Easy 1-hour Pro Mini Classroom Datalogger [Feb 2019]

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

(Last Updated: Oct 2019)

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 highly useful when doing field research. 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 easier than 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.

This variation of the basic 3-component logger is optimized for quick assembly so the soldering has been reduced to adding Pro Mini pin headers and bridging the A4/A5 I2C bus.  An instructor could easily do that ahead of time with about 15 minutes of prep per unit, leaving the solder-less steps for their students. After the header pins are in place, the multi-wire connections to the ProMini are made by simply twisting stripped wire ends together and clamping them under screw terminals.

You must use Lithium AA batteries with a 2-cell unregulated supply because the steep slope of an ‘alkaline’ battery discharge would bring the SD cards below their rated voltage range quickly. While the voltage of a ‘brand new’ Lithium AA is usually 1.8v/cell, that upper plateau provides a sleeping logger voltage of ~1.715 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.

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.

 (NOTE: complete parts list with supplier links are located at the end of this post)

Pro Mini Prep:

Solder the 6 UART pins & test board with the blink sketch:  To upload sketches select (1) TOOLS> Board: Arduino Pro or Pro Mini  (2)TOOLS>ATmega328(3.3v, 8mhz) in addition to the (3)TOOLS> COM port

Remove the power LED (in red). Removing the pin13 LED (yellow square) is optional, and leaving it in place shows you when SD data is being saved via the SPI bus.

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 pins on 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 speed.

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 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 match the D10-13 Pro Mini pins on this side of the board)

Grey (CS) to ProMini D10Orange (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 are about 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: the pass-through port on the RTC is another access point for the I2C bus (note: add male headers on the ‘battery holder side’ for this build)

Optional added step: 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 while the logger sleeps between sensor readings. 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 reset, the logger will keep running)   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.  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 row 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.)

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:

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 output to the serial monitor will be garbled by 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 a color channel 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 batteries (NOTE: regulated builds also leave out the short red jumper that was used to recruit the orphan capacitor on the raw vin line.  The RTC & added sensors now get connected ONLY to the Pro-Mini’s regulated 3.3v rail instead of the battery input line)   We’ve designed the Cave Pearl Logger for maximum flexibility, so you can change components and positions like this to suit the needs of your 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. Otherwise it will reset the RTC to to that first ‘compiler time’ point every time the Arduino starts (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 then upload 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 when you’ve checked that it’s not too far off, upload the examples>blink sketch to remove the clock setting program from memory.

4. Check the SD card is working with CardinfoChanging 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. Most loggers only generate 5 Kb of CSV format data per year when they are running, so you don’t need a really big SD card. In fact older smaller SD cards actually use less power.

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 AND 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 the 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 calculation more accurate. Load CalVref while the logger is running from USB power and then measure the voltage between GND & the positive rail with a good quality voltmeter. Then type that voltage into the serial monitor window entry line & hit enter. Write down the voltage & reference constant which is then 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 output from CalVref.

This calibration brings the starter script’s battery readings within ±15mv of actual but you can skip this procedure if you are only using digital sensors as that the code will still produce reasonably good battery readings with the default 1126400L value. Increasing the rail reading accuracy is only important when you are using ANALOG sensors (which use that rail voltage as the ADC reference voltage) but it has little effect on the logger operation.

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 your 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 60 seconds of kneading you have ~ 60 seconds to work the putty into place. (it will be 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. 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.

 

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.

Many types of sensors can be added to this logger and 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. 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.  Photosynthesis depends 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 alone 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) . We have integrated this into the starter script which 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 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 collumnated 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 to determine plant health after adding an IR LED to the classroom logger.

