building experience we’ve also posted a 4 minute Rapid Review.
The base code requires the RocketScream LowPower.h library to put the logger to sleep between readings and this can be installed via the library manager in the IDE. In addition to the included NTC / LDR combination, the code has support for the BMP/E280, BH1750(lux), and PIR sensors although you will need to install libraries (via the IDEs library manager) for some of them. Sensors are added by uncommenting define statements at the beginning of the code. Each sensor enabled after the single-byte LowBat & RTCtemp defaults contributes two additional bytes per sampling event because every sensors output gets loaded into a 16-bit integer variable.
The basic sensors cover light, temperature, pressure and humidity – so you could teach an introductory enviro-sci course by enabling or disabling those sensors before each lab. Note: while the BME280 is quite good for indoor measurements where very high RH% occurs rarely; SHT30 or AM2315C sensors encapsulated in water resistant PTFE shells are better choices for long term weather stations.
Bmp280 outputs can be saved individually. Total bytes per sampling record must be 1, 2, 4, 8 or 16 ONLY. You may need to add or remove RTC temp or Current Battery to make the byte total correct for a new sensor.
But limiting this tool to only the pre-configured sensors would completely miss the point of an open source data logger project. So we’ve tried to make the process of modifying the base-code to support different sensors as straight forward as possible. Edits are required only in the places indicated by call-out numbers on the following flow diagrams. These sections are highlighted with comments labeled: STEP1, STEP2, STEP3, etc. so you can locate them with the find function in the IDE.
Those comments are also surrounded by rows of +++PLUS+++ symbols: //++++++++++++++++++++++++++++++++++++++++++ // STEP1 : #include libraries & Declare Variables HERE //++++++++++++++++++++++++++++++++++++++++++
In Setup()
2024 note: Additional start-menu options have been added since this graphic was created in 2023, and there are a few additional debugging options that are not displayed unless serial output is enabled.
A UART connection is required to access the start-up menu through the serial monitor window in the IDE. This menu times-out after 8 minutes but the sequence can be re-entered at any timeby closing and re-opening the serial monitor. This restarts the Pro Mini via a pulse sent from the UARTs DTR (data terminal ready) pin. The start-up menu should look similar to the screen shot below, although the options may change as new code updates get released:
If you see random charactersin theserialwindow, you have the baud rate set incorrectly. Set the baud to 500,000 (with the pulldown menu on the lower right side of the serial monitor window) and the menu should display properly after you close & re-open the window. If you Ctrl-A & Ctrl-C to copy data from the serial monitor when the window still has garbled characters displayed, then only the bad starting characters will copy out. On a new logger: Hardware, Calibration & Deployment fields will display as a rows of question marks until you enter some text via each menu option.
The first menu option asks if you want to download data from the logger after which you can copy/paste everything from the serial window into a spreadsheet. Then, below the data tab in Excel, select Text to Columns to divide the data into separate columns at the comma separators. Or you can paste into a text editor and save a .csv file for import to other programs. While this transfer is a bit clunky, everyone already has the required cable and retrieval is driven by the logger itself. We still use the legacy 1.8.x version of the IDE, but you could also do this download with a generic serial terminal app. You can download the data without battery power once the logger is connected to a UART. However, you should only set the RTC after installing a battery, or the time will reset to 2000/01/01 00:00 when the UART is disconnected. No information is lost from the EEprom when you remove and replace a dead coin cell.
A Unix timestamp for each sensor reading is reconstructed during data retrieval by adding successive second-offsets to the first record time saved during startup. It is important that you download old data from a previous run before changing the sampling interval because the interval stored in memory is used for the calculation that reconstructs each records timestamp. This technique saves a significant amount of our limited memory and =(Unixtime/86400) + DATE(1970,1,1) converts those Unix timestamps into Excel’s date-time format. Valid sampling intervals must divide evenly into 60 and be less than 60. Short second-intervals are supported for rapid testing & debugging, but you must first enter 0 for the minutes before the seconds entry is requested. The unit will keep using the previous sampling interval until a new one is set. It helps to have a utility like Eleven Clock running so that you have HH:MM:SS displayed on your computer screen when setting the loggers clock.
The easiest way to measure the supplied voltage while the logger is connected to USB/UART power is at the metal springs in the Dupont cable.
Vref compensates for variations in the reference voltage inside the 328p processor. Adjusting that constant up or down by 400 raises/lowers the reported voltage by 1 millivolt. Adjust this by checking the voltage supplied by your UART with a multimeter while running the logger with #define logCurrentBattery enabled and serial output Toggled ON at a 1 second interval. Note the difference between the millivolts you actually measured and the battery voltage reported on the serial monitor and then multiply that by 400 to get the adjustment you need to make to the 1126400 value for vref. Restart and save this new number with the [ ] Change Vref menu option and repeat this procedure until the battery reading on screen matches what you are measuring with the DVM. This adjustment only needs to be done once as the number you enter is stored in the 328p EEprom for future use. Note that most loggers run fine with the default 1126400 vref although some units will shutdown early because they are under-reading. It’s rare to get two constants the same in a classroom of loggers so you can use student initials + vref as unique identifiers for each logger. If you do get a couple the same you can change the last two digits to to make unique serial numbers without affecting the readings. The battery readings have an internal resolution limit of 16 millivolts, so ±20mv is as close as you can get on screen.
After setting the time, the sampling interval, and other operating parameters, choosing [ ] START logging will require the user to enter an additional ‘start’ command. Only when that second ‘start’ confirmation is received does old data get erased by pre-loading every memory location in the EEprom with zero. A zero-trap is required on the first byte of each record because those preloaded zeros also serve as the End-Of-File markers later during download. (Note: If you leave the default LogLowestBattery enabled that is already done for you) LEDs then ‘flicker’ rapidly to indicate a synchronization delay while the logger waits to reach the first aligned sampling time so the code can progress from Setup() into the Main Loop().
In the main LOOP()
If all you do is enable sensors via defines at the start of the program you won’t have to deal with the code that stores the data. However to add a new sensor you will need to make changes to the I2C transaction that transfers those sensor readings into the EEprom (and to the sendData2Serial function that reads them back later). This involves dividing your sensor variables into 8-bit pieces and adding those bytes to the wire transfer buffer. This can be done with bit-math operations for long integers or via the lowByte & highByte macros for 16-bit integers. The general pattern when sending bytes to an I2C EEprom is:
Wire.beginTransmission(EEpromAddressonI2Cbus); // first byte in I2C buffer Wire.write(highByte(memoryAddress)); // it takes two bytes to specify the Wire.write(lowByte(memoryAddress)); // memory location inside the EEprom
loByte = lowByte(SensorReadingIntergerVariable); Wire.write(loByte); // adds 1st byte of sensor data to wire buffer hiByte = highByte(SensorReadingIntegerVariable); Wire.write(hiByte); // adds 2nd byte of sensor data to the buffer
— add more Wire.write statements here as needed for your sensors —
The saved bytes must total of 1, 2, 4, 8 or 16 in each I2C transaction. Powers of Twobyte incrementsare required because the # of bytes saved per sampling event must divide evenly into the physical page limit inside each EEprom, which is also a power of two in size. The code will display a warning on screen of bytesPerRecord is not a power of two.
Wire.endTransmission(); // Only when this command executes do the bytes accumulated in the wire buffer actually get sent to the EEprom.
The key insight here is that the wire library is only loading the bytes into a memory buffer until it reaches the Wire.endTransmission() command. So it does not matter how much time you spend adding (sensor variable) bytes to the transaction so long as you don’t start another I2C transaction while this one is in progress. Once that buffered data has been physically sent over the wires, the EEprom enters a self-timed writing sequence and the logger reads the rail voltage immediately after the write process begins. The only way to accurately gauge the state of a lithium battery is to check it while it is under this load.
NOTE: The data download function called in setup retrieves those separate bytes from the EEprom and concatenates them back into the original integer sensor readings for output on the serial monitor. So the sequence of operations in the sendData2Serial retrieval functionmust exactly match the order used in the main loop to load sensor bytes into the EEprom.
Adding Sensors to the Logger:
By default, the logger records the RTC temperature (#define logRTC_Temperature) at 0.25°C resolution and the battery voltage under load (#define logLowestBattery). These readings are compressed to only one byte each by scaling after subtracting a fixed ‘offset’ value. This allows about 2000 readings to be stored on the 4k (4096byte) EEprom which allows 20 days of operation at a 15-minute sampling interval.
A typical RTC temperature record from a logger installed into a cave early in the project. The datasheet spec is ±3° accuracy, but most are within ±0.5° near 25°C. If you do a simple y=Mx+B calibration against a trusted reference sensor, the RTC temperatures are very stable over time. The RTC updates it’s temperature register every 64 seconds so there is no benefit from reading it more frequently than once per minute.
That 4k fills more quickly if your sensors generate multiple 2-byte integers but larger 32k (AT24c256) EEproms can easily be added for longer running time. These can be found on eBay for ~$1 each and they work with the same code after you adjust the define statements for EEpromI2Caddr & EEbytesOfStorage at the start of the program.
This Bmp280 pressure sensor matches the connection pattern on this 32k EEprom module. So the two boards can be soldered onto the same set of double-length header pins.
Vertical stacking allows several I2C modules to fit inside the 50mL body tube. Any I2C sensor breakouts could be combined this way provided they have different bus addresses.
The pullup resistors on the sensor modules can usually be left in place as the logger will operate fine with a combined parallel resistance as low as 2.2k ohms. No matter what sensor you enable, always check that the total of all bytes stored per pass through the main loop is1,2,4,8 or 16or you will get a repeating data error when the bytes transmitted over the I2C bus cross a physical page boundary inside the EEprom. This leads to a wrap-around which over-writes data at the beginning of the memory block. Also note that with larger EEproms you may need to slow the serial communications down to only 250k BAUD to prevent the occasional character glitch that you sometimes see with long downloads at 500k.
Perhaps the most important thing to keep in mind is that breadboards connect to the module header pins via tiny little springs which are easily jiggled loose if you bump the logger. Small beads of hot glue can be used to lock sensor modules & wires into place on the breadboard area. ALSO add another drop can help secure the Cr2032 battery in place for outdoor deployments. Some sensors can handle a momentary disconnection but most I2C sensors require full re-initialization or they will not deliver any more data after a hard knock jiggles the battery contact spring. So handle the logger gently while it’s running – no tossing them in a backpack full of books! Many students make additional no-breadboard loggers with fully soldered connections to the sensor modules if their final projects require rough handling. It’s also a good idea to put 1 gram (or 2 half-gram) silica gell desiccant packswith color indicator beads inside the body tube for outdoor deployments. A change in the indicator bead color is the only way to know if moisture is somehow seeping in, potentially causing runtime problems or early shutdown.
The base code also includes the DIGITAL method we developed to read the NTC/LDR sensors. On this new build we used the internal pullup resistor on D8 as a reference to free up another digital pin. The blue jumper wire on D3 (the 2nd external interrupt) can wake the logger with high / low signals. This enables event timing and animal tracking. Pet behavior is a popular theme for final student projects.
The TTP233 can detect a press through 1-2mm of flat plastic but it does not read well through the curved surface of the tube. In open air it triggers when your finger is still 1cm away but the sensitivity can be reduced by adding a trimming capacitor.
The AM312 draws <15µA and has a detection range of ~5m through the centrifuge tube. This sensor has a relatively long 2-4 second reset time and will stay high continuously if it gets re-triggered in that time. Our codebase supports counting PIR detections OR using the PIR to wake the logger for other sensor readings instead of the standard RTC alarm.
These 0.49″ micro OLEDs sleep at 6µA and usually draw less than a milliamp displaying text at 50% contrast. However, like all OLEDs they send wicked charge-pump spikes onto the supply rails. A 220 or 440µF tantalum right next to them on the breadboard will suppress that noise. Sleep the ProMini while the pixels are turned on to lower the total current load on the battery.
These displays run about two weeks on a coin cell if you only turn them on briefly at 15 minute intervals, depending on contrast, pixel coverage, and display time. It might also be possible to completely depower them when not in use with a mosfet like the TN0702N3.
These OLEDs are driven by a 1306 so you can use standard libraries like GriemansSSD1306Ascii which can be installed via the library manager. However, the mini screens only display a weirdly located sub-sample of the controllers 1k memory – so you have to offset the X/Y origin points on your print statements accordingly.
While I2C sensors are fun, we should also mention the classics. It is often more memorable for students to see or hear a sensors output, and the serial plotter is especially useful for lessons about how noisy their laptop power supply is…
If you twist the legs 90°, a standard potentiometer fits perfectly into the 25 tie-point breadboard for ADC control of PWM rainbows on the RGB LED.
Light-theremin tones map onto the squawky little Piezo speaker and alligator clips make it easy to try a variety of metal objects in the Capacitive Sensing lab.
If you run a lab tethered to the UART for power, then your only limitation is the 30-50 milliamps that those chips normally provide. This is usually enough for environmental sensors although some GPS modules will exceed that capacity. If a high-drain GPS module or a infrared CO2 sensor is required then use those you will need one of the previous AA powered loggers from the project.
When running the logger in stand alone mode your sensors have to operate within the current limitations of the CR2032 coin cell. This means sensors should take readings below 2mA and support low-power sleep modes below 20µA (ideally < 2µA). Order 3.3v sensor modules without any regulators – otherwise the 662k LDO on most eBay sensor modules will increase logger sleep current by ~8µA due to back-feed leakage through the reg. Sensors without regulators usually have -3.3v specified in the name, so a GY-BME280-3.3v humidity sensor has no regulator, but most other BME280 modules will have regulators.
The best sensor libraries to choose should support three things: 1) one-shot readings that put the sensor into a low power sleep mode immediately after the readings 2) give you the ability to sleep the ProMini processor WHILE the sensor is generating those readings and 3) use integer mathematics for speed and a lower memory footprint. Many sensors can read at different resolutions using a technique called oversampling but creating high resolution (or low noise) readings with this method takes exponentially more power. So you want your library to let you set the sensor registers to capture only the resolution you need for your application. The library should also have some way set the I2C address to match your particular sensor module as most sensors support different addresses depending on which pin is pulled up. Always have a generic I2C bus scanning utility handy to check that the sensor is showing up on the bus at the expected address after you plug it into the breadboard (and restart the logger).
Logger Operation:
The logger usually draws peak currents near 3.3mA although this can be increased by sensors and OLEDs. The logger typically sleeps between 5 – 10µA with a sensor module attached. Four 5mA*30millisecond (CPU time) sensor readings per hour gives a maximum battery lifespan of about one year. So the logger is usually more limited by memory than the 100mAh available from a Cr2032. The tantalum rail-buffering capacitor only extends operating life about 20% under normal conditions, but it becomes more important with poor quality coin cells or in colder environments where the battery chemistry slows down:
A BMP280 sampling event with NO rail buffering capacitor draws a NEW coin cell voltage down about 50mv during the logging events…
…while the voltage on an OLD coin cell falls by almost 200 millivolts during that same event on the same logger – (again with NO rail buffer cap)
Adding a 1000µF [108j] tantalum rail buffer to that same OLD battery supports the coin cell, so the logging event now drops the voltage less than 20mV.
The code sleeps permanently when the battery reading falls below the value defined for systemShutdownVoltage which we usually set at 2850mv because many 328p chips trigger their internal brown-out detector at 2.77v. And the $1 I2C EEprom modules you get from eBay often have an operational limit at 2.7v. If you see noisy voltage curves there’s a good chance poor battery contact is adding resistance: secure the coincell with a drop of hot glue before deployment.
High & Low battery readings (in mv) from an e360 logging Pressure & Temp from a BMP280 @30min intervals to a 32k I2C EEprom (full in 84 days). This unit slept at 3.2µA and had a 1000uF tantalum rail capacitor. This kept battery droop below 50mv during the data save, stabilizing above 3000mv.
Same configuration & new Cr2032 at start, only this logger had NO rail capacitor and slept at only 2µA. Despite burning less power during sleep, the coin cell voltage droop during data saves was ~100mv, so the battery plateau is closer to the low battery shutdown at 2800mv.
When testing sleep current on a typical batch of student builds, some will seem to have anomalously high sleep currents in the 600-700µA range. Often that’s due to the RTC alarm being on (active low) which causes a constant drain through the 4k7 pullup resistor on SQW until the alarm gets turned off by the processor. Tantalum capacitors are somewhat heat sensitive, so beginners can damage them while soldering and in those cases they my turn into a short. A typical student logger should draw between 3-10µA when it is sleeping between sensor readings, and if they are consistently 10x that, replacing an overheated rail capacitor may bring that down to the expected sleep current. Also check that the ADC is disabled during sleep, as that will draw ~200µA if it is somehow left on. Occasionally you run into a ProMini clone with fake Atmel 328P chips that won’t go below ~100µA sleep no matter what you do. Buteven with a high 100µA sleep current logger, a new Cr2032 should run the logger for about a month measuring something like temperature, and 10 days with a high power demand sensor like the BME280. This is usually enough run time for data acquisition within your course schedule even if the high drain issue does not get resolved.
Occasionally you get an RTC module with a weak contact spring making the logger quite vulnerable to bumps disconnecting power. A small piece of double sided foam tape can be used to make the battery contact more secure before any outdoor deployments (although this is somewhat annoying to remove afterward). Note that on this unit hot glue was used to affix the logger to the printed rail instead of zip ties.
A few student loggers will still end up with hidden solder bridges that require a full rebuild. This can be emotionally traumatic for students until they realise how much easier the process goes the second time round. Once you’ve made a few, a full logger can usually be assembled in less than 1.5 hours. It’s even faster if you make them in batches so that you have multiple units for testing at the same time. Order a box of 200 cheap coin cell batteries before running a course, because if a student accidentally leaves a logger running blink (as opposed to the logger code) it will drain a battery flat in a couple of hours. This happens frequently.
Measuring tiny sleep currents in the µA range can not be done easily with cheap multimeters because they burden to whole circuit with extra resistance. So you need a special tool like the uCurrent GOLD multimeter adapter (~$75) or the Current Ranger from LowPowerLab (~$150 w screen & battery). The only commercial device for measuring tiny currents that’s remotely affordable is the AltoNovus NanoRanger, but that’s more than twice the price of the µC or the Ranger. You can also do the job with an old oscilloscope if you have that skillset. Again, sleep current is more of a diagnostic tool so you can usually run a course without measuring the sleep currents by simply running the loggers for a week between labs while recording the battery voltage. If that graph plateaus (after a week of running) near 3v and just stays there for a long time – your logger is probably sleeping at the expected 3-5µA.
Running the labs:
The basic two module combination in this logger (without any additional sensors) can log temperature from the RTC. This ±0.25°C resolution record enables many interesting temperature-related experiments, for example:
Log the temperature inside your refrigerator for 24 hours to establish a baseline
Defrost your freezer and/or clean the coils at the back of the machine
Log the temperature again for 24 hours after the change
Calculate the electric power saved by comparing the compressor run time (ie: while the temperature is falling) before & after the change
Note that the housing, the air inside it, and the thermal inertia of the module stack, result in ~5 to 10 minutes lag behind temperature changes outside the logger.
The small form factor of the e360 enables other benchtop exercises like this Mason jar experiment. The loggers are sealed in a cold jar taken directly from the freezer with a BMP280 sampling pressure & temp. every 15 seconds. The jars are then placed in front of a fan which brings them to room temp in ~45 minutes. For a comparison dataset, this experiment can also be done in reverse – sealing the loggers in room temperature jars which are then cooled down in a freezer; although that process takes longer.
Through no fault of their own, students usually have no idea what messy real-world data looks like, and many have not used spreadsheets before. So you will need to provide both good and bad example templates for everything, but that’s easy enough if you ran the experiment a dozen times yourself at the debugging stage.
Even then students will find creative ways to generate strange results: by using a cactus for the evapotranspiration experiment or attempting the light sensor calibration in a room that never rises beyond 100 Lux. Deployment protocols (Sensor Placement, etc.) are an important part of any environmental monitoring course, and ‘unusable data’ ( though the logger was working ) is the most common project failure. It is critical that students download and send graphs of the data they’ve captured frequently for feedback before their project is due. Without that deliverable, they will wait until the hour before a major assignment is due before discovering that their first (and sometimes only) data capturing run didn’t work. This data visualization is required for ‘pre-processing’ steps like the synchronization of different time series and for the identification of measurements from periods where the device was somehow compromised. Your grading rubric has to be focused on effort and understanding rather than numerical results, because the learning goals can still be achieved if they realize where things went wrong.
The temperature readings have serious lag issues while the pressure readings do not. A good lesson in thinking critically about the physical aspects of a system before trusting a sensor. With the built-in 4096 byte EEprom, saving all three 2-byte Bmp280 outputs (temp, pressure & altitude) plus two more bytes for RTCtemp & battery, gives you room for 512 of those 8-byte records. If you sample every fifteen seconds, the logger will run for two hours before the RTC’s 4k memory is full. The test shown above was done with a ‘naked’ logger, but when the loggers are used with inside the centrifuge body tube the enclosed temperature sensors have about 15 minutes of thermal lag behind changes in air temperature outside the tube.
Try to get your students into the habit of doing a ‘fast burn’ check whenever the logger is about to be deployed: Set the logger to a 1-second interval and then run it tethered to the Uart for 20-30 seconds (with serial on) . Then restart the serial monitor window to download those few records to look at the data. This little test catches 90% of the code errors before deployment.
Important things to know:
Time: You need to start ordering parts at least three months ahead of time. If a part cost $1 or less, then order 5x as many of them as you think you need. Technical labs take a week to write, and another week for debugging. You can expect to spend at least an hour testing components before each lab. The actual amount of prep also depends on the capabilities of your student cohort, and years of remote classes during COVID lowered that bar a lot. Have several spare ‘known good’ loggers (that you built yourself) on hand to loan outso hardware issues don’t prevent students from progressing through the lab sequence while they trouble-shoot their own builds. Using multi-colored breadboards on those loaners makes them easy to identify later. Measuring logger sleep current with a DSO 138 scope or a Current Ranger will spot most hardware related problems early, but students don’t really get enough runtime in a single course to escape the bathtub curve of new part failures.
Yes, some of that student soldering will be pretty grim. But my first kick at the can all those years ago wasn’t much better and they improve rapidly with practice. As long as the intended electrical contact is made without sideways bridges, the logger will still operate.
Money: Navigating your schools purchasing system is probably an exercise in skill, luck and patience at the best of times. Think you can push through dozens of orders for cheap electronic modules from eBay or Amazon? Fuhgeddaboudit! We have covered more than half of the material costs out of pocket since the beginning of this adventure, and you’ll hear that same story from STEM instructors everywhere. If you can convince your school to get a class set of soldering irons, Panavise Jr. 201s, multimeters, and perhaps a 3D printer with some workshopsupplies, then you are doing great. Just be ready for the fact that all 3D printers require maintenance, and the reason we still use crappy Ender3V2’s is that there’s no part on them that can’t be replaced for less than $20. We bought nice multi-meters at the beginning of this adventure but they all got broken, or grew legs long before we got enough course runs with them. We now use cheap DT830’s and design the labs around it’s burden-voltage limitations. Small tools like 30-20AWG wire-strippers and side-snips should be considered consumables as few of them survive to see a second class. Cheap soldering irons can now be found for ~$5 (which is less than tip replacement on a Hakko!) and no matter which irons you get the students will run the tips dry frequently. The up side of designing a course around the minimum functional tools is that you can just give an entire set to any students who want to continue on their own after the course. That pays dividends later that are worth far more than one years budget.
An inexpensive BMP280 can be used as a temperature reference for the thermistor calibration lab. At a 1 minute interval the logger will run for 16 hours before the 4K EEprom is full. The logger should remain in each of the three water baths for 8 hours to stabilize. Stainless steel washers keep the logger submerged.
With insulated lunch box containers, the 0°C bath develops a nice rind of ice after being left in the freezer over night. No reference sensor is needed for the ice point because that is a physical constant.
All that probably sounds a bit grim, but the last thing we want is for instructors to bite off more than they can chew. Every stage in a new course project will take 2-3x longer than you initially think! So it’s a good idea to noodle with these loggers for a few months before you are ready to integrate them into your courses. Not because any of it is particularly difficult, but because it will take some time before you realize the many different ways this versatile tool can be used. Never try to teach a technical lab that you haven’t successfully done yourself a few times.
A good general approach to testing any DIY build is to check them on a doubling schedule: start with tethered tests reporting via the serial monitor, then initial stand-alone tests at 1,2,4 & 8 hours till you reach a successful overnight run. Follow this by downloads after 1,2 ,4 & 8 days. On the research side of the project, we do fast (seconds) sample-interval runs to full memory shutdown several times, over a couple of weeks of testing, before loggers are considered ready to deploy in the real world. Even then we deploy 2-3 loggers for each type of measurement to provide further guarantee of capturing the data. In addition to data integrity, smooth battery burn-down curves during these tests are an excellent predictor of logger reliability, but to use that information you need to be running several identical machines at the same time and start them all with the same fresh batteries so you can compare the graphs to each other. A summer climate station project with five to ten units running in your home or back yard is a great way to start and, if you do invest that time, it really is worth it.
Common coding errors like mishandled variables usually generate repeating patterns of errors in the data. Random processor freezing is usually hardware/timing related and the best way to spot the problematic code is to run with ‘logger progressed to Point A/B/C/etc’ comments printed to the serial monitor. In stand-alone mode you can turn on different indicator LED color patterns for different sections. Then when the processor locks up you can just look at the LEDs and know approximately where the problem occurred in your code.
Last Word:
The build lab at thebeginning of the course – with everybody still smiling because they still have no idea what they are in for. The course is offered to earth & environmental science students. Reviewing Arduino based publications shows that hydrologists & biologists are by far the largest users of Open Science Hardware in actual research – not engineers! Traditional data sources rarely provide sufficient spatiotemporal resolution to characterize the relationships between environments and the response of organisms.
Why did we spend ten years developing a DIY logger when the market is already heaving with IOT sensors transmitting to AI back-end servers? Because the only ‘learning system’ that matters to a field researcher is the one between your ears. Educational products using pre-written software are usually polished plug-and-play devices, but the last thing you want in higher education is something that black-boxes data acquisition to the point that learners are merely users. While companies boast that students can take readings without hassle and pay attention only to the essential concepts of an experiment, that has never been how things work in the real world. Trouble shooting by process-of-elimination, combined with modest repair skills often makes the difference between a fieldwork disaster and resounding success. So sanitized equipment that generates uncritically trusted numbers isn’t compatible with problem-based learning. Another contrast is the sense of ownership & accomplishment that becomes clear when you realize how many students gave their loggers names and displayed them proudly in their dorm after the course. That’s not something you can buy off a shelf.
And: We made a Classroom Starter Kit part list back in 2016 when we were still teaching with UNOs and a post of Ideas for your Arduino STEM Curriculum. Those posts are nowterribly out of date, but probably still worth a read to give you a sense of things you might want to think about when getting a new course off the ground. Those old lists also predate our adoption of 3D printing, so I will try post updated versions soon. The bottom line is that if you are going to own a 3D printer you should expect to have to completely tear down the print head and rebuild it once or twice per year. While they are the bottom of the market in terms of noise & speed, every possible repair for old Enders is cheap and relatively easy with all the how-to videos on YouTube. Those videos are your manual because Creality broke the cost barrier to mass market adoption by shipping products that were only 90% finished, with bad documentation, and then simply waiting for the open source hardware community to come up with the needed bug-fixes and performance improvements. X/Y models are more robust than bed-slingers, but our Ender 3v2’s have been reliable workhorses provided you stick to PLA filament. If you want a fast turn-key solution then your best option is one of the Bambu Labs printers like the A1 because they are essentially plug&play appliances. But if you enjoy tinkering as a way of learning, that’s Ender’s bread & butter. A bed level touch sensor is -by far- the best way to get that critical ‘first layer’ of your print to stick to the build plate and the E3V2 upgraded with a CRtouch & flashed to MriscoC is an OK beginners rig. Our local computer stores always seem to brand new 3v2’s for about $100 as a new customer deal but these days I buy cheap ‘for parts’ Ender5 S1’s from eBay for about that price and fix them up because 120mm/s is about the minimum speed I have the patience for. Now that multi-color machines are popular, what used to be high-end single-color machines are being discounted heavily so even brand new E5S1’s are selling for about $200 in 2025 if you look around. Having two printers means a jam doesn’t bring production to halt just before fieldwork, and by the time you get three printers the speed issues disappear and you can dedicate machines to different filaments. Keep in mind that no matter how fast your printer is capable of moving the print head, materials like TPU and PETG will still force you to print at much slower rates if you want clean prints with tight tolerances. To reduce noise, I usually put the printer onto a grey 16x16inch cement paver with some foam or squash ball feet under it.
