If you need a logger with a rugged waterproof housing, it’s still hard to beat the crimped-jumpers build released in 2016. However sometimes I just want a quick bare-bones unit for bookshelf test runs while I shake down a new sensor. I can whip up a breadboard combo in about twenty minutes, but they stop working if I bump one of the wires loose. I’ve lost SD cards from this half way through a long term test, and I’ve also run into issues with noise & resistance from those tiny breadboard contacts.

To address this I’ve come up with a new configuration that uses a screw-terminal expansion shield originally intended for the Nano.  This requires a reasonable amount of soldering, and after some practice, between 2-3 hours to finish depending on how many “extras” you embed into the basic three component core. In return for that time you get all the pins broken out, making this approach almost as flexible as a breadboard, and much more physically robust. Pop them into some pre-made boxes and these little guys qualify as deploy-able for relatively stable environments (though I wouldn’t use those pre-made boxes outside unless I could pot everything inside that housing with paraffin wax)

Connection Diagram:

PARTS & MATERIALS

 

Bill of Materials:					$8.55
	Pro Mini Style clone 3.3v 8mHz I always get the ones with A6 & A7 broken out at the back edge of the board.				$2.00
	Nano V1.O Screw Terminal Expansion Board Note: To save time, you can spend an extra $1 for pre-assembled boards by Deek Robot, Keyes, & Gravitech.  Note that bad vendors show photos of the pre-assembled boards in their listing, but then ship you the no-name assemble-it-yourself part kit. That kind of bait-n-switch tactic is very common with dodgy eBay suppliers. Or if you have a board with unusual dimensions, you could fabricate a custom screw terminal shield from scratch.				$1.05
	DS3231 IIC RTC with 4K AT24C32 EEprom (zs-042) Some ship with CR2032 batteries which will pop if you don’t disable the charging circuit!  Also the DS3231N or DS3231SN chips are much better with ± 2-3ppm error, The DS3231M is a very different chip with ± 5ppm, so buy a small number from each vendor until you find out which chips they put on the module.				$1.25
	SPI Mini SD card Module for Arduino AVR Be sure to buy the ones with four ‘separate’ pull-up resistors! The second resistor from the bottom of these boards needs to be removed because it is pulling up the SCK pin, which with our SD driver needs to rest low because the SD card libs uses mode 0.  I found the best results by removing the bottom 3 resistors, and then using the Arduino’s internal pull-ups on mosi & miso.				$0.50
	4xAA 6V Switched Battery Holder The logger works with battery packs holding 3 to 8 AA batteries (with the default MIC5205 regulator). When using non-switched battery holders, I add Amass XT30U Bullet Connectors between the holder and the logger.				$0.75
	CR2032 lithium battery				$0.40
	Sandisk or Nokia Micro SD card 256mb-512mb  Older Sandisk & Nokia cards have lower sleep & write currents. They also are better suited to access via the SPI interface, while larger newer cards are not. Test used cards well  before putting them in service.				$2.00
	Common Cathode Bright RGB LED 5mm  ( & 30kΩ limit resistor)  A brighter bulb lets you use a larger limit resistor for the same light output.				$0.05
	Double Sided Tape,  2x 10MΩ resistors, 28awg silicone wire, header pins, etc…				$0.50
	Donation to Arduino.cc If you don’t use a ‘real’ Promini from Sparkfun to build your logger, you should at least consider sending a buck or two back to the mothership to keep the open source hardware movement going…so more cool stuff like this can happen!				$1.00
Comment:   You might need one of these to get started:                            (not included in the total above)
	3.3V 5V FT232 Module   ***Be sure to set the UART module jumpers to 3.3v before using it!*** and you will need a USB 2.0 A Male to Mini B cable.				$2.75
	Micro SD TF Flash Memory Card Reader Get several, as these things get lost easily. My preferred at the moment is the SanDisk MobileMate SD+ SDDR-103 which can usually be found on the ‘bay for ~$5.				$1.00

VIDEO GUIDE

In 2018, a visiting colleague inquired about the steps involved in building a Cave Pearl logger, and I figured the best way to convey that was to simply built one while they recorded the process. This took about three hours, and those videos are now available on YouTube. The order of operations in the videos is slightly different from the written instructions below, but the changes are relatively minor.  

COMPONENT PREPARATION

Clean absolutely everything. I go over every surface of every module with 90% isopropyl alcohol and cotton swabs until those boards are squeaky clean. Then all the surfaces that I’m not soldering get a layer of conformal coating. (except the SD card spring contacts…)  The pads I do solder get cleaned afterward to make sure there’s not one speck of circuit wrecking flux left, and then I coat those joins too.  I usually get several years of continuous operation of these loggers, and I’m convinced it’s because I clean the parts thoroughly during the build process.  Just as a reference, I recently took a look at the the ADXL345 module (pictured at right) inside my first drip sensor prototype which I built in 2014.  Since it was just a bench prototype, the module did not go through my usual cleaning procedure, and it was never deployed.  So this is what happens when uncleaned boards are simply left sitting on a bookshelf inside the house – just imagine what could happen to the parts you actually deploy in the field…

Bread-board your logger before soldering!

With the quality variation you typically see in these cheap parts, I make a pre-soldering breadboard version of each unique combination to confirm that the components in my build aren’t drawing excessive current – especially the SD cards!  I bin between 10-20% of the low-end sensor modules I get from eBay, simply because they draw excessive power for no obvious reason, as compared to the others from the same batch…
(note: in the photo to the right D2 is hard wired via a jumper to the rtc SQW line – leaving an unused breadboard row which I’ve re-purposed to bridge vcc to the power rail on the opposite side of the board. And in this example, D4 provides GND for the indicator LED)

The Main Board:

In this build the six serial UART I/O pins must have 90 degree angled headers to make more room for the RTC board which will sit on top of the main board later.  Solder those header pins onto your Arduino board, and test it with your UART adapter. Generally speaking, about 10% of the cheap modules I buy from eBay are flakey in some way, and it’s quite annoying to discover that after you’ve assembled a logger. Once you know the board is working, remove the power and pin13 LED resistors.  These limit resistors tend to move around from one manufacturer to the next, so you might have to hunting for them on your particular board.  You also need to remove the RESET switch from the board, or that button will be compressed when you put the SD card adapter into place:

{Click any images to see larger versions.}

Solder the side rows of straight header pins so that they project from the bottom of the board.  I usually skip the two reset pins, so that I can re-purpose those screw terminals later as GND and Vcc (photo 3 below) but if your application needs reset functionality then  solder those headers as normal.  Add wires to the top of the board for the A4 (SDA white) & A5 (SCL yellow) lines of the I2C interface.  Add wires to the A6 & A7 vias so that they project from the bottom of the board.

Once all the pins are in place clean any flux residue from the board with 90% isopropyl alcohol and a cotton swab. The final step for the main board preparation is to trim the pin header solder points on the TOP of the board flush with the surface: D4-D9, D10-D13, and A0-A1.  Then affix some double sided tape  in place over those trimmed pins, which will mate with the bottom surface of the SD adapter.

The RTC Module:

The simplest modification to these DS3231 RTC boards is to remove the charging circuit resistor and power LED limit resistor from the circuit board (indicated with the red squares in the first picture).  LIR2032 rechargeable batteries are nominally 3.6v, and will not charge with this module connected to a 3.3v Arduino. Replacing that with a CR2032 will backup the RTC for many years of operation 

Add two layers of double sided foam tape, so that the thickness matches the top surface of the DS3231, and the inside edge aligns with that side of the chip. These two surfaces will mate with tape on the SD adapter board.

Since the RTC board already has 4.7k pullups on the SDA (data) and SCL (clock) lines, you will not need to add them to your I2C sensors.  This board also has a 4.7k pullup on the SQW alarm line.  We will be connecting SDA, SCL, GND and VCC wires to the small cascade port on the module.

The SD Card Adapter:

This SD card adapter comes with small surface mount pullup resistors on the MOSI, MISO & SCK (clock) lines (removed from the dashed red line area photo 2 below).  The Arduino SDfat library uses SPI mode 0 communication, which sets the SCK line low when the logger is sleeping. This would cause a constant drain (~0.33mA) through the 10K SCK pullup on the module if we did not remove it.  I prefer to pull MOSI & MISO high using the internal pullups on the 328P processor, so those physical resistors on the breakout board can also be removed. But be careful to leave the top-most resistor of the four in place to pull up the DAT1 & DAT2 lines.  This keeps those unused pins on the μSD card from floating when the cards are accessed in SPI mode.

Add jumper wires to each of the headers pins on the bottom of the SD adapter and trim those solder joints till they have a relatively low profile . Then cut away the vertical header pins from the top of the board. Place a strip of double sided tape on the bottom of the SD card module opposite the soldered wires. This strip acts as a spacer to level the SD board when it is placed in contact with on the double sided tape on the mini style Arduino board.

The SPI connections: RED:           3.3v regulated Grey:          Cable select (to D10) Orange:     MOSI   (to D11) Brown:      SClocK (to D13) Purple:      MISO   (to D12) BLACK:     Ground

The Screw Terminal board:

These screw terminal boards are designed for use with Arduino Nano boards, but if you orient the two correctly when you connect them, labels on one side of the shield will be in alignment with promini pins:

 

Drilling a pass-through hole lets you bring the jumpers down to those unused pins, and to make other connections to solder points on the underside of the shield without blocking the M3 mounting holes. It is also possible to fabricate your own terminal board.