One thing to watch out for is that full sun exposure can cook your entire logger: reaching temps above 80°C can cause batteries to leak or fry the SD card. If you have to leave the logger in full sun, consider adding 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 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 adding the RTC mod should get you out to about a year before you fall off the “upper plateau” of the lithium cell’s burn-down curve, and out to two years if your sensors don’t use much juice (…and you have a well behaved SD card).  With the RTC power leg still attached you’ll see sleep currents in the 0.16mA range, so at least 6 months of operation before you come close to triggering the low voltage shutdown embedded in the code. I’m being conservative here because it all depends on your sensors and other additions you make to the base configuration.

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 DS18b , these venerable old sensors draw virtually no power between readings.


TransparentSinglePixl
Bill of Materials: $20.50
Plano 3440-10 Waterproof Stowaway Box
Sometimes cheaper at Amazon as an “add-on” item.  $4.96 at Walmart and there are a selection of larger size boxes in the series. 6″ Husky storage bins are an alternate option.
$5.00
Pro Mini Style clone 3.3v 8mHz
Get the ones with A6 & A7 broken out at the back edge of the 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.
$1.85
DS3231 IIC RTC with 4K AT24C32 EEprom (zs-042)
Some ship with CR2032 batteries already installed.  These will pop if you don’t disable the charging circuit!  
$1.25
CR2032 lithium battery  $0.40
SPI Mini SD card Module for Arduino AVR
Buy the ones with four ‘separate’ pull-up resistors so that you can remove three of them.
$0.50
Sandisk or Nokia Micro SD card 256mb-512mb 
 Test used cards from 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.
$2.00
Small White 170 Tie-Points Prototype Breadboard
These mini breadboards for inside the logger are also available in other colors.
$0.60
Dupont 2.54mm F2F 40wire ribbon cable Without Housing
Cheaper if you get the ones with the black plastic shrouds on the ends, but removing those shrouds by hand is slow  work. Each $2.70 cable will let you make 2 loggers, and you”l need a couple of  6-pin connector shrouds.
$1.55
3×1.5V AA Battery Batteries Holder w Wire Leads
If you are using alkaline  batteries, changing to a 4xAA battery holder doubles the run time.
If you are running an unregulated system on 2 lithium batteries, then you can use a 2x AA 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 add yourself)  
$0.75
3.3V 5V FT232 Module
  *Be sure to set the UART module to 3.3v before using it!* and you will also need a USB 2.0 A Male to Mini B cable.
$2.75
3M Dside Mounting Tape, 22awg silicone wireheader pins, etc… $1.00
Comment:   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.
Or you can get the Yihua 936 soldering 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 1-2 tips per student, per course, because they let them run hot & dry till they oxidize and won’t hold solder any more.  Smaller hand-held heat guns are also available for ~$15.
$50.00
SYB-46 270 breadboards (used ONLY for soldering pins on Pro-Mini )
Soldering the header pins on the pro-mini is MUCH easier if you use a scrap breadboard 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 my regular breadboards &  I write ‘for soldering only’ on them with a black sharpie.  
$1.30
SN-01BM Crimp Plier Tool 2.0mm 2.54mm 28-20 AWG Crimper Dupont JST
I use my crimping pliers almost as often as my soldering iron –  usually to add male pins to components with lead wires too thin for safe connection on a breadboard. But making good crimp ends takes some practice.   Jumper wires that you make yourself are always better quality than the premade ones.
$16.00
Micro SD TF Flash Memory Card Reader
Get several, as these things are lost easily. My preferred model at the moment is the SanDisk MobileMate SD+ SDDR-103 or 104 which can usually be found on the ‘bay for ~$6.
$1.00
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…so more cool stuff like this can happen!
$1.00

.. and the required lithium AA batteries are also somewhat expensive, so a realistic estimate is about $25 for each logger before you add sensors.


Addendum: Diagnosing Connection Problems

Once you’ve run the blink sketch, you know the Promini part of the logger is working, so other issues during the testing stage are due to incomplete connections from the outside to the Promini I/O ports.

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.

For example, 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. 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 ‘cleanly’ 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 to many of the thin copper wires inside the Dupont cable when you did the insulation stripping.