Printer jams like this are inevitable. But if you don’t manage to fix it, a pre-assembled replacement hot end for an Ender3v2 is only $10. I find that it’s better to have multiple slower printers than one high-end machine because then a single jam or failure doesn’t take out all of your production capacity just before a fieldwork trip. Functional prints rarely have to be pretty – so you can speed up production with lower quality settings.
Avoid shiny or multi-color silk filaments for functional prints as they are generally more brittle and crack easily. Prints also get more brittle as they absorb humidity from the air. If that happens, cyanoacrylate glue + baking soda can be used for quick field repairs. It’s worth the extra buck or two to get filaments labeled as PLA pro as they usually have better durability in trade for slightly higher printing temperature (as long as the company was not lying about their formulation). I use a food dehydrator I bought for $10 from a local charity shop to dry out my PLA or PETG filaments if they have been open for more than a month. Really hygroscopic filaments (like PVA, TPU or Nylon) have to be dried overnight before every single print. Most machines work fine with the defaults, but you can get great prints out of any printer provided the bed is leveled, the e-steps & flow are calibrated, and the slicer settings are tuned for the filament you are using. If you are using Cura there is a plugin called AutoTowers Generator that makes specialtestprints. You print a set of those (for temperature, flow rate and then retraction) and set your slicer settings to match the place on the tower where the print looks best. You may have to do this for each brand of filament as they can have quite different properties. I rarely use filaments that cost more than $15/kg roll because functional prints don’t have to be that pretty and you will be tweaking each design at least 10 times before you iterate that new idea to perfection. I stock up on basic black/white/grey on Amazon sale days for only $10/roll. Once you get the hang of it, most designs can use 45° angles and sequential bridging to print without any supports. Conformal coating or a layer of clear nail polish prevents marker ink from bleeding into the print layers when you number your deployment stations.
Our Lux-to-PAR calibration post is a good example of the physical problems that have to be solved before you can collect good data: A printed shield was necessary for to keep the temp sensors cool under direct sun and a ball joint was required for leveling. A ground spike was needed for the mounting. I have started bumping up against the limits of what you can do in Tinkercad with some of our more complex designs, but OnShape is there waiting in the wings when I’m ready.
As a researcher, becoming functional with Tinkercad and 3D printing is a better investment of your time than learning how to design custom PCBs because it solves a much larger number of physical issues related to unique experiments and deployment conditions. That said, it’s not usually worth your time to design things from scratch if you can buy them off the shelf. So people get into 3D printing because what they need does not exist or is outrageously expensive for what it does. All FDM prints are porous to some extent because air bubbles get trapped between the lines when they are laid down – so they are not truly waterproof like injection molded parts unless you add extra treatment steps. UV-resistant clear coating sprays are a good idea for outdoor deployment. I don’t have much use for multi-color printing, but when they advance that to supporting multiple materials I will be interested because being able to print at TPU gasket inside a PETG housing would be quite useful.
If you are just getting started and looking for something to learn the ropes you can find useful add-ons to your printer on thingyverse or, you could try organizing your workspace with the Gridfinity system from Zack Freedman. You will also find lots of handy printable tools out there for your lab bench searching with the meta-engines like Yeggi or STLfinder. A final word of warning: this hobby always starts with one printer… then two… then… don’t tell your partner how much it cost.
Cr2032Internal Resistance vs mAh [Fig6 from SWRA349] Our peak load of ~8 mA while writing data to the EEprom creates a voltage drop across the battery IR. The load induced transient on the 3v Cr2032 can’t fall below 2.775v or the BOD halts the 328p processor. This limits our useable capacity to the region where battery IR is less than 30 ohms. This also makes it critical to control when different parts of the system are active to keep the peak currents low. With this relatively high cutoff we can only use about 100 milliampseconds of power from a typical CR2032.
Reviewers frequently ask us for estimates based on datasheet specifications but this project is constantly walking the line between technical precision and practical utility. The dodgy parts we’re using are likely out of spec from the start but that’s also what makes our 2-module data loggers cheap enough to deploy where you wouldn’t risk pro-level kit. And even when you do need to cross those t’s and dot those i’s you’ll discover that OEM test conditions are often proscribed to the point of being functionally irrelevant in real world applications. The simple question: “How much operating lifespan can you expect from a coin cell?” is difficult to answer because the capacity of lithium manganese dioxide button cells is nominalat best and wholly dependent on the characteristics of the load. CR2032’s only deliver 220mAh when the load is small: Maxell’s datasheet shows that a 300 ohm load, for a fraction of a second every 5 seconds, will drop the capacity by 25%. However, if the load falls below 3μA for long periods, then this also causes the battery to develop higher than normal internal resistance, reducing the capacity by more than 70%. The self-discharge rate increases with temperature due to electrolyte evaporating through the edge seal. Another challenge is ambient humidity which can change PCB leakage currents significantly.
Voltage Under EEprom load VS date [runtime hours in legend] with red LED on D13 driven HIGH for 1.4mA sleep current, 30second interval, 8-byte buffer. These are serial tests performed on the same logger. 1.4mA continuous is probably is not relevant to our duty cycle.
Surprisingly little is known about how a CR2032 discharges in applications where low μA level sleep currents are combined with frequent pulse-loads in the mA range; yet that’s exactly how a datalogger operates. As a general rule, testing and calibration should replicate your deployment conditions as closely as possible. However, normal run tests take so long to complete that you’ve advanced the code in the interim enough that the data is often stale by the time the test is complete. Another practical consideration is that down at 1-2μA: flux, finger prints, and even ambient humidity skew the results in ways that aren’t reproducible from one run to the next. So a second question is “How much can you accelerate your test and still have valid results?” Datasheets from Energiser, Duracell, Panasonic and Maxell reveal a common testing protocol using a 10-15kΩ load. So continuous discharges below 190μA shouldn’t drive you too far from the rated capacity. Unfortunately, that’s well below what you get with affordable battery testers, or videos on YouTube, so we are forced yet again to do our own empiricaltesting.
The easiest way to increase our base load is to leave the indicators on: all three LEDs will add ~80μA to the sleep current when lit using internal the pullup resistors. 80μA is ~16x our normal 5μA sleep current (including sensor draw & RTC temp conversions). A typical sampling interval for our work is 15min so changing that to 1 minute gives us a similarly increased number of EEprom save events. With both changes, we tested several brands to our 2775mv shut-down:
Cr2032 Voltage Under EEprom Load VS Date: Accelerated Cr2032 run tests with 3xLED lit with INPUT_PULLUP for ~80μA sleep current although each unit was slightly different as noted, 1min sampling interval. Blue Line = Average excluding Hua Dao. CLKPR reduced system clock to 1MHz during eeprom save on this test to reduce peak currents to about 6mA. All units had 227E 25V 220μF rail buffering caps.
Note: With the slight variation between each loggers measured sleep current, the times listed here have been adjusted to a nominal 80μA. Also note that the price/cell is highly dependant on vendor & quantity.
Despite part variations these batteries were far more consistent on that 20ohm plateau than I was expecting. This ~15x test gives us a projected runtime of more than two years! That’s twice the estimate generated by the Oregon Embedded calculator when we started building these loggers. We did get a 30% delta between the name brands, but these tests were not thermally controlled and we don’t know how old the batteries were before the test. The rise in voltage after that initial dip is probably the pulse loads removing the passivation layer that accumulates during storage. The curves are a bit chunky because the 328P’s internal vref trick has a resolution of only 11mv, and we index-compress that to one byte which results in only 16mv/bit in the logs.
One notable exception is the Hua Dao cells, which I tested because, at only 14¢ each, they are by far the cheapest batteries on Amazon. My guess they are so cheap because they used a lower grade of steel for the shell which increases the cell resistance. We have many different runs going at any one time, and to make those inter-comparable you need to start each test with a fresh cell. Even if the current run test doesn’t need a batteries full capacity sometimes you just need to eliminate that variable while debugging. You also use a lot of one-shot batteries for rapid burn-in tests so it makes sense for them to be as cheap as possible. Now that I know Hua Dao delivers half the lifespan of name brand cells, I can leverage that fact to run some of those tests more quickly. I had planned on doing this with smaller batteries but the Rayovac Cr2025 I tested ran for 1035 hours – longer than the Hua Dao Cr2032!
Cr2032’s used since January for bench testing.
Testing revealed another complicating factor when doing battery tests: With metal prices sky-rocketing, fake lithium batteries are becoming more of a problem. We’ve been using Sony Cr2032’s from the beginning of the project but the latest batch performed more like the Hua Dao batteries. This result was so unexpected that I dug through the bins for some old stock to find that the packaging looked slightly different:
Fake (left) vs Real (right)
On closer inspection it didn’t take long to spot the fraud:
Fake Sony Battery : laser engraved logo
Real Sony Battery: Embossed logo
More tests are under way so I’ll add those results to this post when they are complete. A couple of the 80μA units have been re-run after removing the 227E 25V220μF rail buffering caps, confirming that the tantalum does not extend overall run time on good batteries very much because their internal resistance rises slowly, but rail caps can more than double the lifespan with poor quality batteries like the Hua Dao. In my 20min tests at 3 volts, [477A] 10v 470μF caps eventually fell to a leakage of about 25nA which was not much more than the 15nA on a [227E] 25v 220μF tantalum. 6v volt [108J] 1000μF rail caps have much higher 980nA leakage which is larger than the DS3231’s typical constant Ibat of 840nA. This shortens overall logger runtime to only 6-7 months: so very large rail caps are only useful with high drain sensors or in cold conditions. It’s also worth mentioning that the spring contacts on those RTC modules are quite weak and may need a bit of heat shrink tubing behind them to strengthen the connection to the flat surface of the coin-cell. A small bead of hot glue on top also helps protect the battery from bump-disconnects.
Northern caves hold near 5-10°C all year round, so the current set is running in my refrigerator. I will follow that with hotter runs because both coin cell capacity AND self-discharge are temperature dependant. We also plan to start embedding these loggers inside rain gauges which will get baked under a tropical sun .
Addendum 2023-08-01
To avoid bump-disconnects in rough fieldwork conditions I sometimes add a bit of double-sided foam mounting tape behind the contact spring. This holds the battery well but is also a significant pain in the backside to remove at the end of the deployment. Usually that’s a good trade to guarantee the data.
This summers fieldwork required all of the units in my testing fleet so I only have a handful of results from the refrigerator burn down tests [@5°C]. The preliminary outcome is that, compared to the room temperature burns, the lithium cell plateau voltage lowers between 80-100mv (typically from 3000mv to 2900mv). Provided the loggers were reading a low drain sensor the ‘cold’ lifespan was only about 20% shorter because the normal 50-70mv battery droop (during sensor reading / eeprom save) only becomes important after the battery falls off its plateau. This is approximately the same lifespan reduction you see running at room temp without a rail buffering capacitor – as the buffer also only comes into play when the battery voltage is descending. This is also the reason why the larger 1000μF rail capacitors usually only provide about 10% longer life than the 220μF rail caps as the reduced battery droop with the larger cap only matters when the cell is nearing end of life. Net result is that increasing to 1000μF rail buffers almost exactly offsets the lifespan losses at colder ambient temps around 5°C . But for longer deployments, at normal room temps, the 1-2μA leakage of a 6v 1000μF [108j] tantalum removes any advantage over the 25v 220μF [227e] which has only 5nA leakage at 3 volts. Also note that some caps seem to need a few hours to ‘burn in’ before they are saturated enough to measure their leakage properly.
The freezer results are an entirely different situation where are only seeing a few days of accelerated 100μA sleep current operation because once you get below -15°C the coin cell plateaus below our 2800mv shutdown voltage. So the logger only operates for the brief span of time where the initial overvoltage on new Cr2032 is still above its nominal voltage. Even a 1000μF cap will not fix that problem so for truly cold weather deployments you’d need a different battery chemistry like lithium-thionyl chloride (LiSOCl2) which is good to -40°C provided you add a large rail buffering capacitor to compensate for it’s current limitations. Loggers drawing the a more normal 2-5μA sleep current run OK in a domestic freezer if the sensor is not too demanding. In fact we use waters 0°C phase transition as a physical reference when calibrating the onboard thermistors – and that’s done on a normal CR2032.
Addendum 2025-01-25
Finally getting the 2024 deployments swapped out, so I have some real-world burn down curves to add here. These units were in a U.S. cave rather than our typical Mexico field sites, so temps were cool enough to have some effect on the cell voltage:
11 months of [°C] temperature variation near the entrance of the cave, read by an NTC thermistor on the logger.
A new NightKonic Cr2032 coin cell was installed at the start of the deployment. This logger had a continuous sleep current of 1.2µA, but the RTC TXCO corrections bump that to >3µA average.
[°C] temperature variation is reduced as you move further into the cave (until eventually there is little of the outside annual climate signal left.)
Again a new NightKonic battery was used with this 1.6µA (continuous) sleeper. Lowest battery under load was saved once per day for these records.
With normal runtime operations the CR2032s plateaued near the nominal 3v with a 220µF rail buffering cap. It’s hard to tell if that slight shift indicates the end of the batteries 20ohm plateau or was just a response to the lower winter temperatures. Assuming that was the end of the plateau, then I’d expect another three (?) months of operation before the internal resistance pushes the voltage droop during EEprom saves to our 2800mv cutoff. Both were deployed with 1 gram of desiccant, which probably was not enough for the 30mL polypropylene tubes (given the color of the indicator beads when the loggers were downloaded). Unsaturated silica gel holds the air near 20%, but these loggers probably spent half of their deployment at high RH. I rarely put desiccant in my testing loggers and on the rare occasion when I forget one of them long enough for the Cr2032 to fall below 1000mv there is “a particular smell” that is quite noticeable when the logger is opened. That makes me wonder if electrolyte off-gassing also affects the color of the desiccant indicators beads.
The take home lesson is: when your accelerated testing indicates a two year lifespan under optimal conditions, you should expect to reach only half of that on a real-world deployment. If I assume a 3.5µA sleep current (ie: the DS3231’s timekeeping average incl. TXCO conversions) with four 3.5mA x 50msec logging events per hour, then the Oregon Embedded calculator gives me an 100mAh battery life of 958 days. Interestingly, the capacity of a 256k EEprom @ 15min interval storing 4bytes/record is also about 680 days. Taken together, these factors mean our loggers should only be expected to operate for a year – plus – a healthy 3-6 month margin if you used a good Cr2032 battery.
One unknown variable is the self-discharge rate of the Cr2032. I’ve seen references to a nominal 1% loss in battery capacity per year under ideal conditions, which would be like adding another 300nA of continuous load. But that loss can be up to 10x larger in high humidity environments.
Another variable that can have a serous impact on lifespan is the number of EEprom save events per day.In my tests a typical EEsave uses between 0.3-0.5 milliamp seconds of power, with sensor readings using a similar amount/record. (in comparison to a ‘good’ loggers sleep-current baseload of about 250-300 milliamp-seconds per day) So a few hundred EEsave events could potentially use power comparable to the sleep current each day. EEproms have to erase and re-write an entire page of memory any time you save a number of bytes less than the hardware page size. If you increase the two wire library buffers, and pre-buffer the sensor data into temporary arrays, you can increase the bytes per EEsave event from 16 to 64 (or even 128) to reduce the number of save events. When you transfer the same number of bytes as the EEproms hardwarepage size then the chip can skip any ‘pre-erase’ steps entirely – cutting ALL save events to 1/2 the time/power they would use saving a smaller number of bytes. This both increases speed and can extend operating lifespan significantly with multi sensor configurations collecting many bytes of data per record over short 1-5 minute sampling intervals.
Pressure testing has been on the to-do list for ages, but the rating on the PVC parts in our older housings meant we weren’t likely to have any issues. However, the new two-part mini-loggers fit inside a thin walled falcon tube, which raised the question of how to test them. There are a few hyperbaric test chambertutorials floating around the web, and we made use of one built from a scuba tank back at the start of the project, but I wanted something a less beefy, and easier to cobble together from hardware store parts. Fortunately Brian Davis, a fellow maker & educator, sent a photo of an old water filter housing he’d salvaged for use with projects that needed pressure tests. Residential water supply ranges from 45 to 80 psi so could replicate conditions down to 55m. That’s good for most of our deployments and certainly farther than I was expecting those little centrifuge tubes to go.
This mini pressure chamber was made from a Geekpure4.5″x10″ water filter housing, 2x male-male couplers, a garden tap, & a pressure gauge with a bicycle pump inlet. (~$70 for this combination) The relief valve & o-ring required silicone grease to maintain pressure.
I first tested 50mL ‘Nunc’ tubes from Thermo. These are spec’d to 14psi/1atm, but that’s a rating under tension from the inside. I put indicator desiccant into each tube so small/slow leaks would be easy to see and used a small bicycle pump to increase the pressure by 5psi per day. These tubes started failing at 25psi, with 100% failure just over 30psi. Multiple small stress fractures occurred before the final longitudinal crack which produced an audible ‘pop’ – often four or more hours after the last pressure increase. If 20psi is the max ‘safe’ depth for these tubes then the 50mL tubes can deployed to about 10m with some safety margin for tides, etc. This result matches our experience with these tubes as we often use them to gab water samples while diving.
[Click photos to enlarge]
As expected, the self-standing 30mL tubes proved significantly more resistant. All of them made it to 45psi and then progressed through various amounts of bending/cracking up to 100% failure at 55psi. Where the caps were reinforced (by JB weld potting a sensor module) the rim threads of the cap sometimes split before the tube itself collapsed:
Silicone grease was added to some of the caps although none of the dry ones leaked before the bodies cracked.
So the 30mL tubes have a deployment range to 25m with a good safety margin. The plastic of these tubes was somewhat more flexible with some crushing almost flat without leaks. This implies we might be able take these a little deeper with an internal reinforcement ring (?)
The next experiment was to try filling the tubes with mineral oil to see how much range extension that provides:
A third logger was submerged using only a sample bag:
The bag was included to test the ‘naked’ DS3231 & 328p chips. We’ve had IC sensors fail under pressure before (even when potted in epoxy ) Although it’s possible the encapsulation itself was converting the pressure into other torsional forces that wouldn’t have occurred if the pressure was equally distributed.
Again we moved in 5 psi increments up to 80 psi – which is the limit of what I can generate with my little bicycle pump. At 50psi some mineral oil seeped from the bag and at 70psi the ~1cm of air I’d left in the 50mL tube caused similar leakage. On future tests I will spend more time to get rid of all the bubbles before sealing the housings.
At 70psi the 50mL tube dented & sank and the lid started seeping oil (but did not crack)
The loggers continued blinking away for several days at 70, 75 & 80psi, but eventually curiosity got the best of me so I terminated the run. We were also getting uncomfortably close to the 90psi maximum test pressure on that polycarbonate filter housing. I was hoping to have some weird artifacts to spice up this post but no matter how hard I squint there really were no noticeable effects in the data at any of the pressure transitions – basically nothing interesting happened. I thought the resistive sensors would be affected but the RTC & NTC temperature logs have no divergence. The LDR looks exactly like a normal LDR record with no changes to the max/mins outside of normal variation. The battery curves are smooth and essentially indistinguishable from ‘dry’ bookshelf tests on the same cells. But I guess in this kind of experiment success is supposed to be boring… right? With mineral oil these little guys can go anywhere I can dive them to – even if the ‘housing’ is little more than a plastic bag.
One thing of note did happen after I removed the loggers from the chamber: I accidentally dropped the 30ml logger on the counter while retrieving it from the chamber and a thin white wisp of ‘something’ started swirling around the clear fluid inside the logger. This developed slowly and my first guess was that the capacitor had cracked and was leaking (?)
By the time I managed to capture this photo, the fine ‘smoke’ seen earlier had coalesced into a larger foam of decompression bubbles.
After emptying that oil, the logger itself went into a red D13 flashing BOD loop for a while but by the time I’d cleaned it up enough to check the rail, the battery had returned to it’s nominal 3v. My theory is a similar off-gassing event was happening inside the battery – briefly causing a droop below the 2.7v BOD threshold. So it’s possible that while the loggers are not depth limited per se using mineral oil, components like the separator in a battery may still be vulnerable to ‘rate-of-change’ damage. After more than two weeks at depth, I had vented the chamber in less than a minute. Of course when retrieving loggers in the real world I’d have to do my own safety stops, so this hazard may only affect loggers that get deployed/retrieved on a drop line.
I’ll run these loggers on the bookshelf for a while to see if any other odd behaviors develop. After that it will be interesting to see how well I can clean them in a bath of isopropyl (?) as I suspect that the mineral oil penetrated deep into those circuit board layers.
Addendum: 2023-05-30
Although the units sleep current was the same as before the pressure testing, the battery in the 30mL tube barely made another twelve hours on the bookshelf before the voltage dropped again – well before the expected remaining run time. So it’s a safe bet that any deployment which exposes coin cells to pressure at depth is a one-shot run. Given how cheap these batteries are, that’s pretty much a given when deploying these little loggers even if they remain dry.
Addendum: 2023-12-01
Short 30ml tubes work well for single-sensor applications, but classroom labs needed to switch between different sensor modules easily. So we added 3D printed rails holding mini breadboards to provide this flexibility, and the 50mL centrifuge tubes provide the space for these additions. They may not have the same depth range, but they are robust enough for most student experiments.
It’s also worth noting that these tests were done with the standard ‘plug-style’ caps that come with the NUNC 50ml centrifuge tubes. A few companies make tubes with an O-ring integrated into the cap(made of silicone or ethylene propylene) which gets compressed when the threads are tightened. Those would provide another layer of moisture resistance at that seal, although they wouldn’t do much to prevent the crush-failures. Unfortunately the Nalgene Oak Ridge high-speed polycarbonate centrifuge tubes that could resist those forces have necks pinched-in to a diameter too small for the modules in our logger.
This ‘two-part’ logger fits nicely inside a falcon tube which can be deployed to 10m water depth. With a bit of practice, complete logger assembly can be done in about 30-60 minutes depending on additions. The 4K EEprom on the RTC board will hold 4096 1-byte RTC temperature readings (~ 40 days worth @ 15 min. intervals) and that’s easily extended with I2C memory chips or modules.
The ‘solderless’ EDU build we released in 2020 provides remarkable flexibility for courses in environmental monitoring. However an instructor still needs to invest about five days ordering parts, testing components, and preparing kits for course being run remotely. (Although only 1/2 that is needed for in-person courses where the students pin & test the parts themselves.) While that’s not unusual for university lab subjects it’s a stretch for other instructors. And thanks to COVID shortages, modules that were only a buck at the beginning of the project might now set you back $5 each. So with all that in mind, we’ve continued development of a ‘lite’ version of our logger with the lowest possible prep. That new baby is now ready for release with data download & control done through the serial monitor window.
Instead of connecting the red channel of the 5mm LEDI use the onboard D13 LED for red. Other libraries and programs often require that LED.
With just three core components our only option was to remove the SD card. Groundwork for this change has been in place for a long time in our research loggers, with sensor readings first getting buffered to the 4k to reduce the number of high-drain SD saves. Getting rid of those power hungry cards opened up the possibility of running the entire unit from the coin cell on the RTC module. But a power budget that small adds some complexity to the base code which must minimize CPU run-time and limit peak current.
Internal Resistance with 6.8mA Pulse Discharge, from the Energiser Cr2032 datasheet. (Note: The 8mhz ProMini only draws about 3.3mA when its running)
Most garden-variety chips have a lower operating limit of 2.7v – so a 3v Cr2032 can only fall about 300mv under load before triggering the brown out detection (BOD) circuit. Voltage droop changes over time because the internal resistance of a coin cell is only 10 ohms when new, but increases to 100 ohms by end of life. According to Maxell’s 1Meg-ohm (3.3µA continuous) discharge test, coin cells should stay at their voltage plateau until they deliver about 140mAh. In our testing, 200uF battery buffering capacitors can extend runtime up to 30% but this varied with the quality of the battery. Of course, if you can reach a year without the rail buffer, then you’ve probably filled the memory. So rail caps may only be necessary with high-drain sensors or in low temperature deployments where the battery chemistry slows. It’s not unusual to see a 50mv delta at the battery terminals for every 15°C change in ambient so a standard coin cell will not power the logger very long at temperatures very far below freezing.
However, theres only so much you can predict about system behavior in the real world – especially with stuff constructed from cheap modules carrying half a dozen unspecified components. So let’s just build one and see how it goes.
Clipping the main VCC leg (the 2nd leg in from that corner) forces the DS3231 to run from the backup power input on the other side of the chip.
Disconnect the modules indicator LED by removing its limit resistor with a hot soldering iron tip.
Remove the 200ohm charging resistor & bridge VCC to the coin cell backup power at the black ring end of diode.
Running from Vbat depowers most of the logic inside the DS3231 and disables the 32kHz output (I usually cut away that header). According to the -M and -SN datasheets, but both RTC chips draw the same averageof 3.0µA timekeeping battery current to keep only the oscillator, temperature compensation & comparator working. The default 4k EEprom draws between 20-40nA when not being accessed. Bridging VCC directly to Vbat also means a 3.3v UART will push some sub-milliamp reverse currents through an older coin cell. Despite dire manufacturer warnings that reverse currents beyond 1µA will heat manganese-dioxide/lithium cells until they explode, I’ve yet to have a single problem (or even any detectable warming) with loggers connected for many days during code development. Drift on these RTCs is usually a loss of ~3-5 seconds per month, but you can reduce this considerably by calibrating the DS3231 Aging Offset Register with a GPS module. The only annoying issue with these RTC’s is that, once enabled, the alarms continue to be generated forever unless you set an ‘unreachable’ next alarm time, or shut down the main oscillator completely.
3 mods to the RTC module It’s a good idea to test of those RTC modules (with the logger base-code) before assembling your logger. After successful testing, I add conformal to the RTC & let dry before joining the two modules.
OPTIONAL: Add another EEprom to the RTC module
With a practiced hand you can add more EEprom memory right onto the RTC module. 64k is the sweet spot for single sensors generating 2-byte integers because they can store ~340 days of data at a 15min. interval. Each additional EEprom adds 0.2 to 0.4µA to the loggers overall sleep current. 64K’s are about $1 up to 256K chips selling for about $3.50 at Digikey. AT-series EEproms larger than 64k, show up on the I2C bus as multiple 64k chips.
For an AT24c512 you only need connect the four pins shown because the chip internally grounds any pin left floating. The rtc module pulls all connected address pins high (setting the lower 4k to 0x57) but an upper 64k EEprom with the connections above would go to 0x50.
Stacked EEproms in a 0x57 & 0x51 configuration. If this soldering is a bit too advanced see the ‘adding sensors’ section for a way to increase storage space with modules. Of course you can stack on those boards as well!
PREPARE the Pro Mini:
Carefully clip away the regulator from the 2-leg side to prevent 80µA of back-leakage thru the regulator under battery power.
Clip away the reset switch. This logger can only be started with serial commands via a UART connection.
Remove the limit resistor for the power indicator LED with a hot soldering iron tip. This is near the regulator area.
An Arduino Pro Mini continues at the heart of our loggers because it is still the easiest low-power option for projects that aren’t computationally demanding. 328p’s normally sleep well below 1µA with the BOD turned off but 17µA with BOD left on. It’s worth noting that component level testing of the tiny sleep current on an isolated ProMini board requires stable battery power, as UART supplied voltages are too noisy for sensitive devices like a Current Ranger. You will occasionally get clones withfake Atmel chipsthat won’t go below ~100µA sleepno matter what you do.
At 8Mhz the ‘official’ lowest safe voltage for the 328p is 2.7v and the BOD cutoff circuit is always on while the processor is running. So you want to stop logging if the rail starts falling below ~2850mv. Keep in mind that there is wide range of actual brownout thresholds, from a minimum of 2.5v to a max. of 2.9v. So if you get a logger that consistently quits while the battery voltage is still high, you might want to check that units actual BOD trigger point with a bench power supply.
Attach UART header pins and trim three of the tails flush to avoid accidental contact later.Do not progress with the build until you have confirmed the ProMini has a working bootloader by loading the blink sketch from the IDE.
OPTIONAL: Add a Thermistor, LDR & Indicator LED
One of my favorite sensor combinations for this logger is an NTC thermistor & CDS cell which adds nothing to the sleep current. Range switching with two or more NTCs could also be done if the max / min resistance values of one thermistor can’t maintain your resolution requirements. We explained how to read resistive sensors using digital pins in 2019; so here I will simply show the connections. Add these passives to the Pro Mini BEFORE joining it to the RTC module, taking care not to hold the iron so long that you cook the components. Each [104] 0.1uF of capacitance you use in this circuit gives you about 3000 raw clock counts for the 10k reference resistor.