ASSEMBLING THE LOGGER PLATFORM

Attach SD adapter to the Pro mini:

The first step is to attach the SD adapter to the board, but this must be done with a slight overhang, so that at least the red Vcc wire on the SD adapter extends beyond the top surface of the pro-mini board.  It’s OK to leave more overhang than I’ve shown here, but if you leave less, the wires on the RTC cascade port might interfere with access to the serial I/O pins.

Place a strip of double sided tape across the SD adapter board as shown, taking care not to cover the hole showing the card lock spring.  When that tape is in place, bring the ground and Vcc lines from the SD board forward and make a gap in wire insulation so you can splice-solder them to the GND & Vcc pins on top of the pro mini board. This procedure simultaneously connects the rails to the SD adapter, and brings power to the I2C cascade port on the RTC module.

Connect the RTC board:

I recommend that you take a bit of time holding the RTC board in place over the SD & mini combination while the protective covering is still on the tape, so that you get a feel for the alignment before you actually try to stick these parts together. With the cascade port oriented towards the Arduino’s serial I/O pins, the topography of the SD adapter fits snugly into place against the DS3231 chip on the RTC module.

After you stick the pieces together, trim and solder the I2C bus wires to the RTC’s cascade port. Note that it is possible to unstick the parts afterwards by gently levering them apart with a screw driver, but be careful you don’t rip the metal shield off of the SD card adapter in the process.

Attach everything to the Screw Terminal Shield:

If you’ve gotten this far, then you can now relax, because all the tricky stuff is done.  Trim and tin the four SD lines and bring the down to the D10-13 SPI screw terminals just below. Note that D12(MISO)  & D13(SCLK) lines must crossover.  Bend the pins on the RTC board downward and solder jumpers onto all but the 32K output line.

Pass the SQW alarm line (in blue) through the hole and solder it to the D2 pin projecting from the underside of the terminal adapter board.  If you left out the reset pins when initially soldering the headers, bridge those unconnected terminal screws to the adjacent Vcc & GND lines.  Then patch A6/A7, and the four I2C lines from the RTC board to the unused pins at the end of the screw-terminal shield.  I generally run these loggers on 3xAA battery packs with a 2x10M ohm voltage divider providing 1/2 of that battery voltage to A0.  So the last step is to add that voltage divider, along with some extra tape to serve as foot pads.

The battery voltage calculation for a divider with equal value resistors is:   float batteryVoltage = float((analogRead(A0)/ 511.5)*3.3);  But the MIC5205 regulator found on most promini style boards will accept anything between 3.4 to 12v input, so you will need size your resistors to convert the peak battery pack voltage into something below the 3.3v aref limit. To cover that whole range, you’d need a pair that puts 1/4 of the battery voltage on A0, and a R1(high side) = 3*R2(low side) combination would do that, changing the 511.5 constant in the equation above to 255.75    With 5205’s dropout potentially rising to 300mV during 200mA SD writes, I usually shut down the loggers when the main battery falls below 3.75 volts. With Meg-ohm size resistors, I leave that divider connected all the time, but there is a wonderful self-disconnecting voltage divider idea over at JeeLabs for those who want to use smaller resistance dividers. Also keep in mind that resistors you get off of eBay are usually +- 5% (no matter what the vendor claims) which can really throw off your battery readings: ALWAYS measure high value resistors with a good quality DVM before making a divider with them. And with resistances in the megohms environmental contamination such as skin oils, soldering flux residue, etc. can easily reduce the effective resistance in time-varying ways.

As we removed the pin13 LED back at the start,  solder a limit resistor onto the ground of a common cathode RGB and connect that to one of the ground connections, with the other legs going to D4R-D5G-D6B.  I usually add a few labels to keep track of the extra terminal connections, and any re-allocated any pins for a specific build. Unfortunately black sharpie marker doesn’t stick to those green terminal shrouds very well. 

In this example I’ve re-allocated the screw terminals that would normally have been connected to the two reset pins, but you could use under-board wires to re-assign any of the terminals in a similar fashion. For example, if your application will not be using the RX/TX pair, those could be turned into extra Ground or Vcc points. I’ve never understood why the pro-mini design breaks out reset twice but leaves the Aref pin hidden, so adding a wire to the little aref stabilizing cap would let you fix that issue.

Your Logger is ready to go!

As this is simply a different physical arrangement of the same core components, you can follow the logger testing procedures described at the end of  the 2016 Dupont Jumper Build , which also provides links to a basic data logger script to help you get started on your project.  For the build described above, the pro-mini’s MIC5205 regulator delivers sleep currents less than 0.25mA (Promini~0.05mA,  sleeping SDcard~0.05-0.09mA & RTC~0.09mA)
That should should reach a year of operation on 4xAA’s. 

While it took me a day to get the first one of these sorted, the second one took less than three hours, and the third took less than 2 hours. I lost count after that, and now these things seem to be multiplying like tribbles.   If you need unobstructed access to the SPI bus, you can move the SD lines to under-side solder connections as we did for the I2C bus.  This also makes the logger a little prettier, but since I’m usually making these in a hurry, I often leave those wires on the surface.

The photos in this series were made with Adafruits 26AWG silicone wire, but if you are adding more bottom-side connections, switch to smaller diameter 28AWG wire, or make the pass-through hole a bit larger to accommodate the extra lines. Switching the  90° I/O header pins to the bottom of the promini board gives you more room for the RTC wiring.

You can make the component stack more rigid by adding a few strategically placed beads of epoxy putty.  In fact you could hold the whole thing together that way, so long as you take care not to bridge any contacts – especially where the DS3231 header pins are near the metal top of the SD adapter. Also keep in mind that the putty sets rock hard in about five minutes, so if you make a mistake with that assembly method then you’ve bricked the unit – so test all your connections before this last step!

If you are careful about placement of the batteries, you can fit this new screw-terminal design onto the abs knockout plugs that I’ve been using as mounting platforms. This means that you can still fit the logger into the inexpensive 4″ housings
that I outlined for earlier builds with room under the platform for a second battery bank if needed.  Given how often makers need to put a shell around their projects, I’m surprised that no one has taken the old B-Squares idea into three dimensions to create a re-configurable snap-together housing system. These videos from the tutorial set walk you through the method we developed in 2016 to build housings from PVC plumbing parts that are robust enough for real world deployments:

Battery Platform & Sensor Connections

In addition to the I2C bus, I’ve started breaking out A0-A3, rather than digital lines, since those A ports can do double duty as either analog or digital I/O with some code-side settings. With screw-terminals on all lines anyway, I only break those out to connectors for quick sensor swaps in the field.

Potting Sensors & Waterproof Housing Pass-Through Connections

One thing to keep in mind for any project built from eBay parts is that most of those boards use cheap Y5V capacitors; which have terrible temp-coefficients compared to X7R/NPO’s.  So you need to test your project extensively if you want it to operate over a wide temperature range. My home freezer tests to date have been running ok, but I’m not relying on the Pro Mini’s oscillator/clock for anything that is timing-critical.  I do expect to see bus timing drift out of spec at the low temperatures.  For loggers built with I2C sensors, stick with 100kHz for your first few builds, then things “just work every time”…75% of the time. The 1.1v internal band-gap also changes significantly with temperature, so if you use it as Aref, expect to see the readings go up, as the temperature goes down.

Addendum 2017-06-21:

I’ve been on a steep learning curve since the beginning of this project, and you don’t have to dig very far to find stuff on this blog that seemed like a good idea at the time, but later turned out to be completely wrong.  I should write some sort of disclaimer,  but instead I’ll pass along a recent forum comment that summarizes the kind of criticism we’ve been getting lately:

“In the old days, an embedded enthusiast would have designed the thing (and think AVR) from the outset to meet objectives / specs, not struggle with integrating the various modules and meeting very-so-so sleep currents (while thinking Arduino). Surely, this is a textbook example of how not to do embedded engineering if you are doing it for a salary.”

It’s good to have someone rattle your cage once and a while, and I’ll admit they have a valid point. ( In addition to the fact that I’m not an engineer, and I don’t get paid…)  People complain like that about the pitfalls of using modules & libraries all the time, but the thing I like about the Arduino platform is that you don’t have to know everything before you can do anything. I’m just figuring it out as I go along.

Still, an affront like that demands some kind of response.  So to defend the honor of my fellow Arduino Kool-Aid drinkers, let’s look at how you might tweak those modules to improve this loggers sleep current performance:

1) Pin Power the RTC:

These DS3231 boards don’t get a lot of love because they have about the worst battery charging circuit ever devised, and an equally useless LED power indicator. But for less than a buck delivered to my door, these boards are considerably cheaper than the raw components they carry: so yeah, I’m going to look under that rock and see what I find.  That charger can be disabled with a simple flick of the soldering iron, and at this point we have years of successful run time using a non rechargeable CR2032.

More interesting is the fact that on a 3.3v system, you can leave the charger in place, cut the Vbat line at the battery terminal, and patch in a 1N4148 diode to match the one already on the module.  After the CR2032 burns down a bit the two circuits balance out, and the main battery then takes over supplying the 3µA timekeeping current: 

This mod cuts your loggers sleep current by about 0.09 mA.  Keep in mind that this only works because the RTC was designed for a controlled switch over to the backup power circuit.  In general, de-powering I2C devices is not a good idea because the pullup resistors keep the SDA and SCL lines high. When a regular I2C connected chip has no power, you could leak current via SDA and SCL through the I2C device to GND. 

