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 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. 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.

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.

Pro Mini Prep:

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 port to match the # 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.

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 match the D10-13 Pro Mini pins on this side of the board)	Grey (CS) to ProMini D10,  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 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: 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:

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.

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 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 it’s not too far off actual…) upload the examples>blink sketch to remove the clock setting program from memory.

Another option would be to try setting the clock’s time using one of the serial window input utilities from Github.

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.

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 CalVref while 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 rail directly 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.

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:

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. 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.  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 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 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:

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 to determine 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.

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Bill of Materials:					$20.50
	Plano 3440-10 Waterproof Stowaway Box Sometimes cheaper at Amazon. $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. 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.				$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. 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.				$2.75
	3M Dside Mounting Tape, 22awg silicone wire, header pins, etc…				$1.00
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 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 guns are 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 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 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.				$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 component lead wires for connection on a breadboard. But making good crimp ends takes some practice.  Once you get the hang of it,  Jumper wires that you make yourself are always better quality than the cheap 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
	Side Shear Flush Wire Cutters & Precision Wire Stripper AWG 30-20 HAKKO is the brand name I use most often for these, but there are much cheaper versions.				$5-10
	Syba SY-ACC65018 Precision Screwdriver Set A good precision screwdriver set makes it so much easier to work with the screw terminal boards.				$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…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 & 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.

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.

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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!

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.

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 .

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.

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.

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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 repurposed because 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:

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.

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.

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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:

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.

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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.