D6=10kΩ 1% metfilm reference resistor , D7=10k 3950 NTC, D8=300Ω (any type), D9=LDR (5528). Note that the LDR is optional, and could be replaced with any other type of resistive sensor up to ~65kΩ. A typical 10kNTC reaches that limit near -10°C and the LDR usually reaches that at night.
The code puts all lines that are not being read into input mode to isolate them from the circuit when reading each individual channel. With these sensors I usually jumper D2->SQW with a longer piece of flexible wire to avoid covering the D13 LED.
A 104 ceramic cap to GND completes the ICU timing circuit. With a 0.1uF as the charge reservoir, each resistor reading takes ~1-2msec in sleep mode IDLE. With this small capacitor, timing jitter produces a band of readings about 0.02°C wide on the output plots if temp. readings are repeated rapidly.
We created a how-to guide on calibrating thermistors which makes use of an online calculator to determine the equationcoefficients. You should never expect the NTC you have in your hands to match the values provided by its manufacturer, but even if it did our method leverages the behavior of the ProMini itself as part of the measuring system. So there is no point buying expensive interchangeable thermistors – the cheapest penny-parts will do just fine. I add a thermistor to all my loggers (even if they will eventually drive I2C sensors) because a good way to test new loggers is to read the NTC at a short one-second interval until the EEprom has been completely filled a few times. After that burn-in you can be sure the core of the logger is reliable before adding other sensors.
[OPTIONAL]Common cathode RGB w red leg cut on A0=GND, A1=Green, A2=Blue. The colors are lit via internal pullups to keep below 50µA. Leaving the RED LED on D13in place is useful to let you know when the bootloader has been triggered and that gets used by default if no other LED is enabled at the start of the logger code. The 1k D13 limit resistor must be left in place to use the onboard LED.
Join the Two Modules:
Resistor legs wrapped in heat shrink extend the A4/A5 I2C bus. These two wires must cross over each other to align with connections on the RTC.
Extend the VCC & GND headers with resistor legs.
Add a strip of double-sided foam tape across the chips on the RTC module and remove the protective backing.
Carefully thread the four I2C jumpers through the RTC modules pass-through port. Press the two boards together onto the double sided tape and solder the four connections.
OPTIONAL: use the VCC & GND wire tails to add an SMD tantalum rail buffering capacitor and trim away any excess wire. Anything from 220 to 1000µF will reduce battery voltage droop as the coin cell ages. 10V 470uF caps provide a good overall balance between buffering ability and leakage currents in the 50nA range. After checking polarity, flip the SMD solder pads to the upper surface for easier soldering. Rail buffering caps can extend runtime by 20-30%, depending on the quality of your coin cell, but they are not necessary for short-term logger operation.
Trim away the (non-functional) 32kHz pin and tin to the SQW header pin. Solder a resistor leg (or a short length of 26AWG wire) to interrupt input D2 on the Pro Mini.Add heat shrink to the D2 jumper & solder it to the SQW alarm header pin.
The minimum 2-module stack usually draws about 1µA constant sleep current, but with it’s TXCO corrections the RTC alone brings the average to at least 3µA. Cheap modules often have leftover flux which can cause current leaks. It’s worth the time scrub these boards with alcohol before assembly. I found no significant difference in sleep current between setting unused pins to INPUT_PULLUP or to OUTPUT_LOW.
The basic two-module combination usually sleeps around 1-2µA continuous. Most of that is the RTC’s timekeeping current as a 328p based ProMini only draws ~150nA in power-down (with regulator removed & BOD off) and the 4k eeprom should be less than 50nA in standby. If we assume four readings per hour at 5mA for 30msec, the battery life calculator at Oregon Embedded estimates a 220mAh battery will last more than 10 years…which is ridiculous. We know from the datasheet that the typical Ibat timekeeping current for the DS3231 is `700nA (dsheet pg7 with EN32KHZ=0 @3v) but TXCO temperature conversions bring the RTC average up to 3µA – which can’t be seen on this direct measurement. And there’s the battery self discharge of 1-3% per year. Perhaps most important there’s the complex relationship between pulsed loads and CR2032 internal resistance, which means we’ll be lucky to get half the rated capacity before hitting the typical 328p brown-out at 2.77v. A more realistic estimate would start with the assumption that the battery only delivers about 110mAh with our logger consuming whatever we measure + 3µA (RTC datasheet) + 0.3µA (coincell self-discharge). For conservative lifespan estimation we can round that all up to about 5µA continuous, with four 5mA*10millisecond sensor readings per hour, and we still getan estimated lifespan of about two years. So the most significant limitation on this logger is usually the EEprom memory. It’s worth noting that newer DS3232 variants of the RTC lets you push the TXCO corrections out to 512 seconds. This lowers the average RTC standby to about 1µA -if- you are willing to spend $10 for the RTC chip alone on DigiKey.
Build video: ( 3 minute rapid review)
Note: the order of operations in the video is slightly different from the photos.
In 2023 the logger base code was updated to match the newer the e360 model, being essentially identical except for the changes required for the alternate NTC and LED connections. The code requires the LowPower.h library for AVR to put the logger to sleep between readings and this can be installed via the library manager.
One important difference between a coin cell powered logger and our older AA powered models is that the battery has a high probability of being depleted to the point of a BOD restart loop. (which causes rapid flashing of the D13 LED) So we use a multi-step serial exchange in Setup() to prevent data already present in the EEprom from being overwritten by an accidental restart.
Setup()
Purple callouts on the following flow diagrams indicate the places that would need to be altered to add a new sensor to the base code. If all you do is enable sensors via defines at the start of the program you won’t have to deal with the code that stores the data. However to add a new sensor you will need to make changes to the I2C transaction that transfers those sensor readings into the EEprom ( with matching changes to the sendData2Serial function that reads them back out later).
Note: The options on the startup menu will change over time as new code updates are released.
A UART connection is required at start-up so those menu-driven responses can occur through the serial monitor in the IDE. These have 8minute timeouts to avoid running the CPU too long during unintentional restarts. The menu sequence can be re-entered at any time simply by closing & re-opening the serial monitor window: This restarts the Pro Mini via a pulse sent from the UARTs DTR (data terminal ready) pin.
If you see random characters in the serial window, you have the baud rate set incorrectly. Reset the baud to 500,000 and the menu should display properly HOWEVER you also need to close & re-open the window. If you copy data from the serial monitor when the window still has garbled characters, then only the bad starting characters will copy out. On a new logger the Hardware, Calibration & Deployment fields will display rows of question marks until you enter some text via each menu option. There are additional debugging options that are not displayed unless serial out is enabled, and this menu will CHANGE over time as I add new features.
The first menu option asks if you want to download the contents of the logger memory to the serial monitor window. This can take up to 2 minutes with 256k EEproms at 500000 baud, which is the fastest rate an 8MHz ProMini can reliably sustain. Then copy/paste everything from the IDE window into an Excel sheet. Then below the data tab, select Text to Columns to separate the data fields at the embedded commas. Or you can paste into a text editor and save as a .csv file for import to other programs. While that process is clunky because the IDE’s serial interface doesn’t export, everyone already has the required cable and data retrieval is driven by the logger itself. And yes, the exchange could also be done with other serial terminal apps like PuTTY with logging turned on, CoolTerm, or Termite with the ‘logfile’ filter. You can also redirect Windows COM port output to a file, although it seems limited to only 19k baud. These options may be required on builds with large memory expansions as the IDE serial monitor starts to forget the initial data in it’s buffer after displaying about 100,000 lines. I have successfully used Termite at 1,000K baud downloading an 8mhz logger, but when the amount of data gets large the ProMini starts limiting you to an effective 500K baud. Functions like itoa() before serial.print() or using Rob Tillaart’s splitDigits4 & splitDigits10 with serial.write() speeds things up considerably. In summary, 250k BAUD is stable, 500k is usually fine with occasional serial dropouts in a very long download, while 1,000k Baud exhibits frequent flaky behaviors with the IDE’s serial monitor. Those character dropouts don’t happen using Coolterm or PuTTY.
Vref compensates for variations in the reference voltage inside the 328p processor. Adjusting the 1126400 (default) value up or down by 400 raises/lowers the reported voltage by 1 millivolt. Adjust this by checking the voltage supplied by your UART with a multimeter while running the logger with #define logCurrentBattery enabled and serial output ON. Note the difference between the millivolts you actually measured and the current voltage reported on screen, and then multiply that difference by 400 to get the adjustment you need to make to vref for accurate battery readings. After you [7] Change Vref and enter the adjusted number it will be used from that point onward. Stop adjusting when you get within ±20mv.
After the start menu sequence the first sampling time is written to the internal EEprom so the timestamps for sensor readings can be reconstructed during data retrieval later by adding offsets added to the starting time. This technique saves a significant amount of our limited EEprom memory and all it takes is =(Unixtime/86400) + DATE(1970,1,1) to convert those Unix timestamps into human readable ones in Excel. It is important that you download the old data before changing the sampling interval via the startup menu option because the interval stored in EEprom is also used to reconstruct the timestamps. Valid sampling intervals must divide evenly into 60 and second-intervals can be set for rapid testing if you first enter 0 for the minutes.
No data is lost from the EEprom when you replace a dead coin cell and you can do the entire data retrieval process on UART with no battery in the logger. But the clock time should only be reset after installing a new battery or it will not be retained. If the time in the serial menu reads 2165/165/165 165:165:85 instead of 2000/01/01 after a power loss then there’s a good chance the RTC’s memory registers have been corrupted & the RTC module needs to be replaced. I’ve managed to do this to a few units by accidentally shorting the voltage to zero when the logger was running from a capacitor instead of a battery.
After setting the clock time and deployment parameters like the sampling interval, the logger will request the user to manually type a ‘start’ command before beginning a run. Only when that second ‘start’ confirmation is receivedare the EEproms erased by pre-loading every location with ‘Zeros’ which also serve as End-Of-File markers during download. The selected LED will then ‘flicker’ rapidly to indicate that the logger is ‘waiting’ until the current time aligns with the first sampling alarm before beginning the run.
Main LOOP()
EEprom writes usually draw about the same 3mA current the ProMini draws during CPU up time. ‘Lowest’ battery voltage is checked immediately after the data transfer because an unloaded Cr2032 will always read nominal – even when it’s nearly dead. Timing of events at that point is critical because EEproms freeze if the rail voltage fluctuates too much while they are actively writing – so don’t change those LowPower.idle sections of the code! Also don’t run OLED screens during sensor readings or EEsaves because they are noisy as heck. Logger shutdown gets triggered at the end of the main loop if the EEprom save brings the rail voltage below the 2850mv systemShutdownVoltage or if the memory is full.
Adding I2C Sensors to the logger:
The minimum configuration for this logger can log the 0.25°C temperature record from the DS3231, index-encoded to one byte per reading. Approximately 4000 of these readings can be stored in the 4k EEprom on the RTC module. This works out to a little more than 40 days at a 15 minute sampling interval.
We made extensive use of RTC temperature records in our cave drip loggers at the beginning of the project. The accuracy spec is only ±3°C, but most were within ±1 of actual @ 25°C and you can calibrate them against more accurate sensors. The RTC only updates its temperature registers every 64 seconds and any temperature sensors inside the body tube will have about 15 minutes of thermal lag relative to outside air.
The 4K AT24c32 on the RTC module fills rapidly with sensors that generate 2 or 4 byte integers. An easy solution is to combine your sensor module with 32k (AT24c256), or 64k (AT24c512) chips so the sensors bring the extra storage space they will need. These EEprom modules can usually be found on eBay for ~$1 each and after you update the EEpromI2Caddr & EEbytesOfStorage defines at the start of the program, all AT series chips will work with the same code as the default 4k.
The headers on this common BMP280 module align with the 32k headers in a ‘back-to-back’ configuration. The tails on the YL-90 breakout extend far enough to connect the two boards. Note this sensor module has no regulator which is preferred for low power operation.
Pin alignment between the YL-90 and this BH1750 module is slightly more complicated as you can’t cover the light sensor.
Clip away the plastic spacers around the header pins. Then wiggle the BH1750 over the headers on the 32k module. Solder the points where the pins pass through the 1750 board. Note: I2C pullups on the sensor boards can usually be left in place on this low voltage system.
I2C pin order on the RTC doesn’t align with the BH1750 module. So you need to make the required cross-overs with a 4 wire F-F Dupont. Soldering those connections is more robust but do that after calibrating the thermistor.
In addition to the NTC / LDR combination, support for both of the sensors shown above is included in the code on Github although you will need to install hp_BH1750 and BMP280_DEV with the library manager to use them. Sensors are enabled by uncommenting the relevant defines at the start of the program.No matter what combination of sensors you enable, thetotal bytes per recordmust be1,2,4,8 or 16. Otherwise you will get a repeating error at download because trying to save data beyond a hardware page-boundary inside the EEprom over-writes previously saved data at the start of that same page/block.
I2C devices are usually rated to sink about 1mA which on a 3v system would require 3300 ohm pullups. This means you can leave the 10k’s on those sensor modules to bring combined bus pullup ( incl. 4k7 on the RTC & 35k pullup on the ProMini pins) to 2930 ohms, which is close to the 3.3k ideal. The open-drain implementation of I2C means that adding more capacitance to the bus will round off the rising edges of your clock and data lines, which might require you to run the bus more slowly with multiple sensors or with longer wires to your sensors. In those cases you can drop the total bus pullup to 2k2 to offset the capacitance.
The 662k LDO regulator on most eBay sensor modules increase the loggers sleep current by 6-8µA due to back leakage. For long deployments this can be removed & then bridging the in->out pads should bring your sleep back to ~1-2µA. That regulator is below spec any time your supply falls below ~3.4v which is > than the initial over-voltage on a Cr2032.
You must use low power sensors with a supply range from 3.6v to our 2.7v BOD cutoff. A good sensor to pair with this logger should sleep around 1µA and take readings below 1mA. You are more likely to find these no-reg sensor modules with low power operation at Sparkfun or on Tindie, than you are on eBay/Amazon. A coin cell simply doesn’t have enough power to supply a high drain CO2 sensor or GPS unless you take heroic measures. Sometimes you can pin-power sensors that lack low current sleep modes although if you do that be sure to check if that creates current leaks in other places such as pullup resistors or the I2C bus may go into an illegal state (idle is supposed to leave SCL & SDA lines high) requiring a power reset of all the sensors on the bus. Choose sensor libraries which allow non-blocking reads so you can sleep the ProMini while the sensor is gathering data and replace any delay() statements in those libraries with 15msec powerdown mode sleeps.
30ml self-standing Caplugs from Evergreen Labware are a good housing option because they have a brace in the cap that just fits four 22gauge silicone jacket wires. The ‘non-sterile’ versions with separate caps are much cheaper to buy than the sterile ones. The outer groove in the lid provides more surface area for JB-weld epoxy, giving you an inexpensive way to encapsulate external sensors. 1oz / 25ml is enough to cover about five sensors. Then clear JB weld can be used as a top-coat to protect optical sensors.
Drill the central channel to pass the I2C wires through the cap. Roughen the upper surfaces with sandpaper to give it some tooth for the epoxy.
Conformal coat the board before the epoxy. Work the epoxy over the sensor board carefully with a toothpick and wipe away the excess with a cotton swab.
We’ve done pressure tests to 45psi and these tubes can be deployed to ~20m depth, although we don’t yet have any data yet on how long they will endure at that pressure. These housing tubes should be replaced every three months if they are exposed to sunlight because UV makes the plastic brittle. Adding a very small amount of silicone grease to only the upper edge of the tube before closing improves the seal with the lid but don’t add too much or the threads will slip. Holes drilled through the bottom stand enable zip-ties to secure the logger. In our cross calibration of a Bh1750 Lux sensor to measure PAR (Photosynthetically Active Radiation) , we wrapped the body tubes with 2″ aluminum foil tape to reduce heat gain inside the body tubes.
We have produced small printable rails for the 30ml tubes often used with this two module logger. So here is a link to that shared model on Tinkercad. That internal rail & an external mounting bracket is posted at Github. The easiest way to secure the logger to these rails is with a drop of hot glue from the underside but I usually twist and solder the legs of a scrap resistor as the tie as I have lots of these lying around:
Insert a scrap resistor into the mounting holes and twist the legs together.
Solder the twisted legs and trim. Angle the joint inwards to avoid scratching the tube.
Angle the I2C headers slightly toward the point of the tube to leave more room for Dupont connectors during the NTC calibration.
There’s room for two or three 0.5gram silica gel desiccant packs in the lid area. Because the ProMini remains exposed, I don’t usually add conformal to the ProMini until the logger has passed all of its pre-deployment run tests. After that I add a generous layer of conformal to everything but the battery contacts and the header pins. Clear nail polish also works for this.
For deeper aquatic deployments, you could use a stronger PET preform for the enclosure. These have very thick walls because they are the blanks that are thermally blow-molded to create soda bottles. You will need to find ones larger than the standard 2L bottle preforms which have an internal diameter of only 21mm. This is just a bit too tight for the RTC module. The 30ml centrifuge tubes shown above have an internal diameter of 26-27mm.
Testing Your New Logger
Make at least two machines at a time. I usually build in batches of six, and for every dozen at least one usually ends up with some kind of issue like an RTC temp. register outside the ±3°C spec, or a ProMini with one of those fake 328p processors that draws too much sleep current. Having more than one logger makes it easy to identify when you’ve got an a hardware problem rather than an error in your code. Even then, no unit is worth more than an hour of troubleshooting when you can build another one in less time. Seriously! The part cost on these things is well below $10, which is often less than you’d pay just to replace the battery on other loggers. This also applies to maintenance: just run till it fails and then replace it. In our experience, most inexpensive sensors have a reliable lifespan of less than two years in the field.
A good general approach to testing any DIY build is to check them with a doubling schedule: Start with rapid UART tethered tests via the serial monitor at second intervals, then initial stand-alone tests for 1,2,4 & 8 hours till you run overnight, followed by downloads after 1day, 2days, 4days, 8days, etc. For those initial burn-in tests, set the interval short enough that the entire memory gets filled. Valid sampling intervals must divide evenly into 60 and second-intervals can be set for rapid testing if you first enter 0 for the minutes.
Occasionally you get an RTC module with a really weak contact spring and this generally shows up as battery readings that jump all over the place, or even as unexpected quits from accidental bumps disconnecting power. A small bit of double sided foam mounting tape behind the spring will usually make the battery connection quite robust.
The shape of the battery burn-down curve during your pre-deployment testing is and excellent predictor of reliability! But to use that information you need to be running several identical machines at the same time, and start those runs with fresh batteries. I use the cheapest batteries I can get for these tests, knowing the better quality batteries I use on deployment will last much longer.
Remember that eBay/Ali/Amazon sensor modules are cheap for a reason and it’s not unusual to see 20% of them rejected for strange behavior or infant mortality. So huddle test each batch to normalize them. Relative accuracy spec for the BMP280 is supposed to be ±0.12 millibar, but when I run a batch of them side-by-side I usually see ±4 millibar between the records. Cheap BMEs sometimes refuse to operate with it’s individual RT/T/Pr sensors set at different oversampling levels, and at the highest resolution (16x oversampling) that sensor may draw more than your power budget can sustain over a long deployment. Real-world installation inevitably exposes the logger to condensing conditions. Sensors with a metal covers (like the BMP/E series) will experience internal condensation at the dew point. Moisture creep is the largest cause of data loss on the project after theft/vandalism. So cleaning leftover flux from all parts with cotton swabs + 90% isopropyl alcohol before & after assembly is always worth your time. So is conformal coating and you can use clear nail polish for that if silicone coatings are hard to find.
And all the other quid-pro-quos about vendors apply: Split your part orders over multiple suppliers with different quantities, ordered on different days, so you can isolate the source of a bad shipment and/or identify suppliers that are OK. Don’t be surprised if that batch of sensor boards you ordered transmogrifies into a random delivery of baby shoes. Amazon is often cheaper than eBay and AliExpress is 1/4 the price of both. Trusted suppliers increase part costs by an order of magnitude but that may still be worth it if you don’t have time for enough test runs to eliminate the duds.
Power Optimization on this Data Logger:
A (relatively high) average sleep current of ~5µA*86400 sec/day would use ~432 milliAmpseconds/day from a Cr2032 that can provide roughly 360,000 mAs of power [100mAh] on its main voltage plateau . Any power saving strategy must be weighed against this daily amount to determine if the complexity it adds to your code will deliver a proportional increase in operating time. I rarely see a sensor sample reading use more than 1 milliamp-second of power – even with relatively high drain sensors like the BME280. So most of the power used by this logger is due to the DS3231 RTC’s 3µA timekeeping current which can not be changed but battery voltage droop during peak current events is usually what triggers the low voltage shutdown and that is affected by code execution.
8MHz ProMini boards draw about 3.5mA when running at 3v. Slow functions like digitalWrite() and pinMode() are replaced with much faster port commands wherever power and/or timing are critical. Pin states, peripheral shutdowns (power_all_disable(); saves ~0.3mA) and short sleeps are used throughout for battery voltage recovery. Waking the 328p from those powerdown sleeps takes 16,000 clock cycles (~2milliseconds @8MHz +60µS if BOD_OFF) and but the ProMini only draws ~300µA while waiting for the oscillator to stabilize. These wakeups only use about 1mAs/day.
The original code released in 2022 used CLKPR to bring the ProMini down to 1MHz (lowering runtime current from 3.5mA to ~1.3 mA) however testing later revealed that the total energy cost per logging event actually increased slightly when the system clock was divided. In addition, I came across several EEproms that would freeze if I lowered the system clock to 1MHz during save. So I have removed the CLKPR calls to make the codebase more portable. I also found that the startup-time also gets multiplied by the CLKPR divider. This might be the only documentation of this on the web, so I’m leaving this information here – even though CLKPR is no longer relevant to the logger:
( Note: For the following images a Current Ranger was used to convert µA to mV during a reading of the RTC’s temperature register at 1MHz. So 1mV on these oscilloscope screen shots = 1µA is being drawn from the Cr2032 )
Here CLKPR restores the CPU to 8MHz just before entering powerdown sleep, and then slows the processor to 1MHz after waking. The extra height of that first spike is due to the pullup resistor on SQW. Cutting the trace to that resistor and using an internal pull-up reduces wake current by 750µA.
Here the 328p was left CLKPR’d down to 1MHz when it entered powerdown sleep(s). Waking the processor now takes 16 milliseconds – wasting a significant amount of power through the 4k7 pullup on SQW while the RTC alarm is still asserted.
Using the 328s internal oscillator to save power is a non-starter because it’s 10% error borks your UART to the point it can’t upload code. Our ICU based timing method also needs the stability of the external oscillator.
That bridge between the coin cell and VCC means UART connection time probably is shortening battery lifespan a bit. Panasonic specifies: “the total charging amount of the battery during it’s usage period must be kept within 3% of the nominal capacity of the battery”, so it’s a good idea to remove the coin cell if you are spending an extended time on serial. But given our tight operational margin we can’t afford to lose 200mv over a Schottky protection diode. A typical solution would address this by ORing the two supplies with an ideal diode circuit but that’s not a option here as ideals usually waste some 10-20 µA. On a practical level it’s easier to just topop in a fresh battery before every long deployment.
EEprom & sensor additions usually push directly measured continuous sleep currents to 2µA (so ~5 µA average when you add the RTC temp conversions)but that still gives a >1 year estimates on 110mAh. With all due respect to Ganssle et al, the debate about whether buffering caps should be used to extend operating time is something of a McGuffin because leakage is far less important when you only have enough memory space for one year of sensor readings. Even a whopper 6.3v 1000µF tantalum only increases sleep current by ~1µA. That’s 1µA*24h*365days or about 10 mAh/year in trade for keeping the system well above the 2.8v cutoff. That means we don’t need to lower the BOD with fuse settings & custom bootloaders. When you only service your loggers once a year, any tweaks that require you to remember ‘special procedures’ in the field are things you’ll probably regret.
Capacitor leakage scales linearly so usethe Falstad simulator to see what size of rail buffer you actually need. Capacitors rated 10x higher than the applied voltage reduce leakage currents by a factor of 50. So your buffering caps should be rated to 30v if you can find them. The 220µF/25v 227E caps I testedonly add ~15nAto the loggers sleep current and these can be obtained for <50¢ each. (& 440uF 10v caps leak around 25nA) High voltage ratings get you close to the leakage values you’d see with more expensive Polypropylene, Polystyrene or Teflon film caps and moves away from any de-rating issues. The one proviso is that as the buffering cap gets larger you will need to add more ‘recovery time’ in the code before the rail voltage is restored after each code execution block. Sleeping for 30msec after every I2C transaction is a safe starting point, but you’ll need a scope to really tune those sleeps for large sensor loads like you see with a BME280 at 16x oversampling. If moisture condenses inside the housing on deployment, and the logger mysteriously increases from 1-2 µA sleep current to something higher then replacing the tantalum rail buffering cap is one of my first diagnostic steps.
In the next three images, a Current Ranger converts every 1µA drawn by the logger to 1mV for display on the ‘scope. The last two spikes are transfers of 16-bytes into the 4K EEprom on the RTC module while the CPU takes ADC readings of the rail voltage. Note that our current code saves readings as a single event at the end of each pass through the main loop, but I forced multiplelargesaves for this test to show the effect of repeated pulse-loads:
A triple event with a temperature sensor reading followed the transfer of two array buffers to EEprom. Battery current with no rail buffering cap. [Vertical scale: 500µA /division, Horizontal: 25ms/div]
Here a 220µF tantalum capacitor was used to reduce the peak battery currents from 2.5mA to 1.5mA for that same event.
Here a 1000µF tantalum [108J] capacitor reduces the peak battery current to 1mA. The 30msec sleep recovery times used here are not quite long enough for the larger capacitor.
Voltage across a coin cell that’s been running for two months with NO buffering capacitor. The trace shows the 2.5mA loads causing a 60mv drop; implying the cell has ~24 ohms internal resistance. [Vertical Scale: 20mv/div, Horizontal: 25ms/div]
The minimal RTC-only sensor configuration reached a very brief battery current peak of ~2.7mA with no buffering cap, 1.5mA with 220µF and less than 1mA with 1000µF. The amount of voltage drop these currents create depend on the coin cells internal resistance but a typical unbuffered unit usually sees 15-30mV drops when the battery is new and this grows to ~200mV on old coin cells. The actual voltage dropalso depends on time, with subsequent current spikes having more effect than the first as the internal reserve gets depleted. The following images display the voltage droop on a very old coin cell pulled from a logger that’s been in service since 2016 (@3µA average RTC backup)
This very old coin cell experiences a large 250mv droop with no capacitor buffer. Note how the initial short spike at wakeup does not last long enough to cause the expected drop. [Vertical: 50mv/div, Horizontal: 25ms/div]
Adding a 220µF/25v tantalum capacitor cuts that in half but triples the recovery time. CR2032‘s plateau near 3.0v for most of their operating life, so the drop starts from there. [Vertical: 50mv/div, Horizontal: now 50ms/div]
A 1000µF/6.3v tantalum added to that same machine limits droop to only 60mv. Recharging the capacitor after the save now approaches 200 milliseconds. [Vertical : 50mv/div, Horizontal: 50ms/div]
According to Nordic Semi: “A short pulse of peak current, say 7mA for 2 milliseconds followed by an idle period of 25ms is well within the limit of a Cr2032 battery to get the best possible use of its capacity.” After many tests like those above, our optimal ‘peak shaving’ solution is to run the processor at 8MHz, breaking up the execution time with multiple 15-30 millisecond POWER_DOWN sleeps before the CR2032 voltage has time to fall very far. (especially necessary if you start doing a lot of long integer or float calculations) This has the benefit that successive sensor readings start from similar initial voltages but those extra sleeps can easily stretch the duration of a logging event out toward 300 milliseconds – putting limits on the loggers maximum sampling rate:
Current drawn in short bursts of 8MHz operation during sensor readings. The final EEprom save peaks at ~2.75mA draw (in this old example with CLKPR 1MHz CPU which we no longer do) [CH2: H.scale: 25msec/div, V.scale 500µA/div converted via Current Ranger]
Voltage droop on that same ‘old’ CR2032 used above reached a maximum of 175mv with NO buffering capacitor across the rail. This battery has about 64 ohms of internal resistance. [CH2: V.scale 25mv/div, H.scale 25ms]
Adding a 220µF tantalum capacitor to the rail holds that old battery to only 50mv droop. The 25v tantalum cap adds only 0.1µA leakage to the overall sleep current. [CH2: V.scale 25mv/div, H.scale 25ms]
EEprom save events are typically around 3.5 mA for 6ms. Without a rail buffer a new coincell will fall about 100mv. With a 200µF rail buffering cap supplying the initial demand the peak current drawn from the coin cell is less than 1.5mA – which limits the overall voltage droop to less than 50mv. Even with very old batteries a typical EEsave event doesn’t usually drop the rail more than 150mv with a rail buffer cap, however the recovery time growssignificantly with battery age –from less than 25 msec when new to more than 150 milliseconds for a full recovery. So old battery logging events look more like ‘blocks’ on the oscilloscope trace rather than the series of short spikes shown above.