2) Buffer your data before saving:

Those DS3231 RTC modules also have a 4K EEprom on the board, and that lets me save data in 32byte page-writes with reasonably simple code. While the I2C bus is dead dog slow by embedded system standards, you can hang oodles of things off those wires without worrying about cable select lines, or some gummy protocol weirdness.  For an extra buck, you can add 32K more memory without any significant changes to your Arduino script. That usually buffers about a week’s worth of data before I need to save to the SD, even though I’m still making the unforgivable programming sin of storing everything in ASCII string variables

Small red-board versions of the AT24C256 tuck nicely into the 12mm gap between the headers, but you could just as easily put an I2C sensor into that space. If you get boards with the address pins broken out (the one above doesn’t), you can connect up to four of these eeproms to the same logger. A side benefit is that the 32K eeproms are rated to 400kHz, while the 4k’s are only 100kHz, so the upgrade also lets you accelerate the I2C bus clock, since the DS3231 is also rated for 400kHz. 

Even larger eeproms are available in the code compatible AT series, but the wire library limits you to writes smaller than their page sizes. If I had the chops, the path to an IC-only logger is obvious.  The AT24C512 is pin-compatible with the SOIC family that includes the 32k AT24C256 and the AT24C32.  So you could also just replace the 4k eeprom that comes with the RTC module with that 64k chip possibly taking it all the way up to the 128k AT24C1024. 

One thing to watch out for with some eeproms like the 24LC512: “When doing a write of less than 128 bytes, the data in the rest of the page is refreshed along with the data bytes being written. This will force the entire page to endure a write cycle, for this reason endurance is specified per page.” So it’s worth the time to buffer your data until you can do a page write with some eeproms. AT24C512’s have the same 128-byte Page size with 16-bit addressing , but partial page writes are allowed. With most larger eeproms you also need to send ‘block addresses’ in addition to memory locations during a save event.

I’d love to switch over to superfast Fram chips, but Adafruits module lists a very high standby current of 27uA, while most eeproms sleep between readings at about 1uA. In a data logging application that kind of power use between cycles removes the benefit of the faster data-saving. However if I was working with really fast data inputs, I’d consider Fram for something like a ramdisk. Paul Stoffregen’s SerialFlash library is another interesting option, as it allows one to write data to an SPI Flash memory chip with a filesystem-like interface similar that on an SD card.

3) Cut power to the SD card:

The Promini clones I’m using have a tap at the back that is conveniently located for ground side switching of the SD cards. This lets me tuck a 2N2222A under the board with that extra eeprom.  Cards hit the regulator pretty hard when they initialize, causing significant voltage drops, and if you find that your unit is not saving properly with this technique it’s probably because those transient lows are causing restarts.  I usually add caps to provide and extra 30μF on the rails to help handle those spikes, and I may bump that even higher for cold climate deployments since cheap ceramic caps have terrible temperature constants. Older Nokia 256 & 512 Mb SD cards have significantly lower writing currents than modern SD cards – often holding between the 50-75 mA range during init.

Code and information about this technique are described in some detail on the
SD power post This is a relatively high risk strategy, but it can cut your sleep current by another 0.1mA.   (Note that while the BJT shown above works fine, on more recent builds I’ve switched to the Supertex TN0702 mosfet for ground side switching with 3.3v logic)

4) Replace the voltage regulator:

The MIC5205 on those pro-mini clones is not very efficient at low power (~10-20%), so replacing that with an MCP1702-3302E/TO can cut your remaining sleep current by more than 50%.  The 10uF caps from the original reg. are still in place on the board, so this upgrade has been working fine with the 1700 just hanging off one side. Also keep an eye on the dropout voltage, which on the MCP1700 series can rise as high as 600mv if you push them to their 250mA maximum – many SD cards will pull up to 200mA for brief periods. This requires a fairly high input cutoff around 4v.  

If soldering in close quarters like this gives you the heebie jeebies, you can dead-bug the reg & voltage divider onto the battery connector like I did for the 2016 builds.  Alternatively you can simply build your logger around a small form-factor boards that already have an MCP1700 series regulator, such as the Rocket Scream Mini Ultra  or the Moteino. The HT7333-A is a less expensive TO-92 LDO regulator with similar specs (250mA max out).

All linear regulators (including LDO’s) are less efficient as the raw input voltage increases. (So a 4xAA 6v battery supplying a 3.3v rail system will be about 55% efficient under load)  For higher drain processors sometimes people use buck converters (typically 90% or better efficiency) to step down higher input voltages, and then feed that ripply output into an LDO to smooth it out  for delicate components.  But switching regulators often fall down to 20% efficiency or less at the low micro-amp sleep currents you see with dataloggers, while the MCP1700 keeps chugging along at around 50% until your load approaches its 1.6μA quiescent current.

(Note:  that the pin maps are different for each board, so you might need to adapt the wiring connections shown in the information above.  Also keep an eye out for fake LDO regulators with high dropout voltages. If you’re not sure what you’ve received, bump the input voltage cut-off to at least 1v. The Promini’s 10uf tantalum caps should be enough to damp oscillations, but it probably wouldn’t hurt to add another on the output for extra stability)

And the result?

These modifications to the basic build plan probably violate some important electrical engineering rules, and I can almost guarantee that nothing will work properly the first time you try it.  But don’t let the fact that you might destroy a few cheap components along the way prevent you from just going for it.  Although it might be best if you don’t show your project to any engineer friends at the beginning… unless they’re working on a new textbook 🙂

Addendum 2017-12-22:  Low sleep current the easy way with a TPL5110

It can take a bit of Kung-Fu to implement the power optimization methods described this post, and if you haven’t quite reached that level the Adafruit TPL5110 Low Power Timer can bring any 3.3v logger down to ~0.02mA for a fiver.  Cfastie has is putting this board through it’s paces over at PublicLab.org It’s too bad the TPL5110 timer IC can’t tolerate more than 5.5 volts, because it would be a god-send for all those UNO based loggers out there, but their minimum input is ~6v.

The only problem I can see with the TPL5110 is that you don’t have precise control over the sleep interval [Timer accuracy 1% (typ)], and temporal inconsistency becomes important for many kinds of analysis on time series data over long periods.  Since the sleep currents I’m seen now are already in the 20uA range, I probably will not bother with full power-down methods till I start needing to run on tiny batteries.  But when I do get there I will probably use the DS3231 RTC alarm (which outputs low) to control  a P-channel Mosfet on the high side of the main battery supply. The trick is finding a mosfet with a low enough gate voltage, so that it will be fully turned off till the RTC alarm fires ( AO3401? ).  When the SQW alarm goes low, it turns the p-mosfet on and powers everything including the mcu board which would then get to work taking samples and storing data. The final step after everything is done would be to re-program the next RTC alarm, and then write zeros to the alarm flag registers (A1F and/or A2F) which would release the SQW line on the gate of the mosfet. (+ you would need a pullup resistor on the gate to make sure the pFet turned off properly – but that’s just a pullup and could be quite large- in the Meg ohm range). The RTC itself would run off a coin cell, and DS3231 RTC will generate such alarms when running on backup power.  Kevin Darrah talks about a slightly more advanced latching kill switch approach that could also be adapted.

My biggest concern about kill switches is that the SD card can be corrupted if power is immediately removed during numerous conditions, so you need to make sure you can safely shut down after buffering all your persistent variables in the eeprom.  Most Arduinos will boot in less than a second, but capturing rapidly occurring events like the sensor interrupts we use for our cave drip counters would be nearly impossible.

Addendum 2018-02-13: 

Brian Davis (another builder who tortures his loggers in caves) passed on a neat idea for using Pro Minis with Nano screw-terminal shields. Simply add male pin headers beyond the edge of the board, and then solder the A4/A5 and A6/A7 connections to the top of those pins. Then pull Vcc from the UART connector. If you run the SQW alarm wire over the top to D2,  you no longer need to drill a hole in the shield to bring wires down to the bottom of the board like I did, and you can still pull the Pro Mini out of the adapter board later if you have to. 

An alternative hack that may be useful to people wanting to break out those pesky offset A4 & A5 pins would be to jump them over to re-purposed A2&3 pins, which you make available with a couple of trace cuts. Of course this reduces the number analog input lines on your logger, so it’s not quite as functional as Brian’s extended headers.

Addendum 2018-05-23:

Well Brian’s been at it again, coming up with more alternative build ideas:

Both of us are working with the limitations of a 2″ housing tube, and using the battery holder as a mounting bracket is a great solution. I also like the addition of the screw terminals on the RTC’s cascade port (Sparkfuns Quic adapter board is also a pin-matched option there if you are using their sensors ) For the Pearls, we wire the DS3231 in the opposite direction, and we have some high drain multi-sensor builds that require ganged battery packs to go the distance.  So for us it makes sense to stick with the ‘stacked sandwich’ arrangement, with the batteries in completely separate containers. With high temperature variations, and long duration deployments, we also see a fair number of battery leaks.  This ruins the terminals forcing us to replace the packs more often than we’d like.

Even so, I’m utterly thrilled to see these creative ideas coming out of the logger building community, as that will ultimately give the open science hardware movement some real legs. And while we are on the topic of build variations, it’s worth noting that there are a host of Pro-Mini style board variations out there that may help with these part re-arrangements:

A4 to A7 on back row	Analog & SPI on the back row(s) Julian Illet uses these in his ammeter build.	328PB =  two hardware SPI serial ports though I’ve had success with software SPI on the analog lines to drive displays.