Scanner lockup can also happen if one of the I2C devices on the bus is simply not working: usually about 1 in 10 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 and SCL header pin on the RTC module. So this electrical connection path is good.

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 go to ‘0’ ohms. If you do not see the meter go to a very low number, (ie: nearly zero) then you don’t have a complete electrical path between those points even if it looks 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 .

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 lift the little plastic tab on the black shroud holding the female Dupont ends in place, and replace any single bad wire.

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.

The ‘process of elimination’ 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.

Sometimes those ST adapters have a a poor connection inside the black female headers which receive the pins on the under-side of the Promini board. Or there may be a bad solder connection to one of the screw terminal posts on the bottom of the shield. If you have only one bad connection, you can jumper externally from a Promini header pin on top, down to the other wires under the corresponding screw terminal. If you strip the threads on a single screw terminal, you can use this same approach to move that set of wires over to one of the three ‘unused’ screw terminals beside the SD card adapter at the end of the board.

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.

4 thoughts on “Easy 1-hour Pro Mini Classroom Datalogger [Feb 2019]

  1. Brian Davis

    I’m consistently impressed that you keep coming up with new and even easier ways of doing this while I struggle at times to keep my own more modest efforts moving along. Thank You for doing a lot of “heavy lifting” for the rest of us!

    I wondered if there might be times in a classroom build you could leave the on-boasrd LED on. Since it is under SW control, couldn’t you just turn it off while not in use? Could you also use it as a light detector? Would there be any particular problems with this approach? I admit I’ve not thought about it much before as I’ve just been disabling it on all my builds, but now I’m curious, especially if it could function as a poor man’s integrated light sensor. Putting a cover over it and removing it, exposing it to light, could serve as a ‘switch’ of sorts in SW for example. For high-altitude balloon projects, you often power them up but then have a “remove before flight” piece of tape that either connects the batteries or in some other way triggers the payload that “the flight is now beginning”… and something like this would work great for that.

    For power-saving, I still haven’t gone unregulated… but I am thinking about the RTC power modification (now that it’s easier… even I can probably *cut* a SMD connection 🙂 ). The more I look at power budgets (I know I should replace the cheap regulator with a better one, that too), the more I’m thinking of (for MY deployments, not for teachers) an EEPROM replacement for the SD card. As an I2C device I could swap out EEPROM chips in the field about as easily as I swap SD cards, the only real issue being having a dedicated set-up back at the computer to interface with the EEPROM via an Arduino and dump the data through the serial terminal to the laptop.

    One caution I found with these “flat” AA battery holders – you sort of want beefy ones. My first deployment used two 2XAA battery holders like these, and after a 5 month deployment in-cave the plastic at the two ends had been stressed enough that it had deformed out; a mild jolt was all it took to knock the batteries right out of the holder (the unit that had been in the surface subjected to thermal swings was in even worse shape). In-class this isn’t likely to be a big problem, but in the field or over time it is.

    The thing I really love about this build? If your battery case wears out, just pull it out and replace it – with this build, that’s a quick and easy job.

    Reply
    1. edmallon Post author

      I love the idea of using the D13 LED as another sensor! And these are usually yellow, so it could add another detection color to the RGB set. My only reservation would be that I don’t know how it might affect the SD controller, since pin 13 is also one of the SPI bus lines. That’s why I removed it in the first place – in case it was causing some kind of odd voltage mis-match on that one line compared to the others. If you go with an all eeprom build then there would be no reason not to use that LED as either a sensor, or as a light activated switch. This could be especially useful if you had sensors that only deliver useful data during the day: First check the LED to measure the overall light level and then only take the other sensor reading if it’s day-time.