This ‘solder-free’ AT24c256 DIP-8 carrier moduleis bulky but it lets you easily set multiple I2C address. Here I’ve removed the redundant power led & pullup resistors. Heliosoph posted a way to combine multiple EEproms into a single linear address range
Even with fierce memory limitations we only use the 328’s internal 1k EEprom for startup index values and text strings that get written while still tethered to the UART for power. EEprom.put starts blocking the CPU from the second 3.3msec / byte, and internal EEprom writing adds an additional 8mAto the ProMini’s normal 3mA draw. This exceeds the recommended 10mA max for a garden variety Cr2032. Multi-byte page writes aren’t possible so data saved into the 328p costs far more power than the same amount saved to an external EEprom. However it is worth noting that reading from the internal EEprom takes the same four clock ticks as an external with no power penalty, while PROGMEM takes three and RAM takes two clock cycles. So it doesn’t matter to your runtime power budget where you put constants or even large lookup tables.
A simple optimization we haven’t done with the code posted on GitHub is to buffer data into arrays first, and then send that accumulated data with larger wire library buffers. All AT-series EEproms can handle the 4k’s 32-byte page-write but the default wire library limits you to sending only 30 bytes per exchange because you lose two bytes for the register location. So to store sensor readings in 32-byte buffer arrays and transfer those you need to increase the wire library buffers to 34 bytes. This has to be done by manually editing the library files:
In wire.h (@ \Arduino\hardware\arduino\avr\libraries\Wire\src) #define BUFFER_LENGTH 34 AND in twi.h (@ \Arduino\hardware\arduino\avr\libraries\Wire\src\utility) #define TWI_BUFFER_LENGTH 34
That twi buffer gets replicated in three places so the wire library would then require proportionally more variable memory at compile time . With larger EEproms you could raise those buffers to 66 bytes for 64 data-byte transfers. It’s also worth mentioning that there are alternate I2C libraries out there (like the one from DSS) that don’t suffer from the default wire library limitations. AT series EEproms always erase & rewrite an entire page block no matter how many bytes are sent, so increasing the number of bytes sent per save event reduces wear and can save significant amounts of power. In my tests, newer larger EEproms tend to use about the same power as smaller older EEproms for identical save events because even though they are re-writing larger blocks, they do these internal operations much faster. So a ‘typical’ EEprom write event uses somewhere between 0.30 to 0.5 millamp-seconds of power no matter how many bytes you are saving. If your daily sleep-current burn is about 300milliampseconds, then it takes a few hundred of those EEprom save events to use the same amount of power. Increasing the transfer payload (with temporary accumulation arrays) from the I2C default of 16 bytes to 64 bytes cuts EEsave power use by 75%. That can extend the loggers operating life with short sampling intervals of 1-5 minutes, or where your sensors generate many bytes per record. Despite several technical references saying otherwise, I saw no significant difference in save duration or power (on the oscilloscope) with EEprom locations prewritten to all zeros or 0xFF before the data save events. One thing that does make an enormous difference is transferring blocks that exactly match the EEproms hardware page size – if you get the alignment perfect then EEproms can write the new information without all the preload & insertion operations you see when saving smaller amounts of data. This cuts both the time and the power for EEprom saving by 50% if you have enough memory for all the pre-buffering that requires.
Because of the code complexity we have not implemented array buffering in the current code-build so that the codebase is understandable for beginners. Every pass through the main loop saves data to the external EEprom and these loggers still have an excellent operating lifespan without it. For many EEprom types, when doing a partial write of fewer bytes than the hardware page size, the data in the rest of the page is refreshed along with the new data being written. This will force the entire page to endure a write cycle, so each memory location in the EEprom may actually get re-written [EEprom hardware page size / Bytes per Save ] times, which for the 4k would typically be 32/4 = 8 times per run. EEproms have a ‘soft’ wear limit of about 1,000,000 write cycles, so even in that worst-case scenario the logger could fill that chip 125,000 times before wearing the EEprom out. But buffering can make the eeprom last longer, or extend operating life in ways other than the battery power that’s saved.
FRAM takes about 1/50th as much power to write data compared to standard EEproms but those expensive chips often sleep around 30µA so they aren’t a great option for low-power systems like this logger unless you pin-power the chips so you can disconnect them during sleep. FRAM can endure more than 10 billion, 50 nano-second, write cycles, making it better suited for applications where rapid burst-sampling is required. The I2C bus is not really fast enough take advantage of FRAMs performance, but with the SD card removed from the logger the four SPI bus connections are now available. Once your code is optimized, the majority of the loggers runtime power is consumed by the 328p burning 3.5mA while it waits around for the relatively slow I2C bus transactions – even with the bus running at 400khz.
Here wires extend connections for the thermistor & LED to locations on the surface of the housing.
No matter what optimizations you make, battery life in the real world can also be shortened by thermal cycling, corrosion from moisture ingress, being chewed on by an angry dog, etc. And you still want the occasional high drain event to knock the passivation layer off the battery.
An important topic for a later post is data compression. Squashing low-rez readings into only one byte (like we do in the base code with the RTC temperature & battery voltage) is easy; especially if you subtract a fixed offset from the data first. But doing that trick with high range thermistor or lux readings is more of a challenge. Do you use ‘Frame of Reference’ deltas, or XOR’d mini-floats? We can’t afford much power for heavy calculations on a 328p so I’m still looking for an elegant solution.
Some Run Test Results
Since we covered adding BM & BH sensors, here’s a couple of burn-down curves for the two configurations described above. Both were saving 4 bytes of data per record every 30 minutes giving a runtime storage capacity of about 150 days. In this test, Battery was logged each time 16-byte buffer-arrays were written to a 32k EEprom. Both loggers have a measured sleep current of ~1.5µA and they were downloaded periodically. Although the curve spikes up after each download, these are runs used the same coin cell battery throughout:
Cr2032 voltage after 11 months @30min sampling interval: BMP280 sensor reading Temp. & Pr. stored in 32k eeprom with NO 220µF rail buffering capacitor. This test run is complete. At x16 oversampling the BMP uses considerably more power than the BH1750.
Coin cell after more than 12 months @30min sampling interval: BH1750 sensor & 32k ‘red board’ EEprom (Sony brand battery: again, with no rail buffer cap). Both of these records show only the lowest battery reading in a given day.
I ran these tests without a rail buffering cap, to see the ‘worst case’ lifespan. A pulse loaded Cr2032 has an internal resistance of ~20-30Ω for about 100 mAh of its operational life, so our 3.5mA EEprom writing event should only drop the rail 100mv with no rail buffer cap. But once the cell IR approaches 40Ω we will see drops reaching 200mv for those events. The CR2032’s shown above have plateaued near their nominal 3.0v, so we will see the rail droop to ~2800mv when the batteries age past the plateau. Again, our tests show that with a 220 µF rail capacitor those drops would be reduced to less than 50mv and with 1000µF the battery droop is virtually eliminated.
Note that the UART download process briefly restores the voltage because the 3.3v UART adapter drives a small reverse current through the cell. I think this removes some of the internal passivation layer, but that voltage restoration is short lived. On future tests I will enable both logCurrentBattery (after wake) and logLowestBattery (during EEwrite) to see if the delta between them matches the drops I see with a scope.
And here we compare our typical logging events to the current draw during a DS3231-SN RTC’s internal temperature conversion (with a 220µF/25v cap buffering the rail). The datasheet spec for the DS3231 temp conversion is 125-200ms at up to 600µA, but the units I tested draw half that at 3.3v. On all three of these images the horizontal division is 50 milliseconds, and vertical is 200µA via translation with a current ranger:
Typical sampling event peaks at 450µA with a 220µF rail buffer cap. This logger slept for 15msec battery recoveryafter every sensor reading or I2C exchange.
Every 64 seconds a DS3231 (-N or -SN) temperature conversion draws between 200 to 300µA for ~150ms. There is no way to change the timing of the RTC conversions.
Occasionally the RTC temp conversion starts in the middle of a logging event, adding that current the peaks.
The datasheet spec for the DS3232-SN temp conversion is 125-200ms at up to 600µA, but the units I tested draw half that at 3.3v. The rail cap can’t protect the coin cell from the SN’s long duration load so temp conversions overlapping the EEprom save may be the trigger for most low voltage shutdowns during deployment. The best we can do to avoid these collisions is to check the DS3231 Status Register (0Fh) BSY bit2 and delay the save till the register clears. But even with that check, sooner or later, a temperature conversion will start in the middle of an EEprom save event. These ‘collisions’ may be more frequent with the -M variants of the chip which do temperature conversions every 10 seconds when powered by Vbat, although they only take 10msec for the conversion instead of 150msec for the -SN. Seeing those conversions on an oscilloscope is one way to verify which kind of RTC you’ve got with so many -SN modules out there today being relabelled -M chips:
DS3231-M Temp. conversion: At 3v, this unit drew 230µA for 10milliseconds, but this occurs every 10 seconds. (1mV on scope = 1µA via Current Ranger)
DS3231-SN Temperature Conversion: At 3v, This chip drew 280µA for 130 milliseconds, every 64 seconds. (1mV=1µA via C.R.)
Given that the average timekeeping current is the same for both chips, we try to use ±2ppm SN’s for longer deployments instead of the ±5ppm M’s. In real world terms ±1ppm is equivalent to about 2.6 seconds of drift per month, and that’s what we see on most -SN RTCs. I’ve also seen occasional comments in the forums of some DS3231M oscillators stopping spontaneously during field deployment. Note that on several of the RTC modules the SQW alarms continue to be asserted even after you disable them in the control register (by setting the alarm interrupt enable A1IE and A2IE bits to zero) and this draws 6-700uA continuously through the pullup on the module.The only way to be absolutely sure the RTC alarm will not fire after a logger shut-down is to turn off the RTC’s main oscillator. We do this in the codes shutdown function, because you can just reset the time via the start menu before the next run. When you remove the coincell, DS3231 register contents are lost – usually becoming zeros when power is restored although the datasheet says they are ‘undefined’ at powerup. The RTC oscillator is initially off until the first I2C access.
If your code hangs during execution, the processor will draw 3.5mA continuously until the battery drains and the logger goes into a BOD restart loop with the D13 red led flashing quickly. The logger will stay in that BOD loop from 4-12 hours until the battery falls below 2.7v without recovering. This has happened many times in development with no damage to the logger or to any data in the EEprom.
Most of the units I’ve tested trigger their BOD just below 2.77 volts. And 10 to 20 millivolts before the BOD triggers the internal voltage ref goes a bit wonky, reporting higher voltages than actual if you are using the 1.1vref trick to read the rail. The spring contact in the RTC module can be weak. That can trigger random shutdowns from large voltage drops so I usually slide a piece of heat-shrink behind it to strengthen contact with the flat surface of the coin cell. The rail capacitor protects the unit from most impacts which might briefly disconnect the spring contact under the coin cell. However hard knocks are a such common problem during fieldwork that we use a drop of hot glue to lock the RTC coin-cell in place before deployment. Normal operation will see 40-50mv drops during EEprom saves up to with 200µF rail buffers. If those events look unusually large or rail voltage recovery starts stretching to 100’s of milliseconds on the scope you probably have poor battery contact. Even with good contact, long duration loads can deplete the rail buffering cap so a 200µF reaches the same v-drop as a ‘naked’ battery after ~8-10msec, and 1000µF after ~15-20msec. In all cases, your first suspect when you see weird behavior is that the coin cell needs to be replaced.
Another thing to watch out for is that with sleep currents in the 1-2µA range, it takes a minute to run down even the little 4.7µF cap on the ProMini boards. If you have a larger capacitor buffering the rail the logger can run for more than 10 minutes after the battery is removed.
More Cr2023 Battery Testing
16x accelerated battery tests averaged about 1250 hours run time before hitting the BOD.
Ran a series of Cr2032 battery tests with these little loggers and was pleasantly surprised to find that even with the default BOD limiting us to the upper plateau of those lithium cells; we can still expect about two years of run time from most name brand batteries with a 200-400uF rail cap. Also keep in mind that all the units in the battery test had BOD’s below 2.8v – about 1 in 50 of the ProMini’s will have a high BOD at the maximum 2.9v value in the datasheet. It’s worth doing a burn test with the crappy HuaDao batteries to spot these high cutoff units more quickly so you can exclude them from deployment. We increased the sleep current for the accelerated test by leaving the LEDs on during sleep, but with a series of different resistors on the digital pins, this logger might be the cheapest way to simulate complex duty cycles for other devices.
Addendum: Build video (w EEprom Upgrade)
We finally released a full build tutorial on YouTube – including how to upgrade the default 4k EEprom with two stacked 64k chips:
Released the classroom version of this 2-module logger, with substantial code simplifications that make it easier to add new sensors and 2Module code build has been updated to match. This new variant has two breadboards supported on 3D printed rails so that sensor connections can quickly be changed from one lab activity to the next. The default code reads temperature via the RTC, and NTC thermistor, Light via an LDR and the Bh1750, and Pressure via a Bmp280. It also has support for a PIR sensor, and a mini-OLED display screen.
6pin Cp2102 UARTS are cheap, with good driver support, but you have to make your own crossover cable.
Macintosh users have been running into a very specific problem with this logger: their USB-c to USB-a adapter cables are smart devices with chips inside that will auto shut-down if you unplug them from the computer while they are connected to a battery powered logger. The VCC & GND header pins on the logger feed enough power/voltage back through the wires to make the chip in the dongle go into some kind of error state – after which it does not re-establish connection to the Mac properly until the adapter is completely de-powered. So you must unplug your loggers at the UART module to logger connection FIRSTinstead of simply pulling the whole string of still-attached devices out of the USBc port.
Last Word:
“If you need one, then you need two. And if you need two, you better have three.” The benefit of loggers this easy to produce is that you can dedicate one to each sensor, since the sensors often cost more than the rest of the unit combined. Then you can deploy duplicates to capture long time-series. This gives you redundancy in case of failure and makes it easier to spot when sensors start to drift. Deploying at least two loggers to every site also lets you use a trick from the old days when even the expensive commercial loggers didn’t have enough memory to capture an annual cycle: Set each logger to sample at twice the interval you actually want, and then stagger the readings (or set one of the logger clocks late by 1/2 of that interval). This way both loggers operate long enough to capture the entire dataset, and you can weave the readings from the two machines back together to get the higher sampling interval you originally wanted but did not have enough memory for. If one of the loggers fails you still get the complete season, but at the longer interval.
RH% gain (Y axis) over time (X axis) in submerged 30mL housings: The upper purple lines were controls with no desiccant added to the logger, the orange curve had 0.5 gram packet and the lowest blue curve had two 0.5 gram packets of small desiccant beads. So 1 to 1.5 grams are adequate for a typical one-year deployments with about 1 to 1.5% rise in RH% per month due to the vapour permeability of the centrifuge tubes. This test was done at typical room temps, but the rate increase bump near the end of the test was due to an 8°C rise – so the diffusion rate is temp dependant. BME280 sensors were used.
Dedicated loggers also provide the non-obvious benefit of reducing the potential for interference between sensors. Cross-talk is particularly common with water quality sensors because the water itself can form a circuit between them. It is nearly impossible to predict this kind of problem if all you did was benchtop calibration of the isolated sensors before your deployment. And even if your base code is robust, and you don’t have any weird ground-loops, it’s not unusual for sensor libraries to conflict with each other in a multi-sensor build.
Hopefully this new member of the Cave Pearl logger family goes some way toward explaining why we haven’t moved to a custom PCB: Using off-the-shelf modules that have global availability is critical to helping other researchers build on our work. And when you can build a logger in about 30 minutes, from the cheapest parts on eBay that still runs for a year on a coin cell – why bother? Bespoke PCBs are just another barrier to local fabrication, with potential for lengthy delays at customs to increase what may already be unpredictable shipping and import costs.
We’ve been having fun embedding these ‘ProMini-llennium Falcons’ into rain gauges and other equipment that predate the digital era. There’s a ton of old field kit like that collecting dust in the corner these days that’s still functional, but lacks any logging capability. Much of that older equipment was retired simply because the manufacturer stopped updating the software/drivers. While IOT visualization apps are all the rage in hobbyist electronics, they may end up creating similar dependencies that open source projects aiming for longevity should avoid. Not to mention the fact that those wireless packet transfers require a power budget orders of magnitude larger than the rest of the logger, while relying on back-end infrastructure that doesn’t exist in the parts of the world where more environmental monitoring is desperately needed.
A capacitive touch switch works through the lid of the housing. This lets you do things like check the battery status without disturbing your experiment on long runs.
I highlighted these cheap OLED screens as a useful addition in the 2020 build tutorial, but given that a typical deployment leaves the logger for long periods of time where nobody will see it, some have been asking if it’s worth sand-bagging the unit with a 20mA drain on every reading cycle. (ie: larger than the rest of the logger combined) So the today I want to explore another addition to the EDU build that makes screens viable without hurting the power budget. In sleep mode these screens only draw about ~20 μA. (even if you leave it’s redundant regulator in place) So the key is only triggering the pixels when there’s someone around to actually see it.
For text output l like the SSD1306Ascii library which is available through the library manager. Grieman’s libraries are some of the best on offer whenever you need low power operation and a small memory footprint. With that installed you can drive the SSD1306 with a basic set of commands:
// Compiler instructions at the start of your program: #include <SSD1306Ascii.h>// includes the main library itself #include <SSD1306AsciiWire.h>// use I2C peripheral inside the Arduino (optional) SSD1306AsciiWire oled;// create a library object called ‘oled’ #defineoled_I2C_Address0x3C// 0x3C or 0x3D depending on manufacturer // basic screen initialization in Setup{} -this MUST be after wire.begin starts the I2C bus oled.begin(&Adafruit128x64,oled_I2C_Address); oled.setFont(System5x7);// fonts specified in setup are included with the compile
// sending information the screen at end of the main Loop after the SD save
// you can also package these up into a stand-alone function (see below) oled.clear(); // erases anything displayed on the screen oled.setCursor(0,2); // (0-127pixel columns,0-7text rows) oled.set1X(); // set single-row font height for labels oled.print(F(“B280 T”)); // standard .print syntax supported oled.setCursor(46,2); // move cursor to column 46, but remain in same row oled.set2X(); // set double-row font height for readability oled.print(bmp280_temp,2); // a float variable, limited to two decimal places oled.set1X(); oled.print(F(“o”)); // a lower case ‘o’ for the degree symbol oled.set2X(); oled.print(F(“C”)); // … etc ... add more here until the display is ‘full’
// display pixels can be enabled at ANY time later . . . oled.ssd1306WriteCmd(SSD1306_DISPLAYON); // turn on the screen pixels delay(10000); // enough time to read the information oled.ssd1306WriteCmd(SSD1306_DISPLAYOFF); // turn OFF the screen pixels
I’m only including the print statements for one line of the the display shown here -> (click to enlarge) but hopefully you see the pattern well enough to lather-rinse-repeat. The pixels do not need to be ‘turned on’ while you load the screen memory, and that data is persistent as long as the screen has power. So if you wanted to tackle more advanced graphic output, you could build plots ‘one line at a time’ in the eeprom without needing a 127 field array to buffer that data.
The TTP223 Touch Switch:
Here I disabled the LED by removing the limit resistor, set the mode to momentary low, and trimmed the header pins so the upper surface is flat. The unmarked solder pads on the upper right are were you would add trimming caps to reduce sensitivity. These switches self calibrate for 0.5 sec at startup, so they need a bit of ‘settling time’ at power-on.
These small capacitive switches can be had for less than ~20¢ each on eBay. The power LED wastes about 8mA, so that needs to be disabled for logger applications. Then you set the operating modes by bridging the A / B solder pads. These switches are so sensitive that moving a finger within 2cm of the surface will trigger them. In applications where the sensor is exposed you probably need to add a 10-20 pF ‘trimming cap’ to prevent self triggering. Noise on your rail can also set them off, and you don’t want loose wires inside the housing near that sensing pad. TTP223 chips do not go rail to rail, but they still work ok driving MOSFETS.
A small square of double sided tape holds the T233 inside the student build just above the batteries. (The outer most rows of the breadboards are power rails)
In our case the sensor will be under 1.5mm of HDPE and another millimeter of double sided foam tape. So the default sensitivity level is almost perfect for sensing through the lid of the housing. With the LED disabled the TTP223 module pulls about 5μA when NOT being triggered, and 100μA when actively signaling the logger. Here I’ve connected the switch using 10cm pre-made jumpers but you want to be careful that you don’t put the switch in a position on the lid where it might cast a shadow over any light sensors as the sun moves through the sky. With our most recent student logger, you also need to shift the indicator led over to R4-GND5-Gr6-Bl7 so that the switch output (orange wire) can be fed into the hardware interrupt on D3. This chip produces ‘clean’ transitions so you don’t need to worry about de-bouncing. These switches seem to work fine without pullup, but I’ve been enabling the internal pull on D3 anyway.
Sensors such as rain gauges generate on/off interrupt signals at any time due to environmental conditions. But no matter how many times that happens, only the RTC’s sampling interval alarm should control the ‘regular’ read cycles in the main loop. To handle multiple interrupt sources we ‘trap’ the processor in a Do-While loop that checks flag variables (set in the associated ISR functions) to see where the wake-up signal originated.
In the code below; if the RTC flag variable is false when the processor reaches the while(flag status check) at the end of the loop, then the program gets sent back to the initial do{ statement. This code assumes you have already programmed the screens memory with data to display:
// when you are about to put the logger to sleep (after setting the RTC alarm)
pinMode( 0, INPUT_PULLUP ); // I always use pin D2 for the RTC alarm signal
rtc_d2_INT0_Flag = false; // D2 interrupt flag – this can only be set true in the ISR
pinMode ( 1 , INPUT_PULLUP ); // Cap. switch output is connected to D3
d3_INT1_Flag = false; // setting false here prevents any display until TTP223 is pressed
EIFR=EIFR;// clears old ‘EMI noise triggers’ on BOTH hardware interrupt lines
do { // The ‘nesting order’ here is critical:
attachInterrupt(1,switchPressed_ISR, FALLING); attachInterrupt(0, rtc_d2_ISR_function,LOW); LowPower.powerDown(SLEEP_FOREVER, ADC_OFF, BOD_ON); detachInterrupt(0); // MUST detach the high priority D2 first detachInterrupt(1); // then detach the lower priority D3 interrupt
// Here we are simply powering the OLED display pixels but any other code put here
// is isolated from the main sequence, so you could trigger ‘special’ sensor readings, etc. if (d3_INT1_Flag ==true){ oled.ssd1306WriteCmd(SSD1306_DISPLAYON); // turn on the screen pixels attachInterrupt(0, rtc_d2_ISR_function,LOW); LowPower.powerDown(SLEEP_8S, ADC_OFF, BOD_ON); // long enough to read detachInterrupt(0); oled.ssd1306WriteCmd(SSD1306_DISPLAYOFF); // turn OFF the screen pixels d3_INT1_Flag =false; // reset D3 interrupt flag }
} while (rtc_d2_INT0_Flag ==false);// if RTC flag has not changed repeat the ‘trap’ loop
if (rtc.checkIfAlarm(1)) { rtc.turnOffAlarm(1); } // processor awake so disable the RTC alarm EIFR=EIFR;// clears leftover trigger-flags from BOTH D2 & D3
// now return to the start of the main loop & capture the next round of sensor readings
// and each attached interrupt requires an ISR function (outside the main loop!) switchPressed_ISR() {
if (d3_INT1_Flag==false)
{ d3_INT1_Flag = true; } // Flag variables set in an ISR must be global & ‘volatile’ }
rtc_d2_ISR_function() {
if (rtc_d2_INT0_Flag==false)
{ rtc_d2_INT0_Flag= true; } }
BOTH the TTP on D3 and the RTC alarm on D2 can wake the logger from sleep – but switchPressed_ISR()does not change the critical flag variable so a wakeup caused by the D3 will not break out of the Do-While loop. Only the rtc_d2_ISR_function() can set the tested flag to true and thus escape from the trap. This general method that works equally well with inputs from reed switch sensors for wind, rain, etc. With a few tweaks the same basic idea can also be used to capture ‘opportunistic’ sensor readings – provided you save those to a different logfile, or tag the ‘extra’ records with something that’s easily sorted from the regular records later in Excel.
Note that the two interrupt sources use different triggers: FALLING for the TTP223, and LOW for the RTC. It doesn’t really affect anything if the switch transition is missed (you can just tap it again), but the whole logger operation is affected if the RTC alarm fails. The RTC must also be able to interrupt the ‘display time’. Fortunately the RTC’s alarm output is ‘latched’ so it will cascade the wake-ups until we exit the do-while loop. The most important thing to know when using HIGH or LOW as your interrupt state is that you MUST detach the interrupt IMMEDIATELY upon waking. If you forget to detach a HIGH or LOW triggered ISR it will just keep on firing until it fills all of the variable memory with stack pointers and crashes your logger.
In addition to preventing the display from using power when nobody is around, having the ability to trigger the display ‘at any time’ is a great way to make sure the logger is OK without opening it – especially if all you want to see is the most recent timestamp & battery level. This is also helpful when students are running labs that can’t be ‘physically disturbed’ ( for example, soil sensors tend to produce significant discontinuities if they get bumped in the middle of a run) Nothing is worse than letting an experiment run for a week, only to find that the system froze up a few hours after it was started.
Handling multiple screens of information
Once you’ve got the basic screen operation working you might want to check our page on using two displays. Keeping each display in different ‘memory access modes’ lets you do some interesting graphical output tricks. But what if you only have one screen and there just isn’t enough room to display everything at one time?
You can use flag variables & switch-case to toggle between different screens of information:
// I use defines in setup{} so the code is more readable, but these are just numbers
#define displaySOILdataNext 0 #define displayTEMPdataNext 1
// set the ‘first screen’ you want to display before entering the do-while trapping loop uint8_tnext_OLED_info = displaySOILdataNext;
do { // our processor ‘trapping loop’ described above
// load the screen’s eeprom at the start of the do-while loop
// each successive press of the touch-switch changes which data gets loaded switch (next_OLED_info) {
case displaySOILdataNext: // case where next_OLED_info = 0
{ sendSOILdata2OLED(); // a separate function to program the display memory next_OLED_info=displayTEMPdataNext; //toggles to send different info on next pass
break;
}
casedisplayTEMPdataNext: // case where next_OLED_info = 1
{ sendTEMPdata2OLED(); // a separate function to load the eeprom next_OLED_info=displaySOILdataNext; //toggle to opposite screen on next pass
break;
} }// terminator for switch (next_OLED_info)
{insert here}all the loop content controlling wake/sleep described earlier
} while (rtc_d2_INT0_Flag ==false); // end of our processor ‘trapping loop’
// and you will need stand-alone functions for each screen of information:
void sendTEMPdata2OLED() { oled.clear(); oled.set2X(); oled.setCursor(0,0); oled.print(TimeStamp+5); // +5 skips first 5 characters of the string because it’s too long
oled.set1X(); oled.setCursor(0,2); oled.print(F(“Temp1: “)); oled.set2X(); oled.setCursor(46,2); oled.print(ds18b20_short_degC,2); // … etc ... add more here until the display is ‘full’ }
void sendSOILdata2OLED() { oled.clear(); oled.set1X(); oled.setCursor(0,0); oled.print(F(“RAIL”)); oled.set2X(); oled.setCursor(70,0); oled.print((float)ads1115_rail*0.000125,3); // … etc ... add more here until the display is ‘full’ }
The nice thing about this method is that you can toggle your way through as many different screens of information as you need simply by adding more ‘cases’ and screen ‘loading’ functions.