I’ve been using the more common variant with GND-A6-A7 on the back row as I bring the I2C lines forward to connect the RTC, but I can see how having those offset pins at the back row could make soldering easier for some builds.

Addendum 2020-04-05:   New Underwater Housings for the Screw Terminal Stack

Happy to present the 2020 iteration of our DIY submersible housings, with fewer PVC parts & faster assembly. Using the power mods described above, with the screw terminal stack, reduced the need for batteries & the space they required. With a double 332 o-ring this build is not quite as robust as the original ‘long body’ version described in the 2018 paper, but it still should get to about 10m. Can’t wait to see the first student builds based on this design make it into the wild later this year.

Addendum 2020-10-20: 

In 2019, a teacher friend asked us for a build with minimal soldering because they didn’t have time in their course, or the budget for soldering stations. So we re-designed the 2016 ‘EDU build’ , adding a screw-terminal shield and a pre-made housing to reduce assembly time.  We also added support to the updated code base for using the indicator LED as a light sensor:

https://thecavepearlproject.org/2020/10/22/pro-mini-classroom-datalogger-2020-update/

The schematic diagram is essentially the same, but the 2020 version shown above is probably a better starting point for people building their first Arduino-based logger than this older 2016  described below. For more background information see: A Flexible Arduino-Based Logging Platform for Long-Term Monitoring in Harsh Environments and the PDF is free to download from the open access journal Sensors.

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Original post from 2016:

Its been almost a year since the last stand-alone logger tutorial, and I continue to receive questions from people adopting the platform in education settings.  That feedback makes it pretty clear that the lack of soldering skills is a significant stumbling block for beginners, so I have come up with a build that uses pre-made DuPont style jumper cables wherever possible.

This build uses a different SD card adapter than previous builds and I’ve changed the resistor location to make the overall assembly easier for beginners.This comes at the expense of having more wires to deal with on the limited real-estate of the knock-out cap platform, and re-positioning the modules to make room for the overhang of the DuPont connector housings. The overall result is a little uglier, and not as robust to knocking about as a unit where every connection is soldered in place, but the platform takes about a third less time to build (~2.5h) with a part cost under ten bucks before you add sensors.

PARTS & MATERIALS

Parts Total					$8.25
	Pro Mini Style clone 3.3v 8mHz				$1.80
	DS3231 IIC RTC with 4K AT24C32 EEprom (zs-042) Some ship with CR2032 batteries which will pop if you don’t disable the charging circuit!				$1.25
	SPI Mini SD card Module for Arduino AVR Be sure to buy the ones with four ‘separate’ pull-up resistors.				$0.50
	20cm Dupont Ribbon Cable 40pin F-F 2.54mm You use ~1/2 of a 20cm cable per logger. Get the ones without the shrouds to save time.				$0.70
	2xAA Battery Holder & 1xAA Battery holder				$0.60/both
	CR2032 lithium battery				$0.40
	4″ Knock-Out Test Cap				$0.40
	Deans Ultra-T Style Battery Connector Plug				$0.30
	2x Nylon M2 12mm standoff, Nut & Screw M2 5mm				$0.40
	Micro SD card 64mb-1gb  Older Sandisk  cards have lower sleep currents. Test used cards well  before putting them in service.				$1.00
	Dupont Crimp Pins & Housings Most parts  cost about a 1-2 cents each, after you buy the crimping tool.				$0.30
	Common Cathode Bright RGB LED 5mm  A brighter bulb lets you use a larger limit resistor for the same light output.				$0.10
	Double Sided Tape,  2x 4.7MΩ resistors, 26awg silicone wire, (104) 0.1uF caps, 4″ cable ties, heat shrink tubing, header pins, etc…etc…				$0.50
	Donation to Arduino.cc It is now possible to build a decent data logger for less than $9 because of the hard work of many people around the world. If you don’t use a ‘real’ Promini from Sparkfun to build your logger, you should at least consider sending a buck or two back to the mothership to keep the open source hardware movement going…so more cool stuff like this can happen!				$1.00
Comment:  And a few tools you might need to get started:                     (not included in the total above)
	CP2102 USB-UART Bridge module This 6-pin connector module was specifically designed for compatibility with the Pro Mini. (note that most 2102 boards are not because they only have 5 pins) and these boards work with Mac & Windows after you install the drivers  Or for a safer option you could use the FTDI chip version. ***Be sure to confirm any UART board is set to 3.3v before using it!***				$2.20
	Yihua 936b soldering station 110v These have enough power to solder the large battery connectors.  Get extra handles & tips.				$23.00
	DT-830D Digital Multimeter				$3.00
	Heaterizer XL-3000 Heat Gun				$14.00
	SN-01BM Crimping tool				$22.00

COMPONENT PREPARATION

CLEANING:  Boards from reputable vendors like Adafruit / Sparkfun are usually clean, but cheap eBay modules are often covered with leftover flux from the manufacturing process and dirty contacts are guaranteed to corrode.  Most of us use 90% isopropyl alcohol to clean this residue, but some prefer Polyclens brush cleaner. A couple of five minute sessions in an ultrasonic bath with Iso90 usually works great, but you can also do it by hand with a cotton swab, etc.   (Never put vibration sensitive components like accelerometers into an ultrasonic bath)

Preparing the Main Board:

Begin by soldering header pins into place on the 3.3v Pro Mini style board.  This is much easier to do if you use a small breadboard to hold the pins in place, however you should not use that breadboard for anything else afterward because the transferred heat will ruin the contacts. I have a few of these old “for soldering only” breadboards lying around.  I prefer to use clone boards with A4-A7 broken out to the edges of the board to make the soldering the pin headers a bit easier, but those are not always available.      {Click on any images to see larger versions.}

Lay out the required header pins as in the images below. Arrange the pins for the sides of the Pro Mini board on a breadboard so that the two 3x rows extend sideways from the main board at the VCC line and the GND line on the analog side of the Arduino. This is a little tricky because you are working from the bottom of the board, not the top, so there is a left right side transformation. Also note that you can add more sidecar headers than I’ve shown here if your logger will need more connections to the power rails to supply sensors.

 {Click on any of the images below to see larger versions.}

Solder the straight riser pins into place along each edge. Then add a tiny amount of flux to the extra header pins and coat the ends of the pins with solder. Tin the wire on one end of a resistor and solder it to the GND header pin on the Pro Mini.

Then weave the resistor leg through the sidecar pins and use that wire to bridge the solder connection to all of the sidecar pins beside the ground pin on the Pro Mini. Repeat the procedure again for the VCC sidecar headers. Use a good amount of solder here to bridge the pins.  The sidecar pins are a bit fragile since they are not attached to the module, and could be accidentally broken when attaching Dupont connectors later on unless you provide some extra mechanical support with solder.

Now that the risers along the sides of the Pro Mini board are in place, we tackle the other riser pins on the board. These are a bit trickier because the A4 & A5 riser pins (needed for the I2C interface) are not aligned with standard breadboard pin spacing. Place the remaining pins into the board face up, then carefully turn the board over onto a flat surface making sure the loose pins stay in place.

It is good to have the six serial UART I/O pins bent slightly away from the other risers to make more room for connecting the UART board. Once the pins are in place clean the flux residue on the bottom of the board with some 90% isopropyl alcohol and a cotton swab.

Before proceeding further with your logger build, take a few moments and connect the board to your computer via the UART adapter to test it out by uploading the Blink sketch. You need to install drivers to get the serial communications adapter working with your operating system, and this part of the process can be challenging for people who have never done that before. There are well written guides at Sparkfun & Adafruit if you are using boards with the FT232RL chip from FTDI. If you are using this CP2102 from Silicon Labs then you need to sort out driver installation on your own, but a quick google search should bring up lots of guides posted around the web. Once you have the drivers installed, the Arduino IDE should display a new com port when the adapter is plugged in to the usb port of your computer. At that point you can upload sketches to your Arduino through the serial adapter after selecting Board: Arduino Pro or Pro Mini   and  ATmega328(3.3v, 8mhz)   in addition to the new com port provided by the drivers. If you are ordering parts from eBay, then buy at least three of everything – it’s not uncommon to see a 10-15% failure rate on these budget parts…that’s why they are so cheap. I also order those backup parts from different vendors, since some will take months to ship, while others get the modules to you in a week.

Once you have confirmed that the board is working,  remove the power LED limit resistor and the resistor for the LED on pin 13. Simply hold the tip of the iron on one side of the small resistor and apply a gentle sideways pressure. When enough heat has conducted through the resistor, the solder will melt and you should be able to simply ‘flick’ the resistor off the board. Disabling these LEDs conserves battery power. (Note: these limit resistors are in different locations from one Arduino compatible board to the next, so you might have to hunt around to find them.)

Power LED limit Resistor

Pin 13 limit resistor

Trim the A4 & A5 pins flush to the board with a pair of side snips. Add two layers of double sided tape to the underside of the Arduino board so that it can be affixed to the platform. Due to the riser pins extending along the side of the pro-mini, two layers are needed: The first layer should fit “between the pins” and the second layer should extend past the pins to the outer edges of the board.