      I also been wondering about a build using 256-Kbyte AT24CM02 EEproms. Although they have been the source of at least half of my problems on the project, the convenience of simply replacing SD cards, desiccants & batteries for on-site turn around has kept me plugging away with them. They also let me use simple code for concatenation/saving and I’m worried that if we went with structs, compression, etc to squeeze the data into the limited space of an eeprom, we’d make the code base too complicated for students/beginners. Using the serial terminal would be the easiest way to do a data dump, but that also assumes you have a laptop on site… which is not always practical in a cave, or a tropical rain storm, etc. I have been thinking about cobbling together an optical modem “downloader” like Onset uses, as this could also save us from removing the underwater units. But then again in the real world, we have to take them out to clean the biogrowth off the units anyway.

      You are quite right about the cheap battery holders, and when I can get an old lot of them on eBay, I much prefer the beefy plastic, or even metal holders from Keystone. There’s no way you’d shake a battery loose from them, but they also cost $3-5 each, and are probably overkill for classroom level loggers. And of course the number one problem with alkalines is that they leak all over the battery holder – so the easy replacement is probably better than spending more money. You can wedge a square of sponge/foam rubber between the batteries and the housing to hold the batteries in better when the lid is closed.

      Reply
      1. Brian Davis

        “…a light activated switch…”

        I was also thinking of this in terms of cave loggers. If there’s light, there’s somebody around, and the logger could go in to a “awake and listening” mode. For running the display only when people are there? Or for turning on a LoRa or low power BT system to look for a host to communicate with? The nice thing is in a completely dark environment, the only way this should trigger is if people are there.

        “…the convenience of simply replacing cards, desiccants & batteries for on-site turn around has kept me plugging away with [SD cards]”

        Agreed, they are extremely easy to collect swap and use, with the downside of being a little touchy and a bit of a power draw. What I’m thinking is a four contact female header that a EEPROM I2C breakout can simply plug in to – instead of swapping cards in the field, you swap entire EEPROM breakouts. Since they’re I2C they’re pretty easy to address and add (your code already tries to autodetect they size that is available).

        I agree this is not something for the casual teaching environment – using pure EEPROM is either going to be less efficient (because of page breaks) or significantly more complex (encoding and page breaks), although data compression and encoding is something that might be handy to teach in more advanced classes (or for advanced students) anyway. One of the things is the availability of authentic small SD cards (between you and I we may have driven up the market price of the Muve music cards, as they seem fairly reliable), while EEPROM breakouts are common… and fairly cheap (per unit, not per Mb… but when was the last time you filled a 2 Gb SD card with a datalogger?).

        An optical window downloader seems an obvious step (I used an Onset logger with this ability for my thesis work back in the 1990’s, and loved it). I’m actually looking at LoRa or low-power BT (especially the second, as it’s so common with the IOT that there are commonly available apps that will let me use my smartphone to get the data). Obviously underwater is not the place for BT or LoRa… but I wonder if you held the device right up next to the datalogger antenna, and slowed the rate down, if you could use that approach.

        Reply
        1. edmallon Post author

          I like the idea of a “removable” eeprom module…that gets me thinking…

          Now that we have power shutdown on the SD cards working reliably, the EEproms probably would use more power – mostly due to the fact that they are pretty slow and this sandbags the system. Back when I started doing eeprom buffering before data saves, I was surprised by how little power benefit I got from their use. Because of this I’ll probably go with Fram if I switch over to an SDcard-less build, on the fastest spi connection I can get.

          Also I tend to use Nokia 256 & 512mb cards now (when I can find them). They don’t sleep much better than the MUVE cards, but they consistently have peak currents in the 50mA range, while the MUVES usually go up to 100mA ish, and no-name cards often hit 130-150mA during write cycles. And especially with the 2xAA lithium units, I’d like to keep currents down during those random housekeeping load events…

          I think the real motivation for the SD cards was the number of times we were burned by commercial equipment suppliers for expensive download cables/adapters/software. Every time a laptop was lost in the field the most expensive part of the replacement was for proprietary software licenses. Just don’t want to setup another situation like that for others with our project.

          Reply

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