Two I2C 0.96″ OLED screens used simultaneously. Left is split Yellow: 128×16 pixels & Blue: 128×48 pixels while the right screen is mono-white. TheCODE ON GITHUBdrives each screen in different memory modes to simplify the functions and uses internal eeprom memory to store fonts/bitmaps.
Oceanographic instruments rarely have displays, because they don’t contribute much to profilers being lowered off the side of a boat on long cables. And most of our instruments spend months-to-years in underground darkness, far from any observer. But a few years ago we built a hand-held flow sensor that needed to provide operator feedback while the diver hunted for the peak discharge point of an underwater spring. That prototype was our first compelling use-case for a display screen, and we used cheap Nokia 5110 LCDs because they were large enough to see in murky water. While driver libraries were plentiful, I realized that Julian Ilett’s shift-out method could pull font maps from the Arduino’s internal eeprom rather than progmem. This let us add those SPI screens to a codebase that was already near the memory limits of a 328p.
Modifications done to the second display:The only thing you must do for dual screens is change the bus address. Here I’ve also removed the redundant I2C pullups (R6&R7, 4k7) and bridged out the 662k regulator which isn’t needed on a 3.3v system. These changes usually bring sleep-mode current on 0.96″ OLEDs to about 2μA per screen. (or ~5μA ea. with the 662k regulator left in place)
Now it’s time to work on the next gen, and the 0.96″ OLEDs that we initially ignored have dropped to ~$2.50 each. Popular graphic libraries for these displays (Adafruit, U8G2, etc) provide more options than a swiss army knife but require similarly prodigious system resources. Hackaday highlighted the work of David Johnson-Davies who’s been using these screens with ATtiny processors where such extravagant memory allocations aren’t possible. HisTiny Function Plotterleverages the ssd1306‘s vertical access mode to plot a graph with a remarkably simple function. These OLED screens support two bus addresses, and that set me to work combining that elegant grapher with my eeprom/fonts method for a two screen combo. Updating Iletts shiftout cascade to I2C would loose some of the memory benefit, but I was already embedding wire.h in the codebuild for other sensors. It’s worth noting that David also posted a full featured plotter for the 1106, but these tiny displays are already at the limits of legibility and I didn’t want to lose any of those precious pixels. Moving the axis labels to the other screen forced me to add some leading lines so the eye could ‘jump the gap’:
My eyes had trouble with single-pixel plots so I made the line 2-pixels thick.
Histogram seems the best underwater visibility mode. Dashed lead-lines toggle off
A little repetition produces a bar graph variant.
The battery indicator at upper left uses the same function as the buffer memory progress bar in the center. Axis label dashes along the right hand side correspond to the lead lines on the next screen. My goal with this layout was ‘at-a-glance” readability. Horizontal addressing makes it easy to update these elements without refreshing the rest of the screen; so there is no frame buffer.
To drive these screens without a library it helps to understand the controllers different memory addressing modes. There are plenty of good tutorialsout there, but the gist is that you first specify a target area on screen by sending (8-pixel high) row and (single pixel wide) column ranges. These define upper left & lower right corners of a modifiable region and the ssd1306 plugs any subsequent data that gets sent into the pixels between those corner points using a horizontal or vertical flow pattern. If you send more data than the square can hold, it jumps back to the upper left starting point and continues to fill over top the previous data. In ALL memory modes: each received byte represents a vertical 8-pixel stripe of pixels , which is perfect for displaying small fonts defined in 6×8-bit blocks. For the taller characters I use a two-pass process that prints the top of the large numbers first, changes eeprom memory offset, and then does a second pass in the next row to create the bottoms of the characters. This is different from typical scaling approaches, but gives the option of creating a three-row (24 pixel high) font with basically the same method, and since the fonts are stored in eeprom there’s no real penalty for those extra bits.
In addition to text & graphs, I need a battery status indicator and some way to tell the operator when they have waited long enough to fill sampling buffers. In the first gen units I used LED pips, but similar to the way vertical addressing made the Tiny Function Plotter so clean, horizontal mode makes it very easy to generate single-row progress bars:
// Set boundary area for the progress bar Wire.beginTransmission(ssd1306_address); Wire.write(ssd1306_commandStream); Wire.write(ssd1306_SET_ADDRESSING); Wire.write(ssd1306_ADDRESSING_HORIZONTAL); Wire.write(ssd1306_SET_PAGE_RANGE); Wire.write(row); Wire.write(row); // page start / end are the same in this case Wire.write(ssd1306_SET_COLUMN_RANGE); Wire.write(barColumnMin); Wire.write(barColumnMax); // column start / end Wire.endTransmission();
//determine column for the progress bar transition int changePoint = ((percentComplete*(barColumnMax-barColumnMin))/100) +barColumnMin;
//loop through the column range, sending a ‘full’ or ’empty’ byte pattern for (int col = barColumnMin ; col < barColumnMax+1; col++) { Wire.beginTransmission(ssd1306_address); Wire.write(ssd1306_oneData); // send 1 data byte at a time to avoid wire.h limits
// full indicator = all but top pixel on, also using the ‘filled’ byte as end caps if (col<changePoint || col<=barColumnMin || col>=barColumnMax) { Wire.write(0b11111110); } else{ Wire.write(0b10000010); }// empty indicator = edge pixels only
Wire.endTransmission(); } }
I mount screensunder epoxy & acrylicfor field units but hot glue holds alignment just fine at the prototyping stage, and it can be undone later.
And just for the fun of it, I added a splash screen bitmap that’s only displayed at startup. Since the processors internal eeprom is already being used to store the font definitions, this 1024 byte graphic gets shuttled from progmem into the 4k EEprom on the RTC modules we use on our loggers. It’s important to note that I’ve wrapped the font & bitmap arrays, and the eeprom transfer steps, within a define at the start of the program:
#define runONCE_addData2EEproms —progmem functions that only need to execute once — #endif
Since data stored in eeprom is persistent, you can eliminate those functions (and the memory they require) from the compile by commenting-out that define after it’s been run. One thing to note is that the older AT24c32 eeprom on our RTC module is only rated to 100kHz, while bus coms to the OLED work well at 400kHz.
You can update the memory in these screens even if the display is sleeping.So a typical logger could send single line updates to the graph display over the course of a day. There’s a scrolling feature buried in the 1306 that would make this perpetual without any buffer variables on the main system. Color might make it possible to do this with three separate sensor streams, and the 1106 lets you read back from the screens memory, providing an interesting possibility for ancillary storage. However with Arduino migrating to newer mcu’s I think creative code tricks like that are more likely to emerge from ATtiny users in future. As a logger jockey, the first questions that comes to mind are: can I use these screens as the light source in some interesting sensor arrangement that leverages the way I could vary the output? What’s the spectra of the output? Could they be calibrated? What’s the maximum switch rate?
Screens are quite readable through the clear 3440 Plano Stowaway boxes used for ourclassroom logger. (here I’ve hot glued them into place on the lid) Avoid taking new sensor readings while information is being displayed on the OLED screens because they ‘pulse’ power to the pixels (100ms default) when information is being displayed.Each screen shown here peaks at about 20 mA (this varies with the number of pixels lit) so you could be hitting the rail with a high frequency load of about 40mA with dual screens – which could cause noise in your readings unless you buffer with some beefy caps. Also note that the screens tiny solder connections are somewhat fragile, so avoid putting hot glue on the ribbon cable.
As a final comment, the code on github is just an exploration of concepts so it’s written with readability in mind rather than efficiency. It’s just a repository of functions that will be patched into other projects when needed, and as such it will change/grow over time as I exploreotherdualscreenideas. Kris Kasprzak has a slick looking oscilloscope example(+more here) , and Larry Banks BitBang library lets you drive as many I2C screens as you have the pins for – or you could try a multiplexer on the I2C bus. The TCA9548 I2C multiplexer lets you drive up to 8 OLED displays or connect several of those cheap Bme280 sensors to your build.
These OLEDs will only get larger and cheaper over time. (& it will be a while before we see that with e-paper) An important question for our project is: which screens hold up best when subjected to pressure at depth? This was a real problem with the Nokia LCDs.
Addendum 2021-04-05:
I finally took a closer look at the noise from those cheap SSD1306 OLED screens, and was surprised to find the usual [104] & [106] decoupling combo simply weren’t up to the task. So I had to go to a 1000uF Tantalum [108] to really make a dent in it. Contrast settings reduced the current spikes somewhat, but did not change the overall picture very much.
The following are before & after graph of a start sequence from one loggers that end with 8 seconds of the logger sleeping while the latest readings get displayed on screen:
Logger current with a SSD1306 0.96″ OLED display: before & after the addition of a 1000uF Tantalum [108]
This unit was running without a regulator on 2x Lithium AA’s , and I had noticed intermittently flakey coms when the screen pixels were on. A forum search reveal people complaining about ‘audible’ noise problems in music applications, and others finding that their screens charge pump was driving the pixels near the default I2C bus frequency of 100kHz. I’ve dozens of these things lying around, and will add more to this post if testing reveals anything else interesting. The cap. pulls about 1A at startup so I’m not even sure I could put one that large on a regulated ProMini without pushing the MIC5205 into over-current shutdown.
Addendum 2023-12-01: Mini OLED display only draws about half a milliamp!
Our 2-Module classroom data logger uses a 50mL falcon tube housing and is powered from a Cr2032. So we had to go hunting for much smaller displays. Mini 0.49″ OLEDs are an excellent low power option that sleeps below 10 µA and will run about two weeks on a coin cell at 15 minute intervals, depending on contrast, pixel coverage, and display time. A 220µF tantalum right next to them on the breadboard protects the rail from the OLEDs charge pump noise.
These micro OLEDs are driven by a 1306 so you can use standard libraries like SSD1306Ascii. HOWEVER they only display a weirdly located sub-sample of that controllers 1k memory – so you have to offset the origin on your print statements accordingly.
2020 update to the Cave Pearl Classroom logger. This is a combination of inexpensive pre-made modules from the open-source Arduino ecosystem, and can usually be assembled by beginners in 1-2 hours.
(Latest Update: Mar 9, 2022)
Covid has thrown a spanner into the works for hands-on learning because even if you have the space to run a ‘socially distanced’ course, your students could still be sent home at any time. With that in mind, we’ve divided the build tutorial from 2019 into separate stages that make it easier to restructure the labs:
1) Component prep: requires the equipment you normally have access to in the lab like soldering irons, heat guns, drills, etc.
2) Logger assembly: can be done remotely with scissors, wire strippers & a screwdriver. All connections are made by Dupont connectors or by clamping wires under screw terminals.
The complete tutorial can be run in person or if students are ‘distance learning’, the instructor can do the soldering (~15-20 minutes per set) and send out kits for the overall assembly. Even that will be challenging through a zoom window, so you might want to add USB isolators to protect tethered student laptops from accidental shorts. One big challenge of running a course remotely is the extra time to test all the parts before sending them out. A ‘breadboard logger’ like the one shown later in this post lets you do that testing quickly. Another challenge is that other USB devices can push the bus too fast for the slow serial UARTS, causing them to drop off the system due to timing conflicts. (this is more common on Apple computers).
During ‘normal’ runs if a student gets a bad component, or accidentally zaps something in one of the labs, then it simply triggers a brief process-of-elimination lesson while they swap in replacements from the storage cabinet until things are working. But remote students won’t have that option unless you send two of every part – which might be viable approach considering how cheap these components are:
This is a variation of the logger describedin our 2018 paperbut we’ve removed the regulator/ voltage divider and added screw terminals + breadboards for faster sensor connections during labs. Bridging the I2C bus over the A2 & A3 pins leaves only two analog inputs on the screw-terminals. However for ~$1 you canadd a 15-bit ADS1115 which provides differential analog readings using a fixed internal reference. So it’s unaffected by changes in battery voltage.
The main components:
(NOTE: complete parts list withsupplier links can be foundat the end of this post) You don’t need the cable glands if you are using sensors that will work inside the housing (light, temp, acceleration, magnetometers, GPS, etc.) Don’t put holes in your housing unless you are sure you need them.
Two FT232 adapters (in red) & a CP2102 UART module (blk) with a pin order that matches the ProMini headers:[DTR-RX-TX-3v3-CTS-GND]
You will need a UART adapter module to program your logger –This must support 3.3v output & is easier to use if the pin order matches the ProMini connections. UART modules available with the FT232, CP2102, & CH340 chips will all work if the chip maker supplies drivers for your operating system. The Arduino IDE will NOT be able to communicate with your logger unless the driver for your UART module is installed on your Windows or MAC operating system. We use FTDI basic UART modules & you can download the driverat the FTDI website. There’s a basic installation guide at Adafruit , a more detailed one at Sparkfun, and PDF guides from FTDI.UART chips can only supply ~50mA so if your sensors need more current you might run out of power causing a restart of the ProMini.(A somewhat common problem when testing high-drain GPS modules or wireless transmitters)
(NOTE: I have connected ProMini’s to UART modules the wrong way round many, many, times, and none have been harmed by the temporary reversal. Also note that FAKE FTDI chips are a common problem with cheap eBay vendors, so it might be safer to buy a SiLabs CP2102, or CH340 UART from those sources. If you are using the ‘cheap ones’ it’s a good idea to include UART modules from two different manufacturers in the student kits to deal with the inevitable driver compatibility issues.)
Component Prep. Part 1: Pro Mini ( 3.3v 8Mhz ) (click any image to enlarge)
Install the UART driver & IDE. Solder the UART pins & test ProMini board with the blink sketch: Set the IDE to (1)TOOLS> Board: Arduino Pro or Pro Mini (2)TOOLS> ATmega328(3.3v, 8mhz) in addition to the (3)TOOLS>COM portto matchthe # that appears when you plug in the serial adapter.
With a known good Promini: Remove the power LED [in red square]. SPI bus clock pulses will flash the pin 13 LED – so leaving the pin13 LED connected will show you when data is being saved to the SD card.
Carefully remove the voltage regulator from the two leg side with snips. System voltage will vary over time, but the logging code we’ve provided on gitHub records the rail without a voltage divider by comparing it to the 328p’s internal bandgap. Lithium AA’s also provide a very flat discharge curve.
Add header pins to the sides but DO NOT SOLDER THE RESET pin HEADERS – we will be using those screw terminals as power rails.
Bridge the two I2C bus connections with the wire leg of a resistor. Connect A4->A2 & A5->A3.
Adding: DIDR0 = 0x0F; in Setup disables digital I/O on pins A0-A3 so they can’t interfere with I2C bus.
Lithium AA batteries are preferred when running a 2-cell unregulated system because the slope of an alkaline discharge curve will reach the ProMini’s 2.7v brown-out with >50% of the battery capacity unused. (note that SD cards are safe down to ~1.8v) While the voltage of a newLithium AA is usually 1.8v/cell, that upper plateau usually settles at ~1.79 v/cell within an hour or two of starting the logger. That briefly dips to 1.6v/cell during >100mA SD card save events at room temp. At temps near 5°C (in my refrigerator) the SD write battery-droop reached about 1.5 v/cell while on the upper plateau. Lithium cells only deliver ~50% of their rated capacity at temps below freezing, but that’s still an improvement. And alkalinebatteries leak quite often – even when they are not fully discharged. To date, leaking batteries have been the most common reason for data loss on our project.
The MIC5205 regulator is not efficient at low currents, so removing it reduces your sleep current by ~50%. However that modification also forces you to deal with a rail voltage that changes over time. Thermal rise from 15 to 45°C will raise your rail voltage by about 100 millivolts on lithium cells that have been in service for a few months. (~ 5mv /°C) If your sensor circuit is a voltage divider that is being powered by the same voltage the ADC is using as a reference then the ADC readings are unaffected by this. However you will need to compensate for this in your calculations if your analog sensor circuits are not ratio-metric. Battery thermal mass will cause hysteresis unless you read your reference resistors under the same conditions. Regulated ProMini’s usually see the rail vary by ~10-20 millivolts over a similar range of temperatures however it’s worth noting the reg/cap combinations on cheap eBay modules can be subject to other problems such as noise; which can be even more problematic wrt the quality of your data. Most chip-based I2C sensor modules carry their own regulators (usually a 662k LDO) and use internal bandgap reference voltages so they are unaffected by the changing rail.
So making students deal with deal with power supply variation right from the start will save them from making more serious mistakes later because every componentin your logger is a temperature sensor.
Component Prep. Part 2: Screw-Terminal Board & SD adapter
At the UART end of the board: Use a tinned resistor leg to repurpose the RESET terminals: Join RST & GND pins on the digital side and link the pins labeled RST, 5v and Vin for the positive rail. (include Vin only if the reg. has been removed! )
Label the screw terminal blocks that were connected under the shield with red & black markers to indicate the power rail connections. We have no reverse voltage protection – so insert AA’s w the correct polarity or the polarized Tantalum caps will burst.
Gently rock the Pro Mini back to front (holding the two short sides) until the pins are fully inserted. Some ST shields have slightly misaligned headers so this insertion can be tricky.
Remove the last three ‘unused’ female headers to make room for the SD adapter which fits perfectly into that pocket
NOTE: The SDfat library uses SPI mode 0 which sets the SCLK line low when sleeping causing a 0.33mA drain through the 10k SCLK pullup on the module.
Remove the bottom 3 resistors from the SD adapter – leave the top resistor near the C1 label in place!
NOTE: The Screw Terminal board we use in this build was designed for the 5v Arduino NANO, so the shield labels don’t match the Pro Mini pinson the ‘analog’ side.(the digital side does match) To avoid confusion may want to tape over those incorrect labels and hand write new labels to match the pattern above. Wire connections in this tutorial will be specified by ProMini pins: D10-13 are used for the SD card, A4/A2 is the I2C Data line, and A5/A3 is the I2C clock line.
Technically speaking, bridging the I2C bus (A4=data & A5=clock) over top of A2 & A3 subjects those lines to more capacitance and pin leakage. (regardless of whether that channel is selected as input for the ADC p257). However in practice, the 4K7 pull-up resistors on the RTC module handle that OK at the 100 kHz default bus speed.
Adding DIDR0 = 0x0F;
in setup disables digital I/O on pins A0-A3 to prevent interference with A0/1 ADC readings and the I2C bus on A2/3. If you want to disable only the digital IO on only A2 & A3 add
bitSet (DIDR0, ADC2D); bitSet (DIDR0, ADC3D);
to setup{}.
Component Prep. Part 3: RTC Module, Indicator LED & Plano 3440 Housing
Remove two SMD resistors from the RTC board with the tip of your soldering iron. Note that this module includes 4k7 pullup resistors on SQW, SCL & SDA – leave those in place!
Optional! Cutting the Vcc leg lowers sleep current by 0.1mA but no I2C bus coms until coincell is installed.
Add 90 degree header pins to the I2C cascade port. Note: Cutting the VCC leg also requires you to ‘enable alarms from the backup battery’ with a registry setting.
Clean flux residue from both the main & cascade header pins with 90% isopropyl alcohol
Use a coin cell to determine the GND leg of a diffused common cathode RGB LED
Solder a 1-2k ohm limit resistor on the common GND. The precise value is not critical.
Add heat shrink, bend & trim the pins for connection to the screw terminal board. Pre-made 5050 modules also work but are not as good in light sensing mode.
Stepped drill bits make clean holes in plastic housings for different thread diameters. We use glands with waterproof DS18b20 sensors on 1m cables.
Centered holes are ‘slightly’ closer the bottom of the box to allow the gland to rotate without hitting the rim. Inner nuts must also be able to rotate for tightening.
Rubber washers are added to both the inside AND the outside surfaces. PG7 sized washers also fit the M12 cable glands we use.
A 3440 Plano Box configured for two external sensors with nylon M12 cable glands for 3-6mm cables. After threading sensor cables I often seal the outside of the glands with a layer of silicone goop or nail polish.
As with the ProMini, it’s worth testing the RTC modules & SD adapters before logger assembly. I keep a breadboard version of the logger handy so I can test several at a time. If the RTC temp register reads too high I throw them out because the clock time is corrected based on those internal readings. You could also set the clock at this point if the coin-cell is already in place. Note the 5050 LED module on the breadboard shown above could replace the 5mm LED in this tutorial.
Forcing the RTC to run from the backup coin cell reduces sleep current by ~0.1mA, bringing a typical “no-reg & noRTCvcc” build to ~0.15 mA between readings. (with most of that for the sleeping SD card) As a rough estimate, Lithium AA’s provide ~7 million milliamp-seconds of power, and your logger will burn ~12,960 mAs/day at 0.15mA. So ‘in theory’ you could approach a year before you fall off the ‘upper plateau’. The clock will reset to Jan 1st, 2000 if you disconnect the RTC’s coin cell with a hard bump, but a couple of drops of hot glue should prevent this. If a reset does occur the time stamps will be wrong, but the logger will continue running the next day once the clock rolls around to the previous hour/minute alarm that was set. A CR2032 can power the RTC for about four years but if you cut the vcc legyou mustset bit six of the DS3231_CONTROL_REG to 1 to enable alarmsor the logger will not be able to wake up.(NOTE: our logger code does this by at startup with the enableRTCAlarmsonBackupBatteryfunction, which only has to run once – the RTC remembers the setting after that)
The soldered components ready for assembly.
Cutting the Vcc leg on the RTC is optional: if you leave the RTC power leg attached you’ll see typical logger sleep sleep currents in the 0.25 mA range, which should still give at least 4 months of operation before you trigger a low voltage shutdown. I’m being conservative here because runtime also depends on sensors and other additions you make to the base configuration.
See our RTC page for more detailed information on this DS3231 module.
Assembly Part 1: The Screw-Terminal Stack
Part 1: This stack is the ‘core’ of your data logger.
It is very easy to get a couple of wires switched around at this stage so work through these instructions slowly & carefully. Connect the Dupont jumpers to the SD module so that the metal retainer clips are facing upwardsafterthe logger is assembled. That way you can diagnose connection issues with the tip of a meter probe on the exposed metal and, if necessary, pull out & replace a single bad wire without taking everything apart. The extra wires you trim from the SD module are re-used.
Add 2-3 layers of double sided foam tape to the sides & center of the screw terminal shield. The tape needs to extend farther the height of the solder points.
Add single wires from the 20cm M-F Dupont cable to the SD adapter.
The ‘metal tabs’ of the Dupont ends are facing the large metal shield over the SD card.
A layer of double sided foam tape secures the connectors & extends over the solder points.
Red=3v3
Grey=CableSelect [d10]
Orange=MOSI [d11]
Brown=CLocK [d13]
Purple=MISO [d12]
Black =GND
Tape the SD adapter into place on the area that’s been cleared at the back of the screw terminal board.
Bend the jumper wires into place, mark the length & trim the original 20cm wires so that the final insulation length is 9cm or less.
Score the wire insulation about 1.5cm back from the wire end but do not remove the insulation.
Gently ‘pull & twist’ the scored insulation away from the wire to wind the thin strandstogether.
‘Fold-back’ about half of the stripped wire to provide more copper surface for the screw terminals to bite on to.
SD Connection pattern:Grey(CS) to ProMiniD10, Orange (MOSI) -> pm D11, Purple (MISO) ->pm D12, Brown (CLK) -> pm D13(NOTE the shield labels say A0-A3 which do not match the D10-13 pattern of the Pro Mini pins)
Bring the red SD wire over to the re-purposed RST power connection
and the black wire to GND on the digital side. At this point you could test the connections by inserting an SD card & running the CardInfo utility.
Connect the legs of the indicator LED at: D3=red, D4=GND, D5=green, D6=blue
Use the male ends of the wires you trimmed from the SD module to break out pin connections: Grey to A0, Brown to A1, Orange to D7, Purple to D8.
Add a layer of heavy duty (30Lb) double sided mounting tape to the back of the 2xAA battery holder. The battery holder wires need to be approx. 6inches/15cm long.
Attach the battery holder wires to RED>Vin & black>GND. The breakout wires from A0/1 & D7/8 should be about 12cm long to comfortably reach the breadboard area.
Checking continuity to the top of the ProMini confirms the header pins & solder connections under the terminal board are good.
Take a moment to check the continuity of the SD module wires. With one probe on the Dupont metal & the other on top of the corresponding ProMini pin – you should read ~1 ohm or less for each connection path. Occasionally you get a bad crimp-end on those multi-wire Dupont ribbons, and it’s easier to replace a bad wire at this stage than it is after the parts are in the housing.
Note: We’ve used the hardware interrupt port at D3 for the red LED channel, but if you have sensors that need that simply shift the LED over by one. Any digital I/O pins can be used for the LED, but 3,5 & 6 have PWM outputs which lets you do multi-color fades with analogWrite()
Assembly Part 2: Add RTC Module Jumper Wires
Attach Dupont jumper wires to the RTC module using White=I2C Data, Yellow = I2C Clock. Blue is the SQW alarm output line. Nothing is attached to the 32k output pin (cutting the Vcc leg disables 32k output)
Use 20cm M-F jumpers on the 6-pin side of the RTC module and shorter 10cm M-F wires on the smaller 4-pin cascade port.
Add a layer of double sided tape to secure the jumper shrouds, and provide housing attachment points for the module.
Add as small piece of 1/16″ heat shrink tubing to reinforce the contact spring. This reduces the chance that the connection will be bumped loose if the logger is dropped.
Write the installation date on the coin cell with a marker. A new coin cell should power the RTC for about 4 years. To avoid accidentally disconnecting the coin cell battery after the logger is assembled ->
Always check the backup voltage by placing the positive probe tipbetween the spring plates rather than on the surface of the cell.
Assembly Part 3: Connect modules inside the housing:
Part 3: This is what you are aiming for. (click any images in the table below to enlarge them)
The final assembly stage can be a bit tricky – sometimes the metal contact flaps under the green screw terminals are ‘sticky’ so take some time to loosen the screws and poke with the sharp end of your tweezers to make sure you can insert the bus wires from the RTC. It’s helpful to mark the wire lengths with a pen before cutting or stripping the RTC connections. When in doubt leave them a bit long. With screw terminals you always have the option of shortening those wire later on.
Attach the screw terminal stack onto the upper left corner of the housing, and the 2xAA battery holder in the lower right corner. These parts sb as far from each other as possible.
Add the first mini breadboard against the back so that it’s rear right edge aligns with the second rear support strut on the housing. Connect the long stack jumpers to that bboard to keep them out of the way.
Tape the RTC module into the lower left side of the housing, The blue board should be up against the front edge of the housing so that you can easily access the nearby screw terminals.
Measure, mark and trim: Red=3.3v, Yellow=>A3/A5 I2C clock, & White=>A2/A4 I2C data with enough wire to twist & fold the stripped ends under the terminals (~10cm of insulation length)
RTC module power & I2C bus connections: All ports on the analog side of the Pro Mini / Shield combination are occupied with a wire connection.
Connect the black wire to the re-purposed RST=GND, and the blue alarm wire to D2, leaving some extra wire length in a loop for strain relief. (5-6cm of insulation length)
Use the other side of the trimmed blue jumper wire to extend the D9 connection over to the breadboard. You want enough wire length that the pins reach back-most row on the breadboard.
Attach a cable mount to the back of the housing, as close to the bottom of the box as possible so that it does not interfere with closing the housing lid. You can trim those plastic mounts with scissors to make them thinner.
‘Loosely’ tie the long wires to the rear mount. Add another cable mount near the center and attach the 2nd b-board leaving equal side of the 2nd board.
Every year at least one student gets confused about the orientation of the connections inside the breadboard and connects all the jumper wires together in the same row – including the red and black power wires. The resulting short circuit usually kills either the Pro Mini, the UART module, and/or possibly even the USB port on the computer it’s connected to:
Each 5-hole row on the top of the breadboard is connected by a metal rail of spring contacts.
Also note that the internal connectors do not cross the ‘gutter’ depression in the middle, so each side of the breadboard board has its own separate set of connections.
Your Logger is now ready for testing!
A typical I2C sensor configuration with: BMP280 pressure, BH1750 lux & 0.96″ I2C OLED display – connected by short jumper wires made with a crimping tool. The combination shown above averages ~10mA with screen & cpu running, and a sleep current of 0.147 mA with a 1Gb Sandisk SD card. Without the SD, the sleep current on this unit was 37µA; with the sensor modules needing 2-3µA each & the sleeping 0.96″ OLED drawing ~7µA. A 25µA sleep current from the ProMini clone hints that the MCU might be fake but with a AA power supply it doesn’t really matter. Anything up to 250uA sleep current for a student build with an SD card connected should be considered good. Watch out for SD cards that don’t go to sleep properly as they can draw up to 30-50mA all the time.
(Note:Most of the time the tests listed below go well, however if you run into trouble at any point read through the steps suggested for Diagnosing Connection Problems at the end of this page.)