The RTC Module:

I don’t recommend using LIR2032 rechargeable batteries with this module as the charging circuit wastes a great deal of power, and a CR2032 will backup the RTC for many years.  Remove the charging circuit resistor and power LED limit resistor from the circuit board indicated with the red squares in the first picture below.  Next add four 90-degree header pins to the I2C cascade port on the RTC board. These four pins will become the connection point for any I2C sensors you add to the data logger:

Since the RTC board already has 4.7k pullups on the SDA (data) and SCL (clock) lines, you will not need to add them to your I2C sensors. This board also has a 4.7k pullup on the SQW alarm line.  The GND and VCC pins (3.3v) on that cascade port can also be tapped to power other sensors you connect to the data logger.

The SD Card Adapter:

This SD card module comes with small surface mount pullup resistors on the MOSI, MISO & SCK (clock) lines (which have already been removed from the dashed red line area in the diagram).  The Arduino SDfat library uses SPI mode 0 communication, which sets the SCK line low when the logger is sleeping. This would cause a constant drain (~0.33mA) through the 10K SCK pullup on the module (second smd from the bottom) if we did not remove it.  I prefer to pull MOSI & MISO high using the internal pullups on the ProMini’s 328P processor, so those physical resistors on the breakout board can also be removed. But be careful to leave the top-most resistor of the four in place to pull up the DAT1 & DAT2 lines. This keeps those unused pins on the μSD card from floating when the cards are accessed in SPI mode.

Then add a layer of double sided tape to the bottom of the SD card module and trim it to size.

Dupont Connectors:

The RTC & SD card modules will be connected to the Arduino by custom jumpers that you assemble from a 20cm, 40pin F-F Dupont style ribbon cable and 0.1″ (2.54mm) crimp connector housings. Test your cable for end-to-end continuity, then cut the cable into three sections.  Then peel off individual 10cm wires and insert them into the black crimp connector ends to create the cables shown. The longer 20cm wires can be used later when we assemble the LED indicator cable, and for other sensor attachments.

For the I2C connections: Blue:          SQW alarm signal. Yellow:     SCL (clock) (to A4) White:       SDA (data) (to A5) RED:          3.3v regulated BLACK:    Ground

For the SPI cables: RED:           3.3v regulated Grey:          Cable select (to D10) Orange:     MOSI   (to D11) Brown:      SClocK (to D13) Purple:      MISO   (to D12) BLACK:     Ground

I’ve left one of the six RTC connector shroud slots empty for future use with the 32K clock signal but you can change that to a 5-pin housing if you wish.  Single 0.1″ (2.54mm) crimp connector housings are shown on the ends of the red, black and blue wires ,but I find that putting heat shrink over single-wire DuPonts help them hold onto the pins better.

ASSEMBLING THE LOGGER PLATFORM

Attach Battery Holders & solder in series

On a 4” PVC knockout cap, arrange your battery holders as close to the edge as possible to maximize room for other parts on the platform, without letting the holders hang off the edge. Mount the battery holders in place with double sided tape.

Join the red wire from the bottom of the single battery holder to the black wire from the double battery holder, so that the three AA batteries are connected in series. Fold the soldered joint and the long RED wire into the gap between the battery holders. Use some heat-shrink to bind them so they stay together inside the gap. Mark and drill 1/8” pair of holes so that you can tie down the wires from the battery pack neatly. Secure the wires to the cap with a zip tie through the holes.

Risers & Tie Downs

Place the RTC board alongside the batteries with one corner near, but not extending over, the edge of the cap. You want the back edge (with the four-wire cascade port) as close as possible to the single battery holder and the outer edge of the platform. The larger 6-pin connector side of the RTC module will be at a slight angle  to make room for the long connector housing that will be placed over those pins. Mark two places for the nylon standoffs and drill 2mm holes through the knockout cap. Thread the nylon standoffs through the holes and attach them to the platform with the plastic nut on the underside of the cap.

Then add two pairs of ¼ inch holes in the platform to be used later to tie-down the connector cables:

Battery Pack Connector

Join the two pieces of the Deans battery connector, and add solder to the end tabs of the female side of the square battery connector. For big connectors like this I turn the iron up to ~700°F (370°C). Be sure to heat the part long enough for the solder to “flow” like a liquid, and for the flux to burn off, but not so long that the nylon starts melting. Then trim the battery wires so that they extend to a mid-point over the battery holders on the platform. Strip about 5mm of insulation from the wires and tin the ends. Build up some extra insulation thickness to protect the wires from bending stress by adding two or more layers of heat shrink to the wires just before the tinned ends. Thread two larger pieces of heat shrink over the wires and then solder the wires to the connector and seal the heat shrink over the exposed metal:

Note: the top of the ‘T’ shape on this connector is the positive (red wire) side. Set your iron back down to ~600°F(315°C) after you get the solder on the big connector tabs. As a general rule: the smaller the component – the less time it can withstand heat. It’s not unusual to see maximum tolerances of 5 sec @ 288C for tiny SMD’s.

Attach modules to the platform

Remove the tape backing from the pro-mini and the SD card adapter and affix them to the platform. As you can see the space is pretty tight. The SD card adapter needs to be close to the battery holder, because the SDcard will extend another 5mm from the holder itself, and this must not extend past the edge of the disk.

Then attach the RTC board to the nylon risers. Check to make sure there is good clearance between the RTC pins and the Arduino pins.

Connect the RTC

After the modules are placed on the board begin the interconnections by affixing the I2C cables to the RTC.  Then add the white & yellow two wire connector to risers on A4 (white) and A5 (yellow) of the promini board. Gather the white wires and pull them into the gap between the RTC module and the battery holders. Trim the wires as a group, so that their length just reaches the single battery holder. Then strip, twist & solder the ends of the wires together, and cover the join with heat-shrink.

Repeat the process with the yellow wires, and thread the joined wire sets through a cable tie that is routed from the bottom of the platform. Do not tighten the cable tie loop too much, as the black and red power lines will go through that later. To finish the RTC connection, plug a single blue wire to pin D2 on the Arduino [int(0)],  and connect it to the blue RTC alarm line as you did for the other lines. Wrap the soldered join with heat shrink and thread it under the cable tie with the I2C bus wires.

Connect the SD card

Now we will join the SPI lines which connect the SD card adapter to four pins on the Arduino.   Connect the 4-wire SPI cable to pins D10-D13 of the Arduino so that the grey wire is connected to D10 and the brown wire is on pin D13.  Attach the 6-wire SPI cable to the SD card adapter so that the black ground wire is closest to the pro-mini board.

Take each matching color pair of wires and fold them over beside the SD adapter board. Strip, twist & solder the ends together and add heat shrink to the join. Then bind them to the platform with a cable tie.

Join the Power Rails

The final step for the platform interconnecting cable is to join the “power rails”. Connect a single red jumper to the VCC pin on the Arduino compatible board, and add a single black jumper wire to the GND pin (behind the yellow wire on A5). Then gather ALL the black wires together and trim them to length along the same channel used for the three RTC wires.  Strip, twist & solder the black wires together.

Repeat the procedure for the red (+3.3v) Vcc lines. After soldering the ends of these bundles together, cinch them out of the way with the same cable tie that you left loosely holding the RTC connection wires earlier:

Battery Connection with Voltage Divider

Note:  The battery connection wires see a lot of handling compared to the other connections on the logger platform.  Since I have crimping pliers to add my own DuPont ends, this is one place on the build where I prefer to use silicone wire that is more flexible than the PVC insulated wire. 

With the main interconnect wires completed the last part of the logger platform build is the Arduino side power connector. Put the male battery connector in a vise with a spare female side connected to prevent the nylon from sagging due to heat, then add solder to the terminal flaps of the male battery connector. You might need to turn your iron up to 700°F to tin the connections the large connector, but turn it back down to 600°F (315°C) before attaching the voltage divider resistors, or you will cook those smaller components.

Build a voltage divider from two 4.7MΩ resistors, bridging across the low side resistor with a 0.1µF (104) ceramic capacitor. Add a black wire to the capacitor side of the divider, a pink wire to the center, and a red wire to the opposite side. Then solder this divider across the pins on the male battery connector, being sure to connect the black wire side to the ground pin, and the red wire side to the (+) pin.  Blow on the solder to cool it down as soon as the join to the connector pins is complete: there is more than enough residual heat stored in the battery connector plates to cook the little capacitor, which then usually becomes a short.

This voltage divider will cut the battery voltage in half, allowing us to read the status of the 3xAA battery pack on any analog input pin (via the pink wire in the photos above) even though the main battery voltage is above the Arduino’s 3.3v Aref.  The 4.7Meg resistors are large enough to limit losses on the bridge (to about 0.6uA), and because they exceed the input impedance limit of the ADC, the capacitor is needed to provide electrons to fill the ADC’s sample and hold capacitor when a reading is needed. The battery voltage calculation for this combination is:   float batteryVoltage = float((analogRead(A0)/ 511.5)*3.3);  The default MIC5205 regulator found on most promini style boards has ~200-300 mv dropout during SD card writes, so when your AA battery pack gets down 3.65 volts, it’s time to shut down the logger.

Add heat shrink to protect the connections, and then plug the connector into it’s mate on the logger platform. Trim the black wire to length for the ground pin beside D2 (with the blue RTC alarm wire) , and then add a female crimp connector to that wire. Do the same for the pink voltage divider wire which gets attached to your choice of analog inputs from A0-A3, and the red battery wire which gets attached to the RAW input pin near the serial/UART terminal.

Indicator LED

Since we removed the pin 13 LED from the pro-mini, we will now construct an external indicator. This is another place where soft silicone wires are preferred, but if you do not have them, peel away 20cm Red, Green, Blue and Black ribbon cable wires and solder them into place on a common cathode RGB LED, matching the wire colors to the correct pin. Then add stiff heat shrink around the connection points to protect the wire from flexing. Place the ends of the Red, Green and Blue wires into a 3-connector housing.  The R-G-B connector attaches to digital pins 4-5-6 on the Arduino.

Cut the black wire near its mid-point, and insert a 20K resistor to limit the current through the LED. I often replace the bottom half of the wire with a softer flexible length of wire with a MALE crimp connector at its end. The male ground pin connects to the ground pin beside the D2 connector.

The MAIN LOGGER PLATFORM is complete

Typical pro-mini loggers built with this design sleep at 0.25mA, before extra sensors are added. At that current draw, the logger should deliver at least eight months of operation on three brand new AA batteries with a 15min duty cycle; depending on sensor load.

Now you can run testing utilities to verify that everything is working. But before you jump into that it’s worth keeping in mind that the entire construction can easily be disassembled for debugging if you suspect a bad connection somewhere, and it is even possible to pop the modules off with a small screwdriver and replace them:

Test continuity on suspect wires with a DVM.  If you carefully lift the little tabs holding the crimps inside the black housings, individual wires can be extracted from the connectors and replaced.  If your crimps don’t feel like they are securely in place when they slide over the riser pins – then you should replace that connector.

SENSORS & HOUSING

You can build a robust, inexpensive waterproof housing for your logger from a couple of 4″ plumbing end-caps. I use Loctite E30-CL to pot sensors on the exterior of the housing and you can see a complete walk through of the process in the  Sensors & Housing post from last year. Note that I did not use the 4-pin Deans connectors you see there on this newer build because they are fairly expensive, and a few people have been having a hard time getting their hands on them. If you shell out for crimping pliers, you can make as many custom connectors as you need for pennies each.  Dupont’s are not as robust, but I’ve had loggers running for years with them, so they are not a bad place to start. If the male pins don’t mate tightly, put a thin layer of solder on them to make them a bit thicker. Quality varies quite a bit, and Pololu sells good crimp pins.

TESTING THE LOGGER

1. Test the LED
Connect your indicator LED, to pins 4(R) – 5(B) – 6(G), and the ground line to one of your spare GND connections. Open Blink with the LED connected to the Arduino. Change the LED to Pin 4. Verify and upload. The red LED should start flashing. Repeat with pins 5 and 6 to test green & blue respectively. Note that with the common ground connection, you can not light up two colors of the LED at the same time.

2. Test the I2C bus
Use an I2C scanner  to determine if your devices are responding on the I2C bus. With the scanner running, specify P to return to output only the addresses that have devices, and specify S to run a Single Scan. The 4k AT24C32 eeprom on the RTC board is at address 0x57 and the DS3231 will show up at address 0x68. The 32K eeprom should show up at 0x50 and other sensors will show up at different addresses. Note that the 4K AT24C32 is only rated for 100khz operation, but the DS3231 is rated for 400khz bus speeds.

3. Test the RTC
Use the settime & gettime scripts to set and check the time on the RTC. Generally, I set my windows computer to UTC before uploading setTime so that the logger is operating on UTC. This is not possible on a Macintosh, so you will have to check if London time is currently in sync with GMT/UTC, or find some other city that is. Alternatively you could avoid the problem by using the new serial setting script from Mr Alvin’s gitHub.

4. Test μSD Card communication
Insert the microSD in the adapter and test it with the CARDinfo utility at the Arduino  playground.  Remember to change the CSelect variable to 10 to get the CARDino to operate properly, as that is the pin we connected to the μSD card Cable Select line.

5. Test the EEprom(s)
A utility to test I2C eeproms was posted by bHogan at the Arduino playground forum, but that version is now quite old and does not compile in the current IDE without editing. I’ve posted an updated version on gitHub. Setting #define EEPROM_ADDR 0x57 will test the 4k eeprom on the RTC board, and if you install the extra 32k eeprom that shows up on the bus at 0x50. The serial window will return  A through T & 33 through 48 if the eeprom being tested is working OK.

6. Check Sleep Current
#include the LowPower.h library from RocketScream at the beginning of a blank sketch and the put the following line at the beginning of the main loop:

 

LowPower.powerDown(SLEEP_FOREVER, ADC_OFF, BOD_ON);

Then enable some internal pull-up resistors in to the setup section of your code:
(be sure to do this before you call sd.begin)

// Always pullup CS:
pinMode(chipSelect, OUTPUT); digitalWrite(chipSelect, HIGH); //pullup the CS pin to tell the SD card go to sleep
//and you may need to pullup MOSI/MISO lines:
//pinMode(MOSIpin, OUTPUT); digitalWrite(MOSIpin, HIGH); 
//pinMode(MISOpin, INPUT); digitalWrite(MISOpin, HIGH);   

Upload that sketch and your logger will be put to sleep as soon as the program runs.  Note: the CS line should have a pullup when using the standard SD libraries, and the MOSI/MISO lines may also need to be enabled to lower the sleep current with some SD cards. But SPI is a complex protocol, and you might have to turn off these pull-ups if you have other SPI sensors attached.

7. Build your datalogger code
A good place to start would be Tom Igoe’s analog pin reading example at Arduino.cc  ( be sure  const int chipSelect = 10; for the build described in this tutorial) and there is a nifty little function  for automatically creating log file names over on Adafruit’s site.  For something a little more advanced, I have prepared a basic data logger script that puts the data logger to sleep and wakes it up again based on timed alarms from the real time clock. These are all just starting points for you to add to as you learn more about programming an Arduino.

The first major thing to tackle to achieve a decent operating lifespan is buffering your sensor data to the 100 kHz 4k EEprom on the RTC board. Examples of the code I use for this eeprom buffering is embedded the I2C eeprom tester example I’ve placed on gitHub. You don’t want to wake up the card more than once per day, since that operation draws up to 100mA, for 200-400ms, which is more than any other run-time power use on the logger.  Older Sandisk cards (<1Gb) tend to have lower sleep currents around 70µA, but it’s not unusual to see newer large capacity cards drawing ~200µA or more during sleep.

You can extend that power saving strategy by adding a larger 32K AT24C256 EEprom to the logger platform. Because all I2C devices use the same bus wires, simply add the four EEprom connections to the bundles as you make them:

In addition to saving power by reducing the number of SD write events, the larger EEprom also extends the operating lifespan by allowing you to safely accelerate the I2C bus to 400khz, and this can reduce run-time duty cycle length significantly.   Because all I2C bus devices are connected in parallel, simply add the four EEprom jumpers to the appropriate wire bundles as you make them.  Alternatively, you could add a high resolution temperature sensor like the TMP102, or the MCP9808 to the logger in exactly the same way.

To fully optimize your code it is helpful monitor current during “logging events”, but this usually requires an oscilloscope.  In this post, I have outlined a method to use an UNO with the serial monitor built into the IDE to capture and display these brief events.

Addendum 2017-06-22

I wouldn’t bother for just one or two builds, but Dangerous prototypes just introduced a Dirty Cables service to order custom cables. Be interesting if I could build something like this logger interconnect wire with their tool. Might be a good way to throw a few kits up on Tindie…

Build Instructions – Part 3:  Sensors & Housing

Note: This is the third tutorial in a series providing detailed build instructions for a DIY data logger based on a Pro Mini style Arduino board.  Part 1 of this series covered preparation of the 3 core components for assembly, and Part 2 described connection of those components into a functioning logger. If you are landing on this page for the first time, it might be a good idea to start at the beginning of the series.

Status indicator: common cathode RGB LED

1. Test the common cathode LED with a 3v CR2032 coin cell battery by connecting the GND (usually the longest wire) to the negative side of the battery and each other wire to the positive side of the cell in turn. Note which lead is for red, green, and blue.  Usually the red lead is the one closest to the FLAT side of the led, and the ground pin is the longest lead but there is variation between manufacturers, etc.

2. Trim and add a 30 K ohm limit resistor to the GND line. On some RGB LEDs, green is 2-3 times brighter than the other colors, so you can add a second 20-30K ohm resistor to the Green LED line if you power budget is tight, although this will make the green indicator hard to see in full sunlight.

 

3. Cut 4” lengths of Red, Green, Blue, and Black wire. Solder them to the respective LED connections and use shrink wrap to protect the joins. Carefully bend the leads to a 90 degree angle to facilitate mounting in the housing cap later.

 

 

4. Trim the LED wires to the same length, then strip, twist and tin the ends.  The LED wires will be connected to the male side of a black WSD1241 Micro 4B plug connector, maintaining the same R, G, B, & GND pattern used on the corresponding logger side connector.  Make sure that the other side of the connector is attached while you are soldering so that the plastic doesn’t melt out of shape.

 

You can connect and test your completed LED with your logger platform running the Blink sketch, by replacing the default pin number 13 in the code with 4, 5 & 6 respectively.

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I2C Sensor Breakout Boards

Many sensors are available on I2C breakout boards. This guide shows pictures of several including an Adafruit MCP9808 temperature sensor and a MS5803-05 pressure sensor, but the four standard bus connections (VCC, GND, SCL & SDA) would be the same for any I2C sensor and they all get connected in parallel. The DS3231 RTC breakout board in this build puts 4.7k pull-ups on the SCL & SDA lines that we tap at the cascade port. So if you are using multiple sensors you may need to remove the pull-up resistors from some of your sensor boards because those resistors also end up in parallel, and this can place too much pull on those bus lines for the sensors to overcome. If you are only adding one sensor to your logger,  you can usually get away without bothering to remove them.