1. If you have not already done so, Install the UART driver. The IDE will NOT be able to communicate with your logger unless the driver for your UART module is installed on your operating system.
2. Installthe Arduino IDE into whatever default directory it wants – we’ve had several issues where students tried to install the IDE into some other custom sub-directory, and then code wouldn’t verify without errors because the IDE could not find the libraries. The programming environment is written in Java, and the IDE installer comes with its own bundled Java runtime so there should be no need for an extra Java installation. However we have seen machines in the past which would not compile known-good code until Java was updated on those machines; but this problem is rare.
If you have not already done so, there are three things you need to set under the IDE>TOOLS menu to enable communication with the logger:
Note: that the “COM’ setting will be different for each computer, so you will have to look for the one that appears on your system AFTER youplug in the UART module.
Using 22AWG solid core jumpers to bridge a set of ‘power rails’ along each side keeps the wiring tidy. If you are using several I2C sensors you could also do this with those bus lines. After testing your prototypes, you can make them permanent by transferring your circuit to solder-able mini breadboards.
The one that’s easy to forget is choosing the 328P 3.3v 8Mhz clock speed. If you leave the 328p 5v 16mhz (default), the programs will upload OK, but any text displayed on the serial monitor will be random garbled characters because of the clock speed mismatch. Also be sure to disconnect battery power(by removing one of the AA batteries)whenever you connect your logger to a computer. There is no power switch on the loggers, which are turned on or off via the battery insertion. Use a screwdriver when removing the batteries so that you don’t accidentally cause a series of disconnect-reconnect voltage spikes which might hurt the SD card.
3. Test the LED – the default blink sketch uses the pin13 LED, but because that pin is shared with the SD card’s clock line it’s recommended that you test the RGB indicator instead by adding commands in setup which set the digital pin 4 act as the ground line for the LED: pinMode(4, OUTPUT); digitalWrite(4, LOW); Then change LED_BUILTIN in the blink code example to the number of a pin connected to your led module. (ie: for Red set it to 3, for Green use 5, or Blue use 6)
4. Scan the I2C bus with the scanner from the Arduino playgound. The RTC module has a 4K eeprom at address 0x56 (or 57) and the DS3231 RTC chip should show up at address 0x68.
The address of the eeprom can be changed via solder pads on the board, so it may have a different address. If you don’t see at least these two devices listed in the serial monitor when you run the scan, there is something wrong with your RTC module or the way it’s connected: It’s very common for a beginner to get some of the wire connections switched around during assembly but with the screw terminals this takes only a few moments to fix.
5. Set the RTC time, and check that the time was set – The easiest method would be to use the SetTime / Gettime scripts from our Github repository, but you first you need to download & install this RTC control library The SetTime script automatically updates the RTC to the moment the code was compiled (just before uploading) so only run SetTime once, and then upload the GetTime sketch to remove SetTime from memory. Otherwise SetTime will keep setting the RTC to the old ‘code compile time’ every time it runs – and one of the quirks of the Arduino environment is that it restarts the processor EVERY TIME you open a serial window. The SetTime script also has a function which enables the alarm(s) while running the RTC from the backup coin cell battery.
Note: the RTClib by Mr. Alvin that we use has the same name as the Adafruit library for this RTC and this can give you compiler errors if you let the IDE ‘auto-update’ all of your libraries because it will over-write the Alvin RTClib with Adafruit’s library of the same name. You then have to uninstall the Adafruit library ‘manually’ before re-installing Alvin’s RTClib again. This problem of ‘two different libraries with the same name’ was common back when this project started many years ago, but back then the IDE didn’t try to update them automatically.
Typical Cardinfo output on a windows computer when the connections are correct. If you format your SD card on an Apple computer there will also be a long list of ‘invisible’ .trash and .Spotlight files/folders at the root of the SDcard that show up with a CardInfo scan.
6. Check the SD card with Cardinfo Note that the SDfat library we use to communicate with SD cards works well with smaller cards formatted as fat16, but ‘some’ Apple users find they can not write to cards in that format, requiring the SD cards to be reformatted as fat32 (note that most Apple systems have no problem with the fat16 SDcards). With either OS you should format the micoSD cards with this SDFormatter utility. With a 15 minute sampling interval, most loggers generate ~ 5Mb of CSV format text files per year. Older, smaller SD cards in the 256-512Mb range often use less power. Note that we apply internal pullup resistors on some of the SD card lines in setup to help the SD cards go into low power sleep modes more reliably.
This logger uses an advanced code trick to read the positive rail voltage to ~11mv resolution by comparing it to an internal 1.1v bandgap reference inside the processor. That internal ref. can vary by ±10% from one chip to another, and CalVref gives you a numerical constant which usually brings the starter script’s rail=battery readings within ±20mv of actual. An accurate rail reading is more important when you are using ANALOG sensors where the positive raildirectly affects the ADC output, but you can skip this procedure if you are only using digital sensors because they use their own internal reference voltages.
Typical CalVref output (click to enlarge)
Load CalVrefwhile the logger is running from USB power and then measure the voltage between GND and the positive rail with a voltmeter. (this voltage will vary depending on your computer’s USB output, and the UART adapter you are using) Then type that voltage into the entry line at the top of the serial monitor window & press the enter key. Write down the reference voltage & constant which is then output to the serial monitor window. I write these ‘chip-specific’ numbers inside the logger with a black marker as they are related only to the 328p processor on the ProMini board used to make that particular logger. You then need to change the line #define InternalReferenceConstant 1126400L in our starter code to match the long number returned by CalVref. Alternatively you could just tweak the value of the reference constant ‘by hand’, increasing or decreasing the value till the reported rail readings match what you measure with a voltmeter. Add or subtract ~400 to/from the constant to raise/lower calculated output by ~1 millivolt. After you’ve done this once or twice you can usually reach the correct value with a few successive guesses.
8. Find a script to run your on logger. For test runs on a USB tether, the simplest bare-bones logger code is probably Tom Igoe’s 1-pager at the Arduino playground. It’s not really deploy-able because it never sleeps the processor, but it is still a useful ‘1-pager’ for teaching exercises and testing sensor libraries. In 2016 we posted an extended version of Tom’s code for UNO based loggers that included sleeping the logger with RTC wakeup alarms. Our current logging “Starter Script” has grown since then to ~750 lines, but it should still be understandable once you have a few basic Arduino programming concepts under your belt.
Using the logger for experiments:
It’s important to understand that this logger was designed a teaching tool rather than a off the shelf, plug-&-play solution. Learning how to solder and getting some experience physically ‘putting things together’ are key outcomes. Wrangling code into shape driving some new sensor combination is another vital part of that process. So perhaps the best piece of advice I can give to new builders is:
Test, Test,and when you think your logger working: Test it Again
It’s nearly impossible to write code without little bugs and the only way to root them out is with multiple test runs. And even if the script you wrote is ‘perfect’ the processors on the sensor modules are also running code that you don’t have access to. For example, the mcu inside your SD card memory is more powerful, and may have more code on it than the ProMini at the heart of this logger. The only way to catch timing errors that might not get triggered until the 10th or 200th(?) pass is to run your code with a short 1 minute sampling interval until it’s crossed those roll-over thresholds many times. Use “Starting sensor X read” & “Finished sensor X read” print statements liberally during early USB tethered tests so you can observe the timing of events.
If you see water condensing like this on the lid of your logger then it’s time to examine the o-ring and add some extra sealant (nail polish or silicone) around the exterior of the cable glands. This logger quit after one week and it only lasted that long because of the desiccant pack.
Same thing applies to the sensor hardware in terms of durability, only now you have moisture to deal with. Everything that can be sealed in adhesive lined heat shrink, or potted in epoxy should be, once that hardware has passed your ‘dry’ tests. As a general rule no $10 sensor is going to be rated past IP68 which at best gives you 2-3 weeks of operation in the real world before water works it’s way in because of pressure imbalances caused by daily thermal cycling. You’d be surprised how easily moisture can wick along the air space ‘between the copper strands’ inside wires.
A doubling schedule works well for testing: Check the logfile at 1 hour, 2 hours, 4 hours, overnight, 1 day, 2 days, 4 days, 8, 16, 32… etc. Move to the next longer test only when the data from the previous run is confirmed. Keep a close eye on that battery burn down rate: Until you get the hang of putting your sensors into low current ‘sleep’ states – getting your first logger builds to run for a couple of months on new batteries should be considered a spectacular success. At every startup watch and wait for the pattern of LED flashes to confirm that the launch went smoothly – it is very easy to insert a battery or SD card crooked by ‘just enough’ that the unit does not start, and it’s very frustrating to discover you have no data a week later.
You never really know how long a sensor is going to last until you’ve deployed it – no matter what the manufacturer says in the data sheet. Even then we usually deploy three of every ‘new’ combination, and if we are lucky we get one complete data set for the year. Batteries leak, critters love to chew on things, and whenever humans come across something they’ve not seen before they will pick it up – especially if you had to invest a good deal of time securing your logger in exactly the correct position in the stream, on the tree, etc. We never deploy anything for real research until it has passed a several week-long rapid sample ‘burn-in’ test.
One positive aspect of the relatively loose fit of the Plano box lid is that it lets you run sensor tests quickly if you jumper your sensor module with thin 28-30 gauge wires:
A BMP280 pressure sensing module on long wires with crimped male dupont ends in the breadboard.
~1″ square of foam mounting tape with wires spaced evenly
Leave the red backing facing up as you fold the tape & wires over the corner edge.
The front corners of the box exert less pressure than the back corner shown here.
The sharp inner edge of the lid would cut the wire insulation if the tape was not there to protect, and even then you can only use this trick a few times.
The tape over the wires has to be replaced every time.
This gives you a chance to do some test runs before you commit to modifying the housing with holes or cable glands. For some indoor experiments this might be all you actually need, though I would still coat the ‘non-sensing’ parts of that dangling breakout with either conformal coating or clear nail polish. My general advice is: Do not put holes in the housing unless you are sure you need them. The most common failure mode for student loggers used in outdoor environments is from moisture seeping into the through the cable gland. Natural heating and cooling cycles creates pressure differences between the inside and outside of the logger that drive this vapor exchange. Moisture then condenses when temperatures fall at night, collecting on leftover flux residue to corrode contacts. An outer layer of self-fusing rubber mastic tape is often used on cable glands by electricians on out-door installations – even when using the expensive ones with soft rubber ‘cages‘.
After 1-2 minutes of kneading to mix the epoxy you have ~ 1 minute to work the putty into place. (it will become rock-hard within ~10 minutes). Be sure to leave yourself enough extra wire/space inside the housing so that you can open and close the lid easily without disconnecting anything after the putty hardens. This seal is not strong enough for underwater deployments, but it should easily withstand exposure to rain-storm events. HOT GLUE also works to seal pass-through ports with smaller wires & cables. Both pass-through methods can be helped by a layer of silicon caulking, nail polish, or conformal coating applied to the outside edges.
For a classroom project you could simply drill small a hole through the lid and stick the sensor/module on top of the housing, sealing the hole with double-sided tape. Thicker pass-throughs can be also be sealed reasonably well with plumbers epoxy putty which is non-conductive, and adheres quite well to metal, glass & plastic surfaces-> This putty is also a quick way to make custom mounting brackets, or even threaded fittings if you wrap it around a bolt (which you carefully remove before the putty hardens completely)
No matter which pass-through method you are using: Silica gel desiccant packs are important for any outdoor deployments and 5-10 gram packets are a good size for this logger.
Don’t subject these loggers to a lot of bashing around by deploying them in a rough surf-wash zone, or swaying freely in the wind off the end of a tree branch. Solid core wires are pretty good if you cut them to exactly the right length , but longer beadboard jumpers are very easy to bump loose, so once you have your prototype working, it’s usually best to re-connect the sensors directly to the screw terminals before deploying a logger where it could get knocked around. In a pinch you can secure breadboard pins with a small drop of hot glue to keep them from wiggling. Also remember that there are six ‘unused’ screw terminals on the base shieldand these can be use to join wires together without soldering.
2019 Logger mounted on a south-facing window. The top surface was covered with white label-maker tape to act as a diffuser.
[Click HERE] to read about the many types of sensors can be added to this loggerThe transparent enclosure makes it easy to do light-based experiments. Grounding the indicator LED through a digital pin allows it to be used as both a status indicator, and as a light sensor. The code we use is apolarity reversal technique that relies on the tiny capacitance inside the LED. (~5 to 20pF & the processor port adds 10pF) This technique requires the LED to connected directly to ProMini inputs because breadboards can add random amounts of changing capacitance. At these sub pF ranges, any humidity that condenses between the pins will also upset the readings, so desiccants are required. And finally the reverse bias decay is affected by the starting voltage, so if you want to use the technique in a rigorous ‘analytical’ setting you should leave the regulator on your logger.
We have integrated this LED sensor technique into the starter script on GitHub. I’ve tweaked the playground version with port commands so the loop execution takes about 100 clock cycles instead of the default of about 400 clock cycles. The faster version was used to generate the following light exposure graph with a generic 5mm RGB LED, with a 4k7Ω limiter on the common ground.
Red, Green & Blue channel readings from the indicator LED (from a regulated logger) over the course of one day (logger photo above) The yellow line is from an LDR sensor the same unit, that was over-sampled to 16-bit resolution. The LED sensor has a logarithmic response and the left axis on the graph is a time- based measurement where more light hitting the LED sensor results in a lower number. Note how the RED signal changes before/after Blue & Green at sunrise & sunset. LED’s work well with natural full-spectrum light, but their limited frequency bands can give you trouble with the odd spectral distribution of indoor light sources. The peak response of LED’s is usually 30–50nm lower than their peak emission wavelength If we assume the Red was Aluminum gallium arsenide (AlGaAs) then that channel probably had an absorption band @ ~680 nm (~15 nm FWHM?) while the blue Indium gallium nitride (InGaN) channel is responding in the UV-A range, the Green channel (probably also InGaN ?) is most likely peaking around 420nm which is blue to our eyes. But without a spectrometer to test, these are just guesses. No temp. or cosine corrections were determined/applied, although blue/green channels tend to have low temperature coefficients because their bandgap is so far from the thermal spectrum. LED absorption bands have very little drift over time.
You can read more about LED based sensing techniques in the post about our leaf testing experimentswhich used two LEDs for a transmission-based variant of the NDVI ratio.
While the LED sensor idea is fun to work with, it’s a relatively slow method that can keep the logger running for many seconds when light levels are low. Figuring out how to take those light readings only during the day is a good coding exercise for students.
Note: VERY FEW light sensors can withstand exposure to direct sunlight. PTFE is an excellent light diffusing material which available in different sheet thickness. The ‘divot’ on the lid of the Plano box is just a bit larger than 55mm x 130mm x 3mm (depth). The “teflon” tape that plumbers use to seal threaded joints can also be used. PTFE introduces fewer absorbance artifacts than other DIY diffusers like ping-pong balls, thermoplastic, or hot melt glue. Most light sensors like the TSL2561 need 3-5mm of that PTFE sheeting to prevent the sensors from saturating in full sun. LED’s have logarithmic response so you lose quite a bit of detail above 40,000 lux unless you add a diffusion layer to attenuate the signal.
Full sun exposure can also cook your logger. Internal temps above 80°C may cause batteries to leak or damage the SD card. So if you are leaving the logger in full sun, add a bit of reflective film or some aluminum foil around the outside to protect the electronics. Of course if you have a light sensor you’ll need to leave some ‘window area’ for it to take a reading.
The RTC has a built-in temperature register which automatically gets saved with our starter scripthowever that record only resolves 0.25°C, so we’ve also added support for the DS18b20 temperature sensor to the base code. A genuine DS18b20 (yes, fake sensors are a thing) draws very little power between readings and you can add many DS18b’s to the same logger.
Addendum: Diagnosing Connection Problems
If you successfully loaded the blink sketch to test the ProMini during your initial assembly, then issues during the testing stage are often due to incomplete connections to the I/O pins.
If you see only “Scanning I2C….. ” but nothing else appears when running the bus scanner, then it means that the ProMini can not establish communication with the RTC module. One common cause of this problem is that the white & yellow wires have been switched around at one end or the other. It’s also easy to not quite remove enough insulation from the wires to provide a good electrical connection under the screw terminals, so undo those connection and check that the wires were stripped, cleaned & wrapped together before being put under the terminals.
Scanner lockup can also happen if one of the I2C devices on the bus is simply not working: usually about 1 in 6 logger builds ends up with some bad component that you have to identify by process of elimination. (These are 99¢ parts from eBay…right?) It only takes a moment to swap in a new RTC board via the black Dupont connector and re-run the scan. If the replacement RTC also does not show up with the I2C scanner then it’s likely that one of the four bus lines does not provide a complete connection between the ProMini & the RTC module.
On this unit I measured 1 ohm of resistance on the I2C clock line between the ProMini A5 pin (on top of the board) and the SCL header pin on the RTC module. So this electrical connection path is good. It’s not unusual for each ‘dry’ connection to add 0.5-1 ohm of resistance to a signal path.
To diagnose: Unplug any power sources to the logger. Set a multi-meter to measure resistance and put one probe lead on the topmost point of the promini header pins, and the other probe on the corresponding header pin of the RTC module. If there is a continuous electrical connection between the two points then the meter should read one ohm or less. Higher resistances mean that you don’t have a good electrical path between those points even if they look connected:
1)the ground (black) wire should provide a continuous path from the ground pin on the digital side of the Promini board to the GND pin on the RTC module 2) the positive power (red) wire should provide a continuous path from the Promini positive rail pin (the one with the bundle of 4 red wires) to the VCC pin on the RTC 3)A4 (I2C data) near the 328P chip on the Promini must connect all the way through the screw terminal board and through the white Dupont wires to the SDA post on the RTC 4)A5 (I2C clock) nearest the UART end on the Promini must connect through through the yellow Dupont wire to the SCL header on the RTC .
You occasionally get a bad Dupont wire where the silver metal end is not in contact with the copper wire inside because the crimp ‘wings’ did not fold properly. With a pair of tweezers, you can ‘gently’ lift the little plastic tab on the black shrouds holding the female Dupont ends in place, and then replace any single bad wire. Be careful not to break the little black tab or you will have to replace the entire shroud.
Everyone uses short male-to-male Dupont jumper wires when they are creating test circuits because they are so convenient. But pre-made jumper wires are usually too long and so they get knocked around when you close the lid of the logger: so before you deploy take the time to convert your flexible-wire test circuit into one with solid core jumpers:
Flexible wires get used during the initial testing stages (when the lid of the logger is open).
Then the circuit gets re-done with solid core wires
Running wires ‘under’ the modules makes it easier to close the lid without disturbing the connections.
Also look at the little jumpers used to bridge the A4>A2 and A5>A3. If you have a ‘cold’ solder join, or an accidental bridge connection to something else, it could stop the bus from working. Re-melt each connection point one at a time, holding the iron long enough to make sure the solder melts into a nice ‘liquid flow’ shape for each solder point.
The connection diagnosis procedures described above also apply to the connections for the SD adapter board. Sometimes you end up with an adapter that has a defective spring contact inside the SD module, but the only way to figure that out is to swap it with another one.
Here a jumper wire from the ProMini pin is by-passing a bad connection on the screw terminal board. This is also how you would break out A6 & A7 if you need them.
Sometimes those screw terminal boards have a poor connection inside the black female headers below the ProMini. It’s also possible to accidentally over-tighten a terminal and ‘crack’ the solder connection below the board – or there may simply be a cold solder joint on one of the terminal posts. If you have only one bad connection, you can jumper from the ProMini header pins on top, down to the other wires under the corresponding screw terminal. If you accidentally strip the threads on a screw terminal, you can use this same approach but move that set of wires over to one of the three ‘unused’ screw terminals at the far end of the board. (beside the SD card adapter) If you’ve gotten through all of the above steps and still have not fixed the problem, then it might be time to simply rebuild the logger with a different screw terminal adapter board.
If you do accidentally kill the ProMini by shorting a pin, etc, you can carefully lever it up away from the screw terminal shield and replace it without having to rebuild the whole logger.
Build two loggers at a time, because that lets you determine whether problems are code related (which will affect both machines the same way) or hardware related. (which will only affect one of your two units) At any given time I usually have 2-3 units running overnight tests so that I can compare the effect of two different code/hardware changes the next morning. As a general rule you want to run a new build for at least a week before deploying to get beyond any ‘infant mortality’, and reach the good part of the bathtub curve.
An I2C OLED is quite readable through the lid of the housing.I often use Griemans text-onlySSD1306Ascii library because it has a low memory footprint and sleeps well. While few loggers need live output when they are deployed, it’s often helpful to view diagnostic messages on battery power during testing. Adding two OLED displays let’s you view text & graphic output at the same time. Adding a capacitive touch switch lets you check your logger’s status at any time.
Addendum: A note about I2C sensors The I²C bus is slow, so topology (star, daisy-chain, etc.) doesn’t matter much, but capacitance does. Both length & number of sensors increase capacitance. If you find that the devices work when you switch to a slower speed(e.g. 50 kHz), then this is probably your issue, and you need to minimize bus length and/or maybe decrease the combined resistance of the pull-ups to 2 kΩ or less. The DS3231 RTC module has 4k7 ohm pull-up resistors on the SDA & SCL lines & the Pro Mini adds internal 50k pull ups when the wire library is enabled. Typical I2C sensor modules usually add another set of 10k pullups so your ‘net pullup resistance’ on the I2C bus wires is usually: 50k // 4k7 // 10k = ~3k. With a 3.3v rail that means the devices draw 3.3v / 3k = 1 mA during communication which is fairly normal( 3mA is max allowed) for total wire lengths below 1m. It’s common for pre-packaged sensors to arrive with housings at the end of about 1m of wire. If each sensor also adds another set of 10k pullups, the resistance generally compensates for the extra wire length, so the combination still works OK. But that depends on the cable too. A very bad cable might not even get to 0.5 meters and a very good cable (little capacitance to ground, no crosstalk between the wires) can go up to 6 meters.
For most sensor types there will be some options that draw much less power than others, and it’s always worth a look at the data sheet to make sure you are using one that will run longer. The best chip based sensors automatically go into low current modes whenever the bus has been inactive, but more often you need to ‘manually’ put the sensors to sleep via specific commands. So it’s also important to check if your sensor library supports those ‘go to sleep’ & ‘wake up’ commands – many common Arduino libraries do not.
Addendum: The importance of moisture protection
I was noodling around in the garden recently and installed a few loggers without desiccants because it was only a short experiment. It rained immediately afterward and I noticed a small amount of moisture condensed inside the plano-box housing. While this didn’t prevent the logger from functioning, it completely disrupted the LED light sensors because the increased humidity provided an alternate discharge path for the reverse bias charge on the LED’s:
Green channel data from a 5mm diffused RGB LED used as light level sensor. This logger was under some leaf cover, so there was considerable variability from the dappled light crossing over the sensor. An arbitrary cutoff of 200,000 was set in the code at low light levels.
After examining the O-ring I decided to add a little silicone to the channel holding the o-ring to improve the seal:
Gently pry the O-ring loose and apply sealant in the groove before replacing.
Bead only needs to be 3-4mm in diameter.
Close the housing & let the sealant set for a few days. The improved seal is especially visible at the corners
If you already have your logger assembled, try to find a silicone sealant that does not off-gas acetic acid (smells like vinegar) which could harm your circuits. If you are simply preparing empty boxes before assembly, then any regular bathroom sealant will do provided you give it about a week to finish curing.
Attach a mounting base to the lid so that a dessicant pack can be secured above the battery holder without interfering with any breadboard jumpers. Use a desiccant pack with color indicator beads, so you can check whether they are still working simply by looking through the transparent lid.
Addendum: If you want to leave the original regulator in place
It’s worth mentioning that an unregulated build will run for many months – even on 2x regular alkaline batteries which reach the system cutoff (at 2750mv) more quickly. The key deciding factor is whether your sensors require tight voltage regulation. The DS18b20 has a nominal low voltage limit of 3v. So if your project is making heavy use of those then there are only a few of modifications to the tutorial shown above to leave the ProMini’s default MIC5205 regulator in place:
Use straight header pins on the RTC modules cascade port to leave more space for the battery holder.
Only bridge the unused RST terminals to the rail connections – Leave the Vin terminal separate for the raw battery input.
Add a 10/3.3 Meg voltage divider to read the raw battery voltage on A0
You will need more space for the extra batteries. You could go with a 3xAA holder but that leaves about 50% or your alkaline battery capacity unused. Or you could keep the standard layout and use 4xAAA batteries.
An alternative would be to add a better regulator to some kind of intermediate battery connector. The the photo on the right shows two ceramic 105’s stabilizing an MCP1702-3302E/TO, while the 10/3.3M ohm divider provides a third output line so the ADC channel can monitor the raw battery voltage. This is the simplest way to retro-fit a unit that was built without a reg, with the added benefit that the new regulator is far more efficient than the original MIC5205 on the ProMini. It’s worth noting that even on a regulated logger you can monitor the rail voltage to determine when the main batteries are depleted because the regulators output will fall if the batteries reach a point below the minimum dropout voltage. If the rail falls under load by more than ~40mv, then it’s probably time to shut down the logger. With the regulator in place you probably don’t need the USB isolator, as the reg. itself cant pass more current than a USB port.
Addendum: Things to keep in mind when ordering parts
When a finished module arrives at your doorstep for less than you’d pay for any of its sub-components – it’s because you are doing the quality control.
My advice is to order at least 5-6 of each of the core components (Promini, RTC, SD module, screw terminal board, etc) with the expectation that about 10% of any cheap eBay modules will be DOA or have some other problem. I build in batches of six, and one logger typically ends up with a bad part somewhere. Having replacement bits on hand is your #1 way to diagnose and fix these issues. Bad parts tend to come “in bunches”, so if you scale up to ordering in quantities of 10’s & 20’s then spread those orders to a few different suppliers so you don’t end up with all your parts from the same flakey grey market production run. Order from different vendors in different odd-number quantities (11, 21, 9, etc.) because that will be the only way you can distinguish which supplier, sent which parts, because nothing on the package will be written in English.
The other thing I can’t stress enough is CLEAN ALL THE PARTS as soon as they arrive. Leftover flux is very hygroscopic, and solder points will start to corrode the moment your logger gets exposed to atmospheric moisture. I usually give everything about 10 minutes in a cheap sonic bath with 90% isopropyl alcohol, rinse with water, and then dry the parts out in front of a strong fan for an hour. Clean parts that can’t take the sonic vibration (RTC modules, humidity sensors, accelerometers, etc) by hand with a cotton swab. Then store parts in a sealed container with desiccant packs till you need them. I also coat the non-sensing/non-contact surfaces with a layer of MG Chemicals 422B Silicone Conformal Coating and let that dry for a day before assembling the loggers. One hint that you may have moisture issues is that the sensors seem to run fine during indoor tests but start to act strangely when you deploy the unit outside.
Used nut containers make excellent “dry storage” once the parts have been cleaned – but any air-tight container will do.
Another insight I can offer is that the quality of a sensor component is often related to the current it draws – if your ‘cheap module’ is pulling significantly more power than the data sheet indicates, then theres a good chance it’s a junk part. Usually if the sleep current is near spec, then the sensor is probably going to work. It is much easier to check low currents with a µCurrent or a Current Ranger.(I prefer the CR for it’s auto-ranging features) Sensors which automatically go into low current sleep modes take time – so you might need to watch the current for several seconds before they enter their quiescent states. A common reason for a short operating lifespan on a logger is an SD card that refuses to go into sleep mode. If there is an SD card connected to your logger you must initialize it (with sd.begin in setup) or it may ‘stay awake’ causing a constant 30-40mA drain and/or may even cause the logger to freeze up. Also with SD cards, if the freshly formatted throughput drops below its rated write speeds when tested with H2testw, then find another card to use. I avoid cards bigger than 2Gb because they usually draw too much current, and it’s rare to need that much space for a logger.
With cheap part variation & beginner soldering skills, student builds range from 0.15mA to .5mA sleep current. But even at that high end you should still get a couple of months of operation of from the logger on fresh batteries.
Pro Mini Style clone 3.3v 8mHz Get the ones with A6 & A7 broken out at the back edge of the board. Just make sure its the 8 MHz 3.3v version because you can’t direct-connect the SD cards to a 5v board. Watch out for non Atmel 328p chips.
$2.20
‘Pre-assembled’ Nano V1.O Screw Terminal Expansion Board by Deek Robot, Keyes, & Gravitech (CHECK: some of them have the GND terminals interconnected) You will also need to have a few 2.0-2.5mm flat head screw drivers to tighten those terminals down. Since this shield is was originally designed for an Arduino Nano many of the labels on ST board will not agree with the pins on the ‘analog side’ of the ProMini.