1. Cut 3-4” lengths of Red, Black, Yellow, and White wires. Strip, twist and solder those wires to red=VCC, black=GND, yellow=SCL, and white=SDA holes respectively. I find that this is much easier to do with the board in a rubber footed panavise, and the wires supported by ‘helping hands’. Where space is limited, I sometimes mark the wires with a sharpie so I can keep track of which leads go to which sensor from the underside of the pvc cap.

2. Trim and clean the soldered side of the board with isopropyl alcohol to remove flux residue. Trim the free ends of the wires to the same length, and strip & tin the ends. We will be adding a 4 pin deans connector later, but for now leave the ends of you sensor wires free. The sensors will be tested once the wires are inserted through the housing cap, and after the sensors pass the testing stage the connector will be attached to the leads

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Mounting sensors on the housing

This part of the build should be done in a well ventilated area (ie: outside or in fume hood if you have one) as pvc solvent fumes are toxic. Make sure your sensor boards have been completely cleaned of any flux residue with q-tips and 90% isopropyl alcohol before potting them!

1. Once lead wires are attached to your sensors, select 1cm tall pvc rings of sufficient diameter (1” dia. & 2x ¾” dia. shown here) to contain your sensors and arrange them on top of a 4” PVC drain cap. Often it is handy to leave room for and extra sensor well, as you may decide to add another sensor to the cap later.

 

2. Roughen the surface of the cap and the inside of the PVC rings with sandpaper to provide some tooth for the epoxy that will be used to pot the sensors later.  Sand the lower edges of the pvc rings so that they  are level and form a tight fit with the top surface of the 4” end cap.

3. Clean both surfaces, and give the two surfaces that will be brought together a light coating with clear pvc primer, then put a thin bead of pvc cement on the lower surface of the pvc ring and bring it into contact with the previously primed area on the large end cap. It is important that you use ‘just enough” of the primer and pvc cement to bond the rings. If you apply more than a thin bead, the solvent will fail to bond.

Very quickly after applying the pvc solvent cement to the ring, put it into place on the surface of the end-cap. You will need to apply firm pressure to each ring with your fingers for about 2-4 minutes until the ring is bonded to the cap surface. PVC cement usually cures to reasonable strength in about 30 minutes, but I like to leave my solvent welds to set over night because residual PVC solvent can interfere with the curing of the potting epoxy and/or turn it an ugly yellow color.

4. Once all of your sensor well rings are in place you will need to drill holes for the wires to pass through the cap for the indicator LED and sensor wires.  A 7/32” bit produces a hole slightly larger than a typical LED, allowing you to insert the led from the inside of the housing.

This size hole is also a reasonable size to accommodate the wires from you sensor breakout boards.

 

 

PVC is generally a very easy material to drill, but beware that the cap can be wrenched out of your hand violently if the bit bites into the cap the wrong way. Also be sure to wear eye protection whenever you operate a mechanized shop tool. Drill bits often break while you are using them, and then the little bits of metal go flying in all directions. If you have never operated a drill press before, seek guidance from someone with experience.

5. With the LED leads inserted through the cap, neatly strip, twist & solder together the four corresponding I2C bus wires at their ends. To these exposed wires attach the ends of an alligator lead adapter cable (photo below) that has your standard I2C bus connector on one end. It’s a good idea to make some of these cables for all of the connector styles you are using on your project so you can connect raw sensor wires to your logger platforms for testing.

6. With the logger connected to your computer via a USB-UART adapter, run a generic I2C bus scanning utility like the excellent multi-speed scanner by Rob Tillaart to see if the devices are reporting an address on the bus. Then run test utilities appropriate to your individual sensor to confirm that your sensor boards are operating normally and delivering data.  The software you use will depend on which sensors you are testing. Libraries are often provided by better sensor vendors like Adafruit & Sparkfun, and the Arduino.cc forums are a good place to look for useful test utilities that others have shared.

It is essential that testing be done at this stage BEFORE you permanently pot the sensors into place with epoxy!

7. Once the sensors are confirmed working, you can solder them more permanently to the male side of your connectors. I usually use the red Deans Micro-plug connectors for the I2C bus lines from the cascade port on the RTC so that I don’t mix them up with the black connectors I use on the LED lines. Be sure to follow the same connection pattern that you used previously. I like the Deans because they are keyed:  I always put the GND and Vcc lines in the same position with these connectors, so that even if I make an error when connecting the sensor cap – no part ever gets exposed to a polarity reversal.

8. The next step is to mount the sensors more permanently on the sensor cap. I do this by potting the sensor breakout board into place with Loctite E-30CL epoxy. (which remains liquid for at least 2 hours) so we first need to seal the wire pass-through holes in the housing.

Cut and knead a pea-sized bead of plumbers epoxy putty until it is well mixed (it will become slightly warm to the touch at that point) and wrap it around the wires underneath each sensor board so that it completely surrounds the wires. Then press the board and the putty wrapped wires into the hole in the housing until it forms a complete seal.  It is important that the sensor breakout board be relatively level, parallel to the top surface of the pvc end cap, so that the epoxy that will be applied later does not accidentally cover the parts of the sensor that need to be exposed to the air. The putty usually takes about 5 minutes to set.  Repeat this procedure for your other sensor boards and let your top surface putty harden (~15min) before proceeding to put putty in the inside of the housings.

9. Cut and knead another larger bead of putting and wrap it around the indicator LED as shown. Before the putty hardens press the led through the appropriate hole from the inside of the housing so that the putty forms a complete seal against the roof of the housing. Smooth the putty until the LED wires are pressed closely up against the roof of the housing and let it set (5min).

10. Cut and knead more beads of putty and press these up against the other sensor wires passing through the housing. The goal is to have the wires laid out flat and smooth, rather than emerging perpendicular to the top of the sensor cap where they would interfere with the logger platform.

Let the plumbers putty cure for 15 minutes before proceeding. It’s worth noting that it is possible to remove this stuff later on if you have a sensor failure, but the results are usually a bit ugly.

11. At this point you have the LED and sensors mounted to the top of the top of the cap, but they need to be potted to become moisture resistant. With an applicator gun and mixer nozzles that allow flow control down to the single drop level, fill the PVC sensor well rings from the bottom with epoxy until the circuitry of the breakout board is covered. I usually use Hysol E-30CL but any epoxy or urethane with very low moisture permeability should work. I even have loggers sealed with JB weld, that survived six month exposures to salt water, but a clear compound lets you inspect the units better as they age.  Let any potting compound completely cure before exposing your loggers to the environment. I usually wait a week or more (while running power consumption tests) before deploying a new unit.

Many sensors such as temperature, magnetometers, and MEMs accelerometers can be completely encased in the potting epoxy and function well. However some sensors, such as those for PRESSURE and RELATIVE HUMIDITY require that certain surfaces remain exposed to the air for them to function properly.

You must be careful not to spill any epoxy onto those sensor areas or they will be ruined!  Fill the sensor wells with epoxy VERY slowly from the bottom and stop when the epoxy just covers the top edge of the sensor breakout board. Sometimes it is helpful to use a thin wire to paint the epoxy over the soldered breakout board traces while avoiding the sensor itself. The image above is from a HTU21 RH sensor and gives some impression of how tricky this can be to do well.  That sensor is approximately 3 mm per side. If any epoxy had entered the small exposed hole at the center of the chip the RH sensor would have been destroyed. (yes, I have lost a few…) Some sensors also require a covering cap, and it is generally easier to use plumbers putty to affix these protective fabric caps into place after the epoxy has cured, rather than trying to do sub-millimeter adjustments with liquid epoxy.

The indicator LED is easier to deal with as it is already sealed inside a clear plastic housing, so it only needs enough epoxy to seal the housing pass through. For sensors on cables, I usually fill the wells to the very top to provide some protection against flexing. You may need to clamp those wires in place so they remain at 90 degrees to the housing while the epoxy cures.  In the first hour or so after pouring the epoxy it’s a good idea to check it periodically and break any large bubbles that rise to the surface with a pin. Slower epoxies often let most of the bubbles escape on their own.

Slower curing epoxies are generally more moisture resistant, but they can take up to 24 hours to harden.  It is worth noting that some epoxies contract while hardening, and I have noticed that this sometimes produces a permanent offset in my pressure sensors.

12. After the epoxy has cured, you complete the logger assembly with a Fernco PQC-104 4″ Qwik Cap rubber boot which will hold the logger platform in place inside the PVC drain cap.

 

The round bottom of this housing is pretty stable when resting on a flat surface, but you can mount the logger more securely by threading a few zip-tie loops around the metal pipe clamp before you tighten it to seal the housing.  Then you can use those loops as tie down points. A 10 gram silica gel desiccant pack tucks nicely under the knockout platform. Get the ones with indicator beads so you know when they have expired.

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Post assembly testing:

1. Test the LED
Open Blink again with the LED connected to the Arduino (Remember to set board and com port). Change the LED to Pin 4. Verify and upload. The red led should start flashing.
Repeat with pins 5 and 6 to test green and blue respectively.