SD card 256mb -to-1Gb Test used cardsfrom eBay before putting them in service. Older Nokia 256 & 512mb cards have lower write currents in the 50-75mA range. This is less than half the current draw on most cards 1gb or larger. I tend to avoid older cards labeled as ‘TransFlash’ because they seem to have more controller artifacts during saves. Small 128mb & 256mb cards under the name Cloudisk have appeared on eBay, and so far they seem to be working ok.
30cm Dupont 2.54mm M2F 40wire ribbon cable Dupont connector hook-up wires might be expected to add an ohm or two of resistance and carry at most 100mA reliably with their thin 28-30 gauge wires. Each 40-wire cable will let you make at least 2 loggers.
$1.55
10cm Dupont 2.54mm M2F ribbon cable Sometimes these 10cm cables are harder to find, so you can just use the longer 20cm wires in a pinch. It’s usually also helpful to have a few Male-to-Male 10cm cables for interconnections on the breadboard.
$1.00
2×1.5V AA Battery Batteries Holder w Wire Leads If you are running an unregulated system on 2 lithium batteries, then you can use a 2x AA battery holder. If you need to keep the regulator in place to stabilize the rail voltage for particularly picky sensors, use alkaline batteries and a 4xAA battery holder. Watch out for ‘cheap’ battery holders with weak plastic at the connection ends which will slowly bend away from the batteries until they pop out in warmer climates. If that happens you can add a zip tie belt around the holder to keep the cells in place when the plastic softens.
$0.50
5mm Common cathode RGB LED Although you might want to use 10mm LEDs to increase surface area when using the LED as a light sensor. They also look better.
3.3V FT232 UART Module (get at least 2-3 modules – they are easy kill with a brief short) *jumper the pads on your UART module to 3.3v output before using it!* You will also need a few USB 2.0 A Male to Mini B cables. You may need to install drivers from the FTDI website depending on your OS. These boards can only supply ~50mA which can be tricky if your sensors need more for sustained periods. If you are running the class via distance learning it’s probably a good idea to also get some CP2102 (c231932) UART boards and send your students one of each type. If they are unable to get the drivers working for the FT232, they have a second option. You may have to hunt around for non-FTDI chip boards with the same pin order as the ProMini[ DTR-RX-TX-3v3-CTS/gnd-GND ] The DTR pin is critical for uploading code, while the CTS (clear to send) is an input pin for the FTDI chip only and CTS is not used by the ProMini (so it’s usually just tied to ground). So many UART adapters only have 5 connections and you have to cross the wires over each other to get the connections sorted out. Watch out for 6-pin UART modules that put a (+)ive power connection in the same physical alignment as the GND connection on the ProMini – those boards can create a short circuit unless you re-route the wires. It’s also worth knowing that UARTs can communicate directly to serial sensors like GPS modules for testing. Premade 30cm 6-pin Dupont jumper cables are also available..
SYB-46 270 breadboards (used ONLY for soldering header pins ) Soldering the header pins on the pro-mini is MUCH easier if you use a scrapbreadboard to hold everything in place while you work. I use white plastic breadboards that only have one power rail on the side since I won’t mistake them for my regular breadboards. I also write ‘for soldering only’ on them with a black marker.
Syba SY-ACC65018 Precision Screwdriver Set A good precision screwdriver set makes it so much easier to work with the screw terminal boards. But there are many cheaper options. The screw terminal boards need 2mm (or less) flat slot tips.
$12.00
Donation to Arduino.cc If you don’t use a ‘real’ Pro Mini from Sparkfun to build your logger, you should at least consider sending a buck or two back to the mother-ship to keep the open source hardware movement going…
$1.00
.. and the required lithium AA batteries are also somewhat expensive, so a realistic estimate is about $25-30 for each logger when you add a couple of sensors. Expect parts from low-end suppliers to take 4-6 weeks to arrive and always order at least 50% more than you actually need so you have spares. If you’re pressed for time everything on this list is also available from trusted first-tier suppliers like Sparkfun, Adafruit, Pololu, etc – but you will pay 5-10x as much, with an additional $10-15 shipping charge unless you pass the minimum order level. Amazon is now in a kind of weird grey zone between the two as many vendors that sell on eBay, are also selling on Amazon for 2-3x the price.
Addendum: Using a more advanced processor
Moteino MEGA based Cave Pearl Logger
After you’ve built a few ProMini based loggers, you might want to try a processor upgrade. The 1284p CPU has twice the speed & 4x the memory, but delivers comparable sleep current & operating life.
Addendum: Low Temp. effects on 2x Lithium powered logger
2x LithiumAA millivolts (blue-left) vs RTC Temp °C(orange-right) on cells that have been in service for 5 months. We will leave this unit running over winter to see how DS18b20 on that logger handles it if/when the cells fall below the DS18’s 3V minimum, and then rise back up again. (click image to enlarge)
A crop of these loggers have been running in our back-yard garden since mid-summer with various sensor combinations. Winter is finally reaching us so we can now observe how the cold affects an unregulated 2x Lithium AA supply. This ‘student build’ sleeps at ~170uA and has been running for five months. The battery curve was virtually flat above 15°C but it is now being quite strongly affected. Peak loads from the SD card are in the range of about 150mA and the unit is running with a 5 minute sampling interval.
Note: 2022-10-01: We’ve had several unregulated 2xLithium cell loggers running over winter now, with temperatures varying from -20°C to +40°C throughout the year. On units where the sleep current is in the low 20uA range, we typically see the voltage supplied by two cells in series vary due to that that 60 degree range from a low of 3350 to about 3550 mv on hot summer days. So about 200mV thermal delta in normal environmental conditions.
According to Energizer: In ultra-low drain applications like these dataloggers, the discharge curve has a distinct two stage profile. The first ‘very flat’ plateau occurs at slightly higher voltage (nominally 1.79V (or ~3.58v for two cells) @ 21°C) is nearly independent of depth of discharge. This unchanging stage lasts for about 2/3 of the batteries lifetime. The second stage occurs at a slightly lower voltage (nominally 1.7V (or ~3.4v for two cells) @ 21° C) where the cell voltage then decreases slowly as a function of depth of discharge. In my longer run tests, when the two lithium AA cells in series have fallen below ~3.1v, it’s time to shut down the logger.
Addendum: Adding a TTP233 Capacitive Switch lets you check your logger ‘any time’
With a capacitive touch switch that works through the housing, you can check the status of your logger at any time.
Our next tutorial post in the student logger series: Enhance your Logger with an OLED & T233 Capacitive Touch Switch is an excellent ‘next step’ for people using this logger in a classroom setting. The method is easily adapted to trigger ‘opportunistic’ readings in environments that require manual control, but it’s also handy when you need to check the battery level on a complex installation that you don’t want to disturb before the end of the experiment.
Addendum: Another fine crop of student loggers this year!
A soil sensing lab where students characterize daily water use by various potted plants.
A 2-Part ‘mini’ logger released in 2023
Dr. Beddows instrumentation students have been building this Plano-boxed logger for years, and the ability to swap sensors or add an OLED screen has allowed continuous course development. But for those wanting a simpler ‘bare-bones’ version we’ve developed a 2 module classroom logger using 3D printed rails. Without the SD card this unit is memory constrained, and data download is handled via through the serial monitor window in the IDE. This model requires fairly detailed soldering so students get a complete practice lab soldering header pins onto perf-board before attempting the assembly. Running from a coin cell required the addition of several more advanced code techniques than the 2020 student logger at the beginning of this post. But for instructors, this is the least expensive option that still provides your students with an opportunity to change the sensors to develop their own projects.
In this tutorial, a logger is built using a 3.3v Moteino MEGA with a 1284p CPU @ 16Mhz, w 4K eeprom,16K SRAM for variables & 128K program space. Considerably more than the 328’s 1K eeprom, 2K ram & 32K progmem. Also has a spare serial port for GPS/NEMA sensors.
In the 2018 paper we tried to convey that it doesn’t matter which processor you use with our system as long as it’s supported by the Arduino IDE. The ATmega family includes several CPUs more capable than the humble 328p in the Pro Mini / UNO. In fact, some have suggested that the 1284 would have been a better choice right from the start, with full code compatibility once you account for the different port / pin mapping. We built several loggers around that chip early in the project, but at the time I was mostly just leaning on the extra memory to compensate for some fairly crude programming. As my skills improved, and I stopped using bloated sensor libraries, that problem went away. So it’s been a while since I needed anything more than the 328p. But we have a new project spinning up this year that calls for burst sampling of high-bit ADC channels, and we’ll need more SRAM to juggle that data with some fairly large buffer-arrays.
Dave’s protoboard build
For those who build from scratch, Dave Cheney shows one approach to starting with the raw chip. At the opposite end of the spectrum, Stroud Research Center’s Mayfly is a good combination if you’d prefer something ready-made. But I still have a a few Moteino MEGAs lying around, and this project has always followed the ‘middle path’ to enlightenment. So we’ll use one of those boards to demonstrate how easy it is to give our classroom logger a processor upgrade:
Add Screw Terminals to the Moteino: (click images to enlarge)
20 screw terminal ports per side, OR you could solder 2.54mm blocks directly to the MEGA. But they are not as robust for multiple reconnects during prototype experiments.
The Moteno MEGA from ‘just fits’ between the rails on the Raspberry Pi adapter. The ProMini XL fits on a Nano shield – but I like having all those extra I/O pins on the MEGA to play with.
Pins on the MEGA board need to be bent ‘slightly’ to align with the header holes on the Rpi shield.
This is the basic version of the MEGA, but it’s also available with flash & radio transceiver options. LowPowerLabs also sellsthe Current Ranger – anextremely usefultoolfor checking the sleep current on my loggers. The shiny surface is due to the conformal coating we put on all our components. Clear nail polish also works once you’ve cleaned all the flux.
At the time of this writing there are a few 1284 boards on the market: the Moteino MEGA , the Megabrick, the Dwee, and the ultra compact Pro Mini XL. Other projects sporting the chip seem to pop up regularly, though fewseem to lastvery long. This is a shame because 1284 has enough juice to do single-chip emulation of early8-bitcomputers. But perhaps the chip never really caught on in Arduino-land because few beginners need that much capability. Sustained interest from dedicated makers means that some version, or at leastDIY instructions, will will be available as long as the chip is in production. If I had to, the terminal expansion board I’ve used here would let me build one of those from the raw chip & a few passives. The trick would be making templates for the boards.txt and pins_arduino.h, and finding a bootloaderthat matches the system clock.
Component Prep & Logger Assembly:
Black =GND, Purple=MISO, Brown=CLocK, Orange=MOSI, Grey=CSelect, Red=3v3
Foam tape holds the Dupont jumpers together & provides accidental contact protection on the header pins. 3.3v system lets you connect the SD card directly
Place the SD module to one side, leaving room for wire pass-through under MEGA Board. Note:a 10k pullup on the CLocK line was removed because mode0 sleeps low
Route all but the GND wire under the main board
Score the insulation at ~15mm, and ROLL THE INSULATION between your fingers to TWIST the thin 30AWG strands together.
Add a 5mm ‘hook’ to provide more wire under the screw terminal contacts
SD power is controlled by switching the GND line with a TN0702 logic level mosfet on D0 -> After <- SPI peripheral is disabled & all SPI bus lines are pulled up.
Diffused common cathode RGB w GND pin bent to identify. No limit resistor is needed if you use INPUT_PULLUP to light each color channel.
Attach the GND wires before putting the MEGA into the Plano 3440 stowaway box being use as a housing.
Here using 3xAA to supply the MCP1700 reg. on the MEGA. For unregulated builds, I use 2x Lithium AAs with flat discharge curves.
Blk(GND), Red(3.3v), White(SDA), Yellow(SCL), Blue(SQW) with Dupont pin headers added to the cascade port. 32K is not usedas our power-mod disables that output.
170 tie point breadboard completes assembly. I2C bus is tapped from the RTC modules 4-pin cascade port.
For comparison, here’s a regulated Pro-Mini based build from 2019 . In 2020 we switched the 328p builds to2xAA with no regulator.
The Moteino MEGA also makes Aref available (purple). Setting that to the internal1.1v bandgap gives you a reference voltage with thermal stability similar to a T431 provided you leave the ADC running long enough to stabilize the cap on aref & account for the internal resistance.
Now all I have is a bit of code tweaking to change the I/O in the codebase so those commands match the Moteino’s pinmap.
To complete the trim on this prototype I’ve added a 16-Bit ADS1115(I’ll be giving that ADC a serious workout soon & will post any interesting results). Pinning the add-on module vertically preserves space on that tiny breadboard. With the ADS I won’t be using the internal ADC for much other than battery tracking. However analog pins will safely tolerate mid-range voltages which cause serious leakage problems on normal digital I/O pins. This lets you use a resistor bridge for scaled ranging techniques.
With all that extra memory to play with I’ll finally get to try some graphinglibraries with that OLED – a luxury I can’t usually afford with ProMini based builds (though tiny plotter works in a pinch). And rumor has it that you can bit-bang I2C on ‘any’ GPIO pins, driving SSD1306 displays up to 10 frames per second. We have plenty of I/O lines to spare now for those experiments.
All of Atmels ‘-P’ variants have low power modes, and the logger shown here sleeps below 20μA (without the OLED, which draws ~25uA while sleeping) – that’s the about the same as I usually get with the 328p based units. The 1284 draws more runtime current (~17mA), but you can reduce that considerably by throttling the system with a prescaler. If you then accelerate the I2C bus by the same factor with TWBR, your sensors are none the wiser. In fact I’ve had no problem polling the 1115 with the system clock brought all the way down to 62.5 Khz with clock_prescale_set(clock_div_256);
I guess the last thing on my list is a name for this beast. I had been thinking of calling it the ‘MEGA pearl’, but a quick google search convinced me otherwise. So I’m open to suggestions.
Well that didn’t take long: a few have already pointed out that I missed the Microduino Core + in the ‘currently available’ list. I’m sure there’s more so I’ll add them if I hear about others with a physical footprint that’s small enough to work as a drop-in replacement with the student build. I skipped the 644p in the preamble, but it’s another good low-power option if you are looking for more program memory. The 1284 is pin compatible with the 644, ATmega16, ATmega32 and probably a lot more.
Now that I’m playing with the 1284 again, I’ll also post any interesting projects I come across using these more powerful processors, such as this acoustic impulse marker or this body fat analyzer.
Addendum 2020-05-21: Using the ADS1115 in Continuous Mode for Burst Sampling
As promised, here’s a first look at that ADS1115 ADC module. For day to day tasks, it’s already something of a work-horse for Makers. But I wanted to push it out to the max settings to see if it’s ready for some typical research level applications. The answer is a tentative yes, but only at the fastest 860 sample per second speed. And with small signals EMI is more of a problem than the limitations of the ADC itself. I’m sure that’s not news to the analog hardware hackers out there, but I’m kind of a digital kid who’s still learning as I go along.
Dupont jumper variant of the “fully soldered’ Classroom Data Logger from the Cave Pearl project: This version uses dupont jumpers to reduce assembly time to about 1 hour
Note:An updated version of the classroom logger was released in 2020:CLICK THIS LINK to view the newest build tutorial.
It’s only been a couple of weeks since the release of the 2019 EDU logger, and we’re already getting feedback saying all the soldering that we added to that tutorial creates a resource bottleneck which could prevent some instructors from using it:
“Our classroom has just two soldering stations, and the only reason there are two is I donated my old one from home. So we simply don’t have the equipment to build the logger you described. And even if we did, some of my students have physical / visual challenges that prevent them from working with a soldering iron safely…”
Or goal with that design was to give students their first opportunity to practice soldering skills that are needed when repairing kit in the field. However helping people do science on a budget is also important – so that feedback sent us back to the drawing board. After a little head scratching we came up with a new version that combines the Dupont jumper approach we used in 2016, with this flat-box layout. In the following video, I assemble one of these ‘minimum builds’ in about one hour. To put that in perspective, the fully soldered version takes 2 – 2.5 hours for someone with experience.
Note: After you’ve seen the video to get a sense of where you are headed, it’s usually much better to work from the photos (below) when assembling your logger. Youtube videos make it look easierthan it actually is when you are just starting out. So the first one you build could take several hours as you figure out what you are doing, the second will take half as long, and the third one you make usually takes less than two hours. With some practice you can easily make 4-5 of these things a day.
This variation of the basic 3-component logger is optimized for quick assembly, so the soldering has been reduced to adding header pins and bridging the A4/A5 I2C bus to the outer terminals. An instructor could easily do that ahead of time with about 15 minutes of prep per unit, leaving only the solder-less steps for their students. After the header pins are in place, connections to the central Pro Mini are made by simply twisting stripped wire ends together and clamping them under screw terminals.
You should useLithiumAA batteries with a 2-cell unregulated supplybecause the slope of an alkaline batteries discharge would bring the system down to ProMini’s brown-out of ~2.7v when only 40% of the battery capacity has been used. (note the SD card safe down to 1.8v) While the voltage of a ‘brand new’ Lithium AA is usually 1.8v/cell, that upper plateau provides a sleeping logger voltage of ~1.76 v/cell once the batteries settle, and that briefly dips to about 1.625v/cell under load during data saves. At low temps of about 5°C (in my refrigerator) those SD card voltage dips go down to about 1.525 v/cell. Unloaded readings of 1.5v on a Li AA = battery is dead.
This time reduction involves a few trade-offs, and bringing the I2C bus over to A2 & A3 leaves only two analog ports readily accessible ( although A6 & A7 are still available if you add some jumpers). Removing the regulator & battery voltage divider adds ~30% more operating life, but it also forces you to deal with a changing rail voltage as the Lithium AA batteries wear down. The daily variation is usually quite small, but for quantitative comparisons on monthly scales you will need to correct for the change in rail voltage over time if your sensor circuits are not ratio-metric. To do this voltage compensation multiply your raw sensor readings by the the ratio of (3300mv) / (current rail voltage). Here 3300mv is just an arbitrary comparison point, which you could replace with any rail voltage reading from the data saved by your logger. Batteries have a lot of mass, so thermal lag in battery voltage can also cause hysteresis for analog temperature sensors unless you read the reference under the same conditions.
Solder the UART pins & test ProMini board with the blink sketch: Set the IDE to (1)TOOLS> Board: Arduino Pro or Pro Mini (2)TOOLS> ATmega328(3.3v, 8mhz) in addition to the (3)TOOLS>COM portto matchthe # that appears when you plug in the serial adapter board.
Once you know you have a working Promini board: Remove the power LED [in red]. Removing the pin13 LED [yellow square above]is optional. Leaving the pin13 LED in place lets you know when data is being saved to the SD card because the SPI bus SClocK signal flashes the LED.
Remove the voltage regulator with snips. Your system voltage will vary over time, but our starter script records that rail voltage without a voltage divider.
Add pin headers to the sides & Serial input end of the Pro Mini.
Bridge the two I2C bus connections for side access with the leg of a resistor. Connect A4->A2 & A5->A3.
Adding DIDR0 = 0x0F; in Setup disables digital I/O on pins A0-A3 so they don’t interfere with I2C bus coms.
NOTE: The Screw Terminal board we use in this build was designed for the 5v Arduino NANO, so the shield labels don’t match the actual 3.3v Pro Mini pinson the ‘analog’ side.(the digital side does match) To avoid confusion may want to tape over those incorrect labels and hand write new labels to match the pattern above. Wire connections in this tutorial will be specified by ProMini pin labels: D10-13 are used for the SD card, A4=A2 is the I2C Data line, and A5=A3 is the I2C clock line, A0 & A1 are not used.
Technically speaking, bridging the I2C bus (A4=data & A5=clock) over to A2 & A3 subjects those lines to the pin capacitance and input leakage of those analog pins (regardless of whether that channel is selected as input for the ADC p257). But in practice, the 4K7 pull-up resistors on the RTC module can easily handle that at the 100 kHz default bus speed. Adding DIDR0 = 0x0F; in setup disables digital I/O on pins A0-A3 to prevent interference with the I2C bus. If you want to disable the IO on A2 & A3 ‘only’ add bitSet (DIDR0, ADC2D); bitSet (DIDR0, ADC3D); to your code.
Also note: On the UART adapter in the picture above, the USB to TTL adapter pins are in the reverse order to the Pro Mini board. This is a fairly common issue with clones and if the blink sketch never uploads flip the adapter around and try again. I have connected 3.3v ProMinis to UART modules the wrong way round many, many times, and not one of them has been harmed by the temporary reversal.
Screw-Terminal Component Stack:
Add 3 layers of double sided tape so the tape is thicker than the solder pins.
Align RX&TX corner pins before inserting. The GND points on the screw terminal board may be interconnected (via the back-plane) & must match the ProMini’s GND pins.
Gently rock the Pro Mini back to front (holding the two short sides) until the pins are fully inserted. Some ST shields have mis-aligned headers so this insertion can be tricky.
Remove the last three ‘unused’ pin headers to make room for the SD adapter
Note: Screw-terminal board labels do not match the ProMini pins on the ‘analog’ side
Remove bottom 3 resistors from the adapter – leave the top one in place!
Separate Dupont Cable wires & click them into a 6-pin shroud.
Cable Color Pattern: Black =GND, Purple=MISO, Brown=CLocK, Orange=MOSI, Grey=CSelect, and Red=3v3
Use foam tape to attach SD module to the Screw Terminal board. Metal tabs should be visible on top surface.
Measure, cut & strip the 4 SPI bus wires (NOTE the ‘Nano’ ST board labels say A0-A3 which does not matchthe D10-13 Pro Mini pins on this side of the board)
Grey (CS) to ProMiniD10, Orange (MOSI) -> pm D11, Purple (MISO) ->pm D12, Brown (CLK) -> pm D13
Add three jumper wires to the red power line from the SD module, one with a male end pin. I often add Dupont ends with a crimping tool, rather than using a pre-made jumper.
Strip & twist the 4 red power wires together & add heat-shrink for strain relief. Bundling wires like this is easier if you make the stripped area a bit longer.
A short red jumper goes to RAW(pm)=VIN(st) to recruit the orphan capacitor.
Long red jumper bridges power to ‘unused’ screws on other side of the Terminal board, and the wire with the Dupont male end will go to the breadboard.
Add 2 extra wires to the black GND wire from the SD module (1 with a male Dupont end ). Jumper one black wire across the Promini to an unused terminal beside the red power wire.
The GND bundle completes Pro Mini / ST board / SD module stack. The ‘pinned’ Red & Black ‘pinned’ jumpers shown here areabout 1 inch too short to reach the breadboard easily… make yours longer.
Note:You could connect the battery holder lead wires directly into the multi-wire Vcc & GND bundles: skipping the 2 jumpers crossing over to the other side of the ST shield. But adding those jumpers provide extra Vcc/GND connection points & the ability to easily replace the battery holder later if you have a battery leak.
I always try to make my Dupont connectors so that the metal & plastic retainer clips are accessible (in this case facing upwards) after the logger is assembled. That way you can diagnose bad wire connection with the tip of a meter probe, and if necessary, pull out & replace a single bad wire in the Dupont connector without taking everything apart.
RTC module:
Remove two SMD resistors from the RTC board with the tip of your soldering iron.
The DS3231 modules often have flux residue – clean this off with 90% isopropyl.
Cable: Blk(gnd), Red(vcc), White(sda), Yellow(scl), Blue(sqw). Shroud retainer clips face up & there is no wire on 32K output.
First tape layer
Next two tape layers
OPTIONAL: adding header pins to the cascade port provides a convenient attachment point for I2C sensors later.
Optional: After removing the two SMD resistors on the module, you can clip the Vcc leg on the RTC chip which forces the clock to run entirely from the backup coin-cell battery. This reduces the loggers overall power use by 0.09mA bringing a “no-reg & noRTCvcc” build below 0.1mA when the logger sleeps between sensor readings (this should run for more than 2 years on fresh lithium AA cells) . But the risk is that if you bump the RTC backup battery loose, that disconnection resets the clock time to Jan 1st, 2000. (note: while the time stamps will be wrong after that kind of reset, the logger will continue running after the next hour/min alignment occurs with the ‘old’ alarm time) A couple of pieces of soft 1.6mm heat shrink tubing under the spring makes the negative coin-cell connection stronger, an a touch of hot melt glue will secure the battery on the top edges. A CR2032 can power the RTC about four years but you have to set bit six of the DS3231_CONTROL_REG to 1 to enable alarms when running from the coin-cell. (our starter code does this by default) This modification also disables the 32.768 kHz output pin on the RTC. Visit our RTC page for more detailed information on this clock module.
Final Assembly: (Note: references here are to pin numbers/labels on the ProMini which do not match ST board labels on the analog side)
Attach the Pro Mini stack & RTC to housing with the double-sided tape.
Trim white & yellow I2C wires from the RTC & add 1 extra wire with dupont ends for each I2C line to bring the bus over to the breadboard
Attach yellow SCL line from the RTC beside the red 3v rail (ie to A3=A5 on the ProMini) then the white SDA line from the RTC to A2=A4.
The four extra jumper wires with male Dupont ends on Vcc, GND, & both I2C lines. These get patched over to the breadboard so you can add I2C sensors.
Each wire must be plugged into its own separate vertical column on the breadboard. Add a 2nd layer of foam tape to the bottom of the bread-board before attaching.
The RTC power line joins that short red jumper on RAW(pm)=VIN(st) at the end of the screw-terminal board.
Some of the box bottoms have slight bowing. If any component doesn’t stick well enough: add another layer of foam tape.
Attach the RTC’s black ground wire to GND & the blue SQW alarm line to ProMini pin D2
Attach 2xAA battery holder with 2 layers of foam tape. Trim wires to length. Use black 30lb Mounting Tape for extra strength.
Battery wires join the black & red jumpers from other side of the terminal board. All six of the ‘unused’ screw terminals we clipped earlier can be used to make secure ‘dry wire’ connections in this way.
Connections complete except the indicator. 5050 LED modules often come with pre-installed limit resistors which make them safer for the classroom. But you can use a raw 5mm LED if you only light the LED with the INPUT_PULLUP command.
A ‘common cathode’ RGB LED module on pins Red=D6, Green=D5, Blue=D4, & GND=D3.
Your Logger is ready for testing!
(Note:Most of the time the tests listed below go well, however if you run into trouble at any point read through the steps suggested for Diagnosing Connection Problems at the end of this page.)
Install the Arduino IDE into whatever default directory it wants to be in – we’ve had several issues where students tried to install the IDE into some other custom sub-directory, and then code will not verify without errors because it doesn’t know where to look for libraries. If you have not already done so, there are three things you need to set under the IDE>TOOLS menu to enable communication with the logger:
Note: that the “COM’ setting will be different for each computer, so you will have to look for the one that appears on your system AFTER you plug in the UART module.
The one that’s easy to forget is choosing the 328P 3.3v 8Mhz clock speed. If you leave the 328p 5v 16mhz (default), the programs will upload OK, but any text displayed on the serial monitor will just be a random bunch of garbled characters because of the clock speed mismatch. Also be sure to disconnect battery power (by removing one of the AA batteries) whenever you connect your logger to a computer.
1. Test the LED – Edit the default blink sketch, adding commands in setup which set the digital pin 3 connected to the ground line of the LED to “OUTPUT” and “LOW” pinMode(3, OUTPUT); digitalWrite(3, LOW);
Since we removed the ‘default’ indicator led on the Pro Mini board, you will also need to change LED_BUILTIN variable in the blink code example to one of the pins connected to one of the color channels on your led module. (in this example change LED_BUILTIN to either 4, 5, or 6)
2. Scan the I2C bus with the scanner from the Arduino playgound. The RTC module has a 4K eeprom at address 0x56 (or 57) and the DS3231 RTC should show up at address 0x68.
The address of the eeprom can be changed via solder pads on the board, so sometimes it moves around. If you don’t see at least these two devices listed in the serial monitor when you run the scan, there is something wrong with your RTC module or the way it’s connected: It’s very common for a beginner to get at least one set of wire connections switched around during the assembly. With the screw terminals this takes only a few moments to fix. Also have a spare RTC module on hand in case you get a defective module…which is fairly common.
NOTE: Some sensors really need the stability provided by the on-board voltage regulator. Here is an alternative arrangement of parts for the classroom logger that leaves the 3.3v regulator in place on the ProMini and powers the logger from 4xAAA alkaline batteries (NOTE:regulated builds also leave out the short red jumper that was used to recruit the orphan capacitor after the reg was clipped. The RTC & added sensors now get connected ONLY to the Pro-Mini’s regulated 3.3v rail ) We’ve designed the Cave Pearl Logger for maximum flexibility, so you can easily change components and positions like this to suit the needs of your design / experiment.
3. Set the RTC time, and check that the time was set – The easiest method would be to use the SetTime / Gettime scripts from our Github repository, but first you need to download & install this RTC control library The SetTime script, automatically updates the RTC to the time the code was compiled (just before uploading) so you only run SetTime once, and then immediately upload the GetTime sketch to get the SetTime code out of memory after it’s done it’s job. Otherwise SetTime will reset the RTC to the ‘code compile time’ every time the Arduino restarts (and the Arduino restarts EVERY TIME you open the serial window…)
There are dozens of other good Arduino libraries you could use to control the DS3231, and there is also a script over at TronixLabs.com that lets you set the clock without installing a library. [ in 24-hour time, & year with two digits eg: setDS3231time(30,42,21,4,26,11,14); ] The trick with Tronix’s “manual” method is to change the parameters in the line: setDS3231time(second, minute, hour, dayOfWeek, dayOfMonth, month, year); to about 2-3 minutes in the future, and thenupload that code until about 20 seconds before your computers clock reaches that time(this compensates for delay caused by the compilers processing & upload time). Open the serial window immediately after the upload finishes, and when you see the time being displayed (and it’s not too far off actual…) upload the examples>blink sketch to remove the clock setting program from memory.
4. Check the SD card is working with Cardinfo – Changing chipSelect = 4; in that code to chipSelect = 10;Note that this logger requires the SD card to be formatted as fat16, so most 4GB or larger High Density cards will not work because they get formatted as fat32. Most loggers only generate 5 Kb of CSV format data per year when they are running, so you don’t need a big SD card. In fact older, smaller SD cards in the 256-512mb range often use less power if it’s a good name brand like Sandisk or Nokia.
Zip-Tie Mounting Bases are an easy way to add attachment points inside your logger to secure sensor cables, or desiccant packs. These adhesive cable tie mounts come in many varieties, and cost about 10¢ each at most hardware stores.
5. Optional: If you are running with no regulator & using analog sensors: Calibrate your internal voltage reference with CalVref from OpenEnergyMonitor.
This logger uses an advanced code trick to read the positive rail voltage to ~11mv resolution by comparing it to an internal 1.1v reference inside the processor. That internal ref. can vary by ±10% from one chip to another, and CalVref gives you a constant which will make the rail=battery voltage reading more accurate. Load CalVrefwhile the logger is running from USB power and then measure the voltage between GND and the positive rail with a good quality voltmeter. (this voltage will vary depending on your computer’s USB output, and the UART adapter you are using) Then type that voltage into the serial monitor window entry line & hit enter. Write down the reference voltage & constant which is output to the serial monitor window. I usually write these ‘chip-specific’ numbers inside the logger with a black sharpie. You will need to add that info to the core data logger code later by changing the line #define InternalReferenceConstant 1126400L to match the long number returned from CalVref. Alternatively you could just tweak the value of the constant ‘by hand’, increasing or decreasing the value till the reported rail readings match what you see with a voltmeter. After you’ve done this once or twice you can usually get within 15mv of actual with about 10 trials.
This calibration brings the starter script’s battery readings within ±15mv of actual but you can skip the CalVref procedure if you are only using digital sensors, as the script will still produce reasonably good battery readings with the default 1126400L value. Increasing the rail reading accuracy is more important when you are using ANALOG sensors which use that rail voltage to drive your sensors – so the +ive raildirectly affects their output if you are not reading a ratiometric circuit.
6. Find a script to run your on logger. For test runs on a USB tether, the simplest bare-bones logger code is probably Tom Igoe’s 1-pager at the Arduino playground. It’s not really deploy-able because it never sleeps the processor, but it is still useful for teaching exercises and testing sensors after you set chipSelect = 10; In 2016 we posted an extended version of Tom’s code for UNO based loggers that included sleeping the logger with RTC wakeup alarms. Our latest logging “Starter Script” has grown in complexity to ~750 lines, but it should still be understandable once you have a few basic Arduino programming concepts under your belt.
After 1-2 minutes of kneading to mix the epoxy you have ~ 60 seconds to work the putty into place. (it will become rock-hard within ~5-10 minutes). Be sure to leave yourself enough extra wire/space inside the housing so that you can open and close the lid easily without disconnecting anything after the putty hardens. This seal is not strong enough for underwater deployments, but it should easily withstand exposure to rain-storm events. This putty is also a quick way to make custom mounting brackets, or even threaded fittings if you wrap it around a bolt (which you carefully remove before the putty hardens completely)
In the previous tutorial we attached external sensors with a cable gland passing through the housing and epoxying them into a pvc cap. Cheap cable glands are notoriously unreliable and I always add a second rubber oring on the inside – and often over-seal the outside with conformal coating. But for a simpler classroom project you could simply drill small a hole through the lid and stick the sensor/module on top of the housing, seal the hole with double-sided tape. Thicker pass-through wires can be sealed reasonably well with plumbers epoxy putty from the hardware store. This putty is non-conductive, and adheres quite well to both metal & plastic surfaces.
Remember that breadboard connections are very easy to bump loose, so once you have your prototype circuits working, its usually best to re-connect the sensors directly to the screw terminals before deploying a logger where it could get knocked about. In a pinch you can secure breadboard pins with a ‘tiny’ drop of hot glue to keep them from wiggling around.
There is no power switch on the loggers, which are turned on or off via the battery insertion. Use a screwdriver, or some other tool, when removing the batteries so that you don’t accidentally cause a series of disconnect-reconnect voltage spikes which might hurt the SD card.
Using the logger for experiments:
Logger mounted on a south-facing window and held in place with double sided tape. Here the top surface of the housing was covered with two layers of white label-maker tape to act as a light diffuser. PTFE is another excellent light diffusing material available in different sheet thicknesses. The ‘divot’ on the lid of the Plano box is just a bit larger than 55mm x 130mm x 3mm (depth). The “teflon” tape that plumbers use to seal threaded joints can also be used in a pinch. PTFE introduces fewer absorbance artifacts than other diy diffusers like ping-pong balls, or hot melt glue. Most light sensors like the TSL2561 need 3-5mm of that PTFE sheeting to prevent the sensors from saturating in full daylight. LED’s don’t saturate that badly, but you still lose all the useful detail in your data at peak brightnesses > 80,000 lux unless you add a diffusion layer to attenuate the light.
Many types of sensors can be added to this loggerand the RTC has a built-in temperature register which automatically gets saved with our starter script. The transparent enclosure makes it easy to do light-based experiments. Diodes, LED’s & solar panels are basically the same device. So grounding the indicator LED through a digital pin allows it to be used as both a status light, and as a light sensor. The human eye is maximally sensitive to green light so readings made with that LED channel approximate a persons impression of overall light levels. Photosynthesisdepends on blue and red light, so measurements using LED’s that detect those two colors can be combined for readings that compare well to the photosynthetically active radiation measurements made with “professional grade” sensors. In fact Forest Mimms(the man who discovered the light sensing capability of LEDs in the first place) has shown the readings from red LED’s can be used as a reasonable proxy for total PAR. Photoperiod measurements have important implications for plant productivity, as do measurements of transmittance through the plant canopy.Chlorophyll fluorescence is another potential application, and the response of plants to UV is fascinating.
The original code for using LEDs as sensors is from the Arduino playground.This polarity reversal technique does not require the op-amps that people typically use to amplify the light sensing response but it does rely on the very tiny parasitic capacitance inside the LED. (~50-300pF) This means that the technique works better when the LED is connected directly to the logger input pins rather that through the protoboard (because breadboards add random capacitance) . Another thing to watch out for is moisture condensation inside your logger housing: this provides an alternate discharge path for the reverse-charge on the LED, which effectively shorts out the light level reading.
We have integrated this into the starter scriptwhich you can download from GITHUB. I’ve tweaked the playground version with port commands so the loop execution takes about 100 clock cycles instead of the default of about 400 clock cycles. The faster version was used to generate this light exposure graph with a generic 5mm RGB LED, with a 4k7Ω limiter on the common ground which was connected to pin D3:
Red, Green & Blue channel readings over the course of one day from the indicator LED in the logger photo above. The yellow line is from an LDR sensor the same unit, that was over-sampled to 16-bit resolution. The LED sensor has a logarithmic response and the left axis on the graph is a time- based measurement where more light hitting the LED sensor results in a lower number. Note how the RED signal changes more quickly than Blue & Green at sunrise & sunset. LED’s work well with natural full-spectrum light, but their limited frequency sensing bands can give you trouble with the spectral distribution of indoor light sources. The peak spectral response of LED’s is usually around 30–50nm lower than their peak emission wavelength So the blue channel is actually recording in the near-UV range, the Green channel is responding at ~ 420nm (blue) and the red channel is actually responding to a wide band of yellow-green light.
Characterizing light absorption and re-emission is also a primary technique in climate science . For example, measuring light intensity just after sunset with LEDs inside a heat-shrink tube pointed straight up can provide a measure of suspended particles in the stratosphere. An “ultra bright” LED has more than enough sensitivity to make these columnated readings, in fact on bright sunny days you usually have to place the LED/sensor beneath a fair thickness of white diffusing material (like PTFE tape) to prevent it from being completely saturated. Older LEDs that emit less light can sometimes be easier to work with because they are less sensitive, so the discharge time does not go to zero in high-light situations. Other sensor experiments are possible with LED’s in the IR spectrum which can be used to detect total atmospheric water vapor. A light transmission-based variant of the NDVI ratio can be used todetermine plant health after adding an IR LED to the logger.
Of course, full sun exposure can also cook your little logger: temps above 80°C may cause batteries to leak or damage the SD card. If you have to leave the logger in full sun, add a bit of reflective film or a layer of aluminum foil around the outside to protect the electronics. Though if use a light sensor you’ll need to leave a little un-covered window for it to take a reading.
You might also find it handy to add a few holes to the Plano box as tie-down points:
and it’s always a good idea to add desiccant packs inside the box to prevent condensation. If you use a desiccant with color indicator beads, you can check whether they are still good simply by looking through the transparent housing.
If you clipped the RTC leg, your logger should pull less than 0.1mA while sleeping. Back of the envelope for Lithium AA’s is about 7 million milliamp-seconds of power, with your logger burning about 8600 mAs/day at 0.1mA. So clipping the voltage regulator and cutting the vcc leg on the RTC should easily get you out to about a year before you fall off the “upper plateau” of the lithium burn-down curve, and more likely to ~two years if your sensors don’t use much power (…and you have a well behaved SD card). With the RTC power leg still attached you’ll see sleep currents in the 0.18-0.22 mA range, so at least 6 months of operation before you come close to triggering the low voltage shutdown (which is embedded in the logger code). I’m being conservative here because it all depends on your sensors and other additions you make to the base configuration. For every type of sensor the market, there will be some brands that draw much less power than others, and it’s always worth a look at the data sheet to make sure you are buying one that will run longer on a given set of batteries. Good sensors automatically go into a low current sleep mode whenever the bus has been inactive for a period of time.
While the LED sensor idea is fun to work with, it’s a very slow method that keeps the logger running for many seconds per reading when light levels are low -> so reading all three color channels will probably cut your operating life in half again. Figuring out how to only take those light readings during the day is a good coding exercise for students that saves quite a bit of power. The RTC’s temperature record is pretty crude, so we’ve also added support for the DS18b20 temperature sensor in the base code. If you have a genuine DS18b20 (yes, crummy fake knock-off sensors are a thing) , these venerable old temperature sensors draw virtually no power between readings.
Pro Mini Style clone 3.3v 8mHz Get the ones with A6 & A7 broken out at the back edge of the board. Just make sure its the 3.3v version because you can’t direct-connect the SD cards to a 5v board.
$2.20
‘Pre-assembled’ Nano V1.O Screw Terminal Expansion Board by Deek Robot, Keyes, & Gravitech (CHECK: some of them have the GND terminals interconnected) You will also need to have a few small flat head screw drivers to tighten those terminals down. Since this shield is was originally designed for an Arduino Nano many of the labels on ST board will not agree with the pins on the ‘analog side’ of the ProMini.
Dupont 2.54mm F2F 40wire ribbon cable Without Housing Cheaper if you get the ones with the black plastic shrouds on the ends. Dupont connector hook-up wires might be expected to add an ohm or two of resistance and carry at most 100mA reliably with their thin 28-30 gauge wires. Poorer quality crimped ends can add significant contact resistance. Each 40-wire cable will let you make at least 2 loggers, and you”l need a couple of 6-pin connector shrouds.
$1.55
2×1.5V AA Battery Batteries Holder w Wire Leads
If you are running an unregulated system on 2 lithium batteries, then you can use a 2x AA battery holder. If you need to keep the regulator in place to stabilize the rail voltage for particularly picky sensors, use alkaline batteries and a 4xAA battery holder.
$0.50
5050 LED module (with built-in limit resistors) (Alternatively, you can also use cheaper 5mm diffused LEDs with a 4K7 limit resistor on the GND line – that you solder into place on the LED yourself)
$0.75
3.3V 5V FT232 Module *set the UART module to 3.3v before using it!* and you will also need a few USB 2.0 A Male to Mini B cables. You may need to install drivers from the FTDI website depending on your OS. Get at least 2-3 of these as they do wear out and you can kill them with a short circuit. These boards can only supply about 50mA which can be tricky with sensors that draw more than that.
You might need some extra tools to get started:(not included in the total above)
2in1 862D+ Soldering Iron & Hot Air station Combination
a combination unit which you can sometimes find as low as $40 on eBay. Oryou can get the Yihua 936soldering iron alone for about $25. While the Yihua is a so-so iron, replacement handles and soldering tips cost very little, and that’s very important in a classroom situation where you can count on replacing at least 1-2 tips per student, per course, because they let them run dry till they oxidize and won’t hold solder any more. Smaller hand-held heat-shrink gunsare also available for ~$15.
$50.00
SYB-46 270 breadboards (used ONLY for soldering header pins ) Soldering the header pins on the pro-mini is MUCH easier if you use a scrapbreadboard to hold everything in place while you work. I use white plastic breadboards that they only have one power rail on the side since they do not look like regular breadboards & I write ‘for soldering only’ on them with a black sharpie to make sure I don’t get the heat-damaged boards mixed up with the good ones.
Donation to Arduino.cc If you don’t use a ‘real’ Pro Mini from Sparkfun to build your logger, you should at least consider sending a buck or two back to the mother-ship to keep the open source hardware movement going…so more cool stuff like this can happen!
$1.00
When a finished module with 20 sub-components arrives at your doorstep for less than you’d pay for any one of those components individually, then the tradeoff is that you are the quality control.
.. and the required lithium AA batteries are also somewhat expensive, so a realistic estimate is about $25 for each logger before you add sensors & tools.
My advice is to order at least 5-6 of each of the core components (Promini, RTC, SD module, screw terminal board, etc) with the expectation that about 10-20% of any cheap eBay modules will be DOA due to some quality control problem. I build loggers in batches of six, and one unit typically ends up with a bad part somewhere. Having replacement parts on hand is your #1 way to diagnose and fix these issues. I’ve noted over the years that bad parts tend to come “in bunches”, so if you scale up to ordering in quantities of 10’s & 20’s then spread those orders to a few different suppliers so you don’t end up with all your parts coming from the same flakey production run.
The other thin I can’t stress enough is CLEAN ALL THE PARTS as soon as they arrive. Leftover flux is very hygroscopic, and pretty much guarantees that solder joints will start to corrode the moment your logger gets exposed to any atmospheric moisture. So I usually give everything about 10 minutes in a cheap sonic bath with 90% isopropyl alcohol, rinse with water, and then dry the parts out in front of a strong fan for an hour. Store parts in a sealed container with a dessicant pack till you need them. I clean parts that can’t take the sonic vibration (like RTC modules, humidity sensors, accelerometers, etc) by hand with cotton swabs. Then I coat everything I can with a layer of MG Chemicals 422B Silicone Conformal Coating and let that dry for a day before assembling the loggers. One hint that you have moisture issues is that the sensors seem to run fine in the house but start to act weird when you deploy the unit outside.
Used nut containers make excellent “dry storage” once the parts have been cleaned – but any air-tight container will do.
Another insight I can offer is that the quality of a sensor component is often related to the current it draws – if your ‘cheap module’ is pulling much more power than the data sheet for that sensor indicates, then it’s probably a junk part. If the sleep current is on spec, then your part is probably going to work OK. It is much easier to check low current devices with at Ucurrent or a Current Ranger. Keep in mind that sensors which automatically go into low current sleep modes usually take some time to do so after the bus has gone quiet – so you might need to watch the sleep current for 30 seconds or more before they fall down into their quiescent state. With SD cards, if the throughput drops below the typical write speed for that model with H2testw, then you should just find another card to use with your logger.
I strongly recommend that you build at least two loggers at a time, because that lets you isolate whether problems you run into during testing are code related (which will always affect both machines the same way) or hardware related. (which will usually only affect one of your two units) At any given time I usually have 2-3 units running overnight tests so that I can compare the effect of two different code/hardware changes the next morning. As a general rule you want to run a new build for at least a week before deploying it to make sure you’ve made it past any ‘infant mortality’, and moved onto the good part of the bathtub curve.
Addendum: Diagnosing Connection Problems
If you loaded the blink sketch to test the Prominin during your initial assembly, you know that part of the logger is working, so issues during the testing stage are due to incomplete connections from the outside to the Promini I/O ports. Before you start your troubleshooting it’s worth remembering that after you’ve built a few of these units you can simply rebuild the whole logger in about a hour. So it really doesn’t make sense to spend much more time than that – especially when a few five-minute part swaps are likely to fix the problem. The components are only worth a couple of bucks each, and your time is more valuable than that!
This is the connection pattern inside the white breadboard. Take care that you don’t connect the breadboard jumper wires together by accidentally plugging them in to the same row or – the I2C bus will stop working. NEVER CONNECT the GND & 3.3v wires or the short circuit will likely kill your logger, and possibly even the computer they are connected to . . .
If you see only “Scanning I2C….. ” but nothing else appears when running the bus scanner, then it means that the ProMini can not establish communication with the RTC module. The most common cause of this problem is that the white & yellow wires were switched around at one end or the other. It’s also easy to not quite remove enough insulation from the wires to provide a good electrical connection under the screw terminals, so undo those connection and check that the wires were stripped, cleaned & wrapped together ‘clean’ before being put under the terminals. Those screws need to be clamped down relatively tight on the thin Dupont wires and if you are not careful, you might have accidentally cut away some of the thin copper wires inside the Dupont cable when you stripped the insulation.
Scanner lockup can also happen if one of the I2C devices on the bus is simply not working: usually about 1 in 6 logger builds ends up with some bad component that you have to identify by process of elimination. (These are 99¢ parts from eBay…right?) It only takes a moment to swap in a new RTC board via the black Dupont connector and re-run the scan. If the replacement RTC also does not show up with the I2C scanner then it’s likely that one of the four bus lines does not have a complete connection between the ProMini & the RTC module.
On this unit I measured 1 ohm of resistance on the I2C clock line between the ProMini A5 pin (on top of the board) and the SCL header pin on the RTC module. So this electrical connection path is good. It’s not unusual for each ‘dry’ connection to add 0.5-1 ohm of resistance to a signal path.
To diagnose this: set a multi-meter to measure resistance and put one probe lead on the topmost point of the promini header pins, and the other probe on the corresponding header pin of the RTC module. If there is a continuous electrical connection between the two points then the meter should read one ohm or less. Higher resistances mean that you don’t have a good electrical path between those points even if they look connected:
1)the ground (black) wire should provide a continuous path from the ground pin on the digital side of the Promini board to the GND pin on the RTC module 2) the positive power (red) wire should provide a continuous path from the Promini positive rail pin (the one with the bundle of 4 red wires) to the VCC pin on the RTC 3)A4 (I2C data) near the 328P chip on the Promini must connect all the way through the screw terminal board and through the white Dupont wires to the SDA post on the RTC 4)A5 (I2C clock) beside A4 on the Promini must connect through through the yellow Dupont wire to the SCL header pin on the RTC .
For ‘single-wire’ connections under the screw terminal with those thin dupont wires, strip enough insulation that you can ‘fold-back’ some extra wire to give the terminal more wire to bite onto.
You occasionally get a bad Dupont wire where the silver metal end is not in contact with the copper wire inside because the crimp ‘wings’ folded over plastic insulation. With a pair of tweezers, you can ‘gently’ lift the little plastic tab on the black shroud holding the female Dupont ends in place, and then replace any single bad wire. Be careful not to break the little black tab or you will have to replace the entire shroud.
Also look at the little jumpers used to bridge the A4>A2 and A5>A3. If you have a ‘cold’ solder join, or an accidental bridge connection to something else, it could stop the bus from working. Remelt each connection point one at a time, holding the iron long enough to make sure the solder melts into a nice ‘liquid flow’ shape for each solder point.
A note about I2C sensors: The I²C bus is slow, so topology (star, daisy-chain, etc.) doesn’t matter much, but capacitance does. Length and number of devices increase capacitance. If you find that the devices work when you switch to a slower speed (e.g. 50 kHz), then this is probably your issue, and you need to minimise bus length and/or maybe decrease the combined resistance of the pull-ups to 2 kΩ or less. The DS3231 RTC module has 4k7 ohm pull-up resistors on the SDA & SCL lines & the Pro Mini adds internal 50k pull ups when the wire library is enabled. Typical I2C sensor modules usually add another 10k so your ‘net pullup resistance’ on the I2C bus wires is usually: 50k // 4k7 // 10k = ~3k. With a 3.3v rail that means the devices draw 3.3v / 3k = 1 mA during communication which is fairly normal( 3mA is max) for total wire lengths below 1m. It’s common for pre-packaged sensors to arrive with housings at the end of about 1m of wire. If each sensor also adds another set of 10k pullups, the resistance generally compensates for the extra wire length, so the combination still works OK. But that depends on the cable too. A very bad cable might not even get to 0.5 meters and a very good cable (little capacitance to ground, no crosstalk between the wires) can go up to 6 meters.
The connection diagnosis procedures described above also apply to the connections for the SD adapter board. Sometimes you end up with an adapter that has a defective spring contact inside the SD module, but the only way to figure that out is to swap it with another one.
Here a jumper wire from the Promini pins is by-passing a bad connection. This is also how you would break out A6 & A7 connections if you need them.
Sometimes those screw terminal boards have a a poor connection inside the black female headers below the Promini. It’s also possible to accidentally over-tighten a terminal and ‘crack’ the solder connection below the board – or there may simply be a cold solder joint on one of the terminal posts. If you have only one bad connection, you can jumper from the Promini header pins on top, down to the other wires under the corresponding screw terminal. If you accidentally strip the threads on a screw terminal, you can use this same approach but move that set of wires over to one of the three ‘unused’ screw terminals at the far end of the board. (beside the SD card adapter) If you’ve gotten through all of the above steps and still have not fixed the problem, then it might be time to simply rebuild the logger with a different screw terminal adapter board.
Also note: The environment is written in Java, and the IDE installer comes with its own bundled Java runtime, so there should be no need for an extra Java installation. However we have seen machines in the past which would not compile working code due until Java was updated on those machines; but this problem is rare. More likely some permission limitation meant that you’ve ended up installing the IDE in a directory other than the default, and now it can’t find its libraries.
Addendum 2020-02-02: Refining the build
The ‘solderless’ classroom build includes a breadboard for flexibility, but if you are building a logger for a single sensor application you can leave out the breadboard, and add some under board jumpers to avoid those finicky multi-wire bundles:
Using the tined leg of a resistor to add some under-board connections to give you more 3.3v & GND attachment points. Clean any residual solder flux thoroughly!
To repurpose those screw terminals, leave the corresponding pins un-soldered so they can be removed.
Here, we are repurposing the TX, RX, & RST terminals as GND connections. DO not repurpose TX & RX if you need serial comms with NEMA sensors like GPS.
Use a Black marker to label the repurposed GND points. NOTE: The GND-labeled terminals can not be repurposedbecause they are bridged via the back plane of the screw terminal board itself.
Label the connections with red. NOTE: ONLY join the Vraw & Vcc like this if you have removed the regulator. If you are keeping the reg. in your build then Vraw must remain separate, & you can only repurpose the reset pin for one extra 3.3v rail.
In a no-regulator build, you now have three (+)ive rail connections. This is handy if you accidentally strip one of the terminals by over-tightening.
Adding header pins to the RTC module’s cascade port provides a more convenient attachment point for I2C sensors, and removing the breadboard leaves room for a desiccant pack:
Example of a typical “dedicated sensor” build without the breadboard, and with the under-board jumpers to provide extra (+)ive & GND terminals. This photo also shows a mosfet controlling the SD ground line, and a thermistor is tucked under the Promini board – but it’s probably better to tackle those additions when you have a few successful logger builds under your belt.
‘Single shroud’ 40-wire 20cm FF Dupont cables are ~$1 each – 1/3 the cost of the ones without housings.
We deliberately put students through the creation of custom sized Dupont connectors because it’s a useful thing to know about, however single shroud wires also work on the RTC and SD module because you are using the double sided tape to hold everything in place. I also tend to use RAW diffused LEDs as indicators, and simply light the individual channels with INPUT_PULLUP mode which uses the internal pullup resistor to limit current. However a single mistaken OUTPUT-HIGH can kill your Arduino if your not careful with that approach. I also leave one of my battery leads long enough that I can tap in to that line & measure sleep currents later on with a Current Ranger.
MCP1702-3302E/TO dead-bug hanging off a Deans style T-plug connector.
The unit pictured above has had its regulator removed so it can run more efficiently on 2x AA lithium batteries. But if you later find out that your sensors need strict voltage regulation, you can always replace the battery holder with a 4xAA, and add a regulator in-line on a battery connector. In the photo (right) two ceramic 105’s stabilize the MCP1700, while a 10/3.3M ohm divider provides a third output line so the ADC can monitor the raw battery status. This is the simplest way to retro-fit a regulator onto a unit that was built without one.
You occasionally get a plano box where the plastic ridges of the housing don’t mate well with the red rubber o-ring. In those cases it’s relatively easy to pull the o-ring out of its groove in the lid, and add some gasket compound to the recessed trough before carefully putting the o-ring back in. The additional height then compensates for casting error in the housings, though don’t add to much or you won’t be able to close the lid. One positive aspect of the relatively loose fit on that lid is that it lets you run prototype tests quickly if you jumper to your sensor module with thin 28-30 gauge wires:
This is a BMP280 pressure sensing module. Wires must extend beyond the edge
~1″ square of foam mounting tape with wires spaced evenly
Leave the red backing facing up as you fold the tape & wires over the corner edge.
The front corners of the box exert less pressure than the back
The sharp inner edge of the lid would cut the wires insulation if the tape was not there to protect
The tape has to be replaced every time.
This gives you a chance to do overnight run tests before you commit to modifying the housing with holes, putty, or pass-throughs with cable glands. And for indoor sensor deployments this might be all you actually need to do, though I would still coat the exposed solder joints on that dangling breakout module with either conformal coating or clear nail polish to prevent oxidation.
Addendum 2020-07-29: Better moisture protection
I was noodling around in the garden recently and installed a few loggers without desiccants because it was only a short experiment. It rained immediately afterward and I noticed a small amount of moisture condensed inside the plano-box housing. While this didn’t prevent the logger from functioning, it completely disrupted the LED light sensors because it provided an alternate discharge path for the reverse bias charge:
Green channel data from a 5mm diffused RGB LED used as light level sensor. This logger was under some leaf cover, so there was considerable variability from the dappled light crossing over the sensor. An arbitrary cutoff of 200,000 was set in the code at low light levels.
After examining the O-ring I decided to add a little silicone to the channel holding the red o-ring to improve the seal:
Gently pry the O-ring loose and apply sealant in the groove before replacing.
Bead only needs to be 3-4mm in diameter.
Close the housing & let the sealant set for a few days. The improved seal is especially visible at the corners
If you already have your logger assembled, try to find a silicone sealant that does not off-gas acetic acid (smells like vinegar) which could harm your circuits. If you are simply preparing empty boxes before assembly, then any regular bathroom sealant will do provided you give it about a week to finish curing.
Addendum 2020-05-11: Using a more advanced processor
After you’ve built a few ProMini based loggers, you might want to try a processor upgrade. The 1284p CPU has twice the speed & 4x the memory, but still delivers comparable sleep current / longevity.
Underwater Arduino Data Loggers 