2. Test the I2C bus
I usually use Rob Tillaart’s multi-speed scanner to determine if the devices are responding on the I2C bus:  https://github.com/RobTillaart/Arduino/tree/master/sketches/MultiSpeedI2CScanner
With the scanner running, specify P to return to output only the addresses that have devices, and specify S to run a Single Scan. The RTC breakout board has an AT24C32 eeprom at address 57 and the DS3231 RTC will show up at address 68. Your other devices should report other bus addresses as per their data sheets.

Note:  the AT24C32 is only rated for 100khz operation, but the DS3231 is rated for faster bus speeds up to 400khz. Also note that the I2c address of the DS3231 can be changed by joining solder pads on the breakout board if you have an address conflict with your sensor.

3. Test the DS3231 RTC
I use the library from https://github.com/MrAlvin/RTClib because it allows me to use sub minute alarms during debugging, but there are many good DS3231 libraries out there to use. The setTime & getTime code examples that came with older versions of MrAlvins lib provide a good way to test your RTC functionality.

• Open & upload setTime File/Examples/RTClib/settime (from the link provided above) to set the RTC to your computers current time. Generally I set my computer to UTC before uploading this program so that the logger runs on UTC. The arduino restarts every time the serial window is opened so do not launch the serial monitor when settime is running or you will create a delay offset in your clock time.
• Open & upload getTime File/Examples/RTClib/gettime  (from the link provided above) to check that the RTC has been set. Expect about a 10-15 second offset between your computers time and that read back from the RTC, as a result of the time needed between compiling the code and executing it on the Arduino.

Note: After the time is set you can try to make a sketch that reads the temperature register from the DS3231 with the example provided by Coding Badly at: http://forum.arduino.cc/index.php?topic=22301.0   While the resolution of the DS3231 is a modest ±0.25C, I have been quite impressed with its overall accuracy when compared to other much more capable temperature sensors.

4. Test μSD card communication
Insert a microSD card into the carrier and test it with the CARDinfo utility at: https://www.arduino.cc/en/Tutorial/CardInfo  You will need to change the CSelect variable to pin 10, and I usually have to add #include SPI.h to get CARDinfo to compile properly. After uploading the sketch to the Arduino, information about card size, files, etc. are reported to the serial monitor reported if the card is connected.

Note: While CARDinfo is a very useful testing utility, it depends on the SD.h library that I almost never use any more. I much prefer Greiman’s SdFat.lib for loggers that are deployed as it uses less memory.

5. Test the Eeprom on the RTC breakout board
There is a utility to test AT style I2C eeproms posted by bHogan at http://forum.arduino.cc/index.php/topic,18501.0.html  Set the EEPROM address to 57 and the serial window will return written A through T and 33 through 48 if the eeprom is working.

Note: Because of latency delays, wear leveling, and other factors the SD card often uses a significant amount of the power budget for a data logger. Buffering your data to the eeprom is a good approach to improving your operating lifespan.  If you use page-writes, rather than saving data to individual locations in the eeprom you can store 28 characters of data with the same time delay as saving a single byte (a page is actually 32 bytes, but some gets used in coms overhead & for carriage return characters, etc.). So I think about that 4K eeprom in terms of “pages”, where my first 16 characters of the first page is always the time stamp: “YYYY/MM/DD,00:00”. That means if your actual sensor data only needs 10 characters of space you could store a reading in one “page” of the eeprom which would let you store 128 readings/pages in the 4K eeprom on the RTC board before you had to flush the data out to the SD card. If you have a fairly typical 15 min sample cycle (96 readings per day) you would be “waking up” the SD card less than once per day to write data, as opposed to 96 times per day. This kind of 100 to 1 reduction in SD card write events results in a substantial improvement in the performance of your data logger. I2C eeprom write cycles use about 3mA for 5-6 ms,  and this often keeps your arduino processor awake an drawing power as well (usually @ 4-5 mA). My tests to date with much larger 32K eeproms buffering five days worth of data have only lead to a 10-15% overall improvement in the power budget when compared to one day buffering.  So you do eventually reach a point of diminishing returns with the eeprom buffering strategy, but definitely take advantage of the eeprom on that RTC board if you can.

6. Test your other sensors
This will depend on which sensors you are using, and which libraries you have to work with.

7. The final and perhaps most important test is to check how much current the logger draws while it is in sleep mode. The LowPower.lib from Rocketscream is a good way to do put your Arduino into deep sleep modes. If you want to see that library in action, I posted a bare bones logger script for the UNO based logger tutorial on the project’s Github which will run fine on this build, and that is the most minimal example you are likely to find of logger that sleeps, wakes on alarm, takes a reading, and then goes back to sleep.  A typical logger built with Pro Mini form factor boards will draw between 0.20 to 0.25 mA depending on the attached sensors and how much sleep current your SD card draws.

A logger that sleeps just below a quarter of a milliamp usually runs for about eight months on a set of 3x AA batteries before falling below the 3.4v minimum for the voltage regulator.  If your sleep current is higher than that you should look at whether you have a bad SD card, or if you have a sensor breakout board has a voltage regulator wasting large amounts of power. Since you already have the pro-mini style board delivering regulated 3.3v, try to buy sensors that accept 3.3v directly on their pins so that you don’t waste power with other support circuitry.  Also try buy sensors that can be put into low current sleep modes easily. The really good sensor IC’s sleep automatically if there is no I2C bus traffic so you don’t even have to do anything for them. If you are using analog sensors, try to de-power any voltage dividers during sleep with a transistor or a logic level mosfet.

Also keep in mind that each AA cell in bank of three series cells can only go down to 1.15v before you reach the 3.4v minimum input for your regulator. If you look at alkaline battery discharge curves that implies that you are only going to have access to about 75% of the battery capacity. With 2000 mAh being fairly typical for a AA, that means you should conservatively estimate that your power supply will only deliver about 2000*.75= 1500 mAh. I usually get more than that from fresh batteries, and lithiums give you a slight boost because of their flatter discharge curves, but they also tank very quickly at the end of their lifespan, so be sure you check them before doing any SD writing.  If the power fails in the middle of a data writing event – you loose all the data on the card. 

Bumping these loggers up to 4x AA batteries in series should get you past a year even if your logger is on the high side at 0.25mA sleep current.

8. Program and run your logger

If you have made it through all the tests, then the next step is to run your logger with a basic bare-bones script and then when you are ready, you can dig into the other more complicated code builds from the project.  I’m not going to hold everyone’s hand through all that, however I will add that the content of these tutorial pages by Nick Gammon were immensely useful when I was starting out, and I still refer to them frequently:

1. 1. http://gammon.com.au/interrupts –  on AVR microcontroller interrupts
   2. http://www.gammon.com.au/forum/?id=11504 – on AVR timers and counters
   3. http://www.gammon.com.au/forum/?id=11497 – on Power saving techniques

Once you have digested that information, a good learning exercise would be to start with the bare bones script, and see if you could copy & paste only the parts that you need for your sensor out of the more complicated code examples. Keep doing that, one piece at a time, and eventually you will understand the entire thing. And there are plenty of other great datalogger examples out there if you Google around and many of them were created by people who actually know what they are doing, most especially the moderators at the Arduino.cc forums. 

With all that in mind, Part 4 of this series covers techniques for optimizing power consumption, but there is some more complicated coding, and trickier soldering, so its probably a good idea to build a couple of the basic loggers and get them running properly, before trying all the things in the final tutorial.

Build Instructions Part 2:  Connecting core components on a logger platform

Note: In Part 1 of this assembly guide we covered the preparation of the three main components being assembled here. If you have not seen that page yet you should probably start there.

1. On a 4” PVC knockout cap, arrange your battery holders as close to the edge as possible to maximize room for other parts, without letting them hang off the edge. Mount them in place with double sided tape.

 

 

 

 

2. Join the black and red wires so that the three AA batteries are connected in series. Fold the solder join and the remaining red wire into the gap between the battery holders. Mark and drill 1/8” pair of holes to tied down the wires from the battery pack. Secure the wires to the cap with a zip tie through the holes.
       

3. Place the RTC alongside the batteries with one corner near, BUT NOT EXTENDING OVER the edge of the cap. Mark two places for the M2 nylon standoffs and drill 2mm holes through the knockout cap. Thread the standoffs through the holes and attach them to the platform with a plastic nut on the underside of the cap.

 

4. Place the RTC board on the standoffs and fasten it into place with the battery side down. Then carefully drill two 1/8” or larger holes near the cascade port of the RTC board and fasten the I2C bus connector cable to the platform. Leave some play in the wires to that they are not under tension when the zip tie is tightened

5. With the RTC mounted on the nylon risers, add a thin layer of solder to each of the connector pins on the end of the RTC board.

6. The next step is to use  double sided tape to adhere both the Arduino, and the SD card adapter to the platform. The SD card must not hang off the end of the platform (so it’s a good idea to do this step with the card inside the holder to see the spacing) and the Arduino must not extend off of the platform. Do not push directly on the metal part of the SD card holder or you could damage it.

7. Once the components are in place it is time to start soldering the SPI bus wires to the bent riser pins on the Arduino.  The trick is to hold the wire in place with tweezers, and trim each wire to length while still leaving a bit of play in the wires. Don’t cut them too short or you will not be able to solder the ends in place on the Arduino.

With the colors we used on the SD card adapter earlier:

• Grey wire is the spi CSelect line connect this to pin D10
• Green wire is the spi MOSI line connect this to pin D11
• Orange is the spi MISO line connect this to pin D12
• White wire is the spi SCLock line connect this to pin D13

If you used different colors on the SD adapter board you will have to figure out which of your colors correspond to each SPI line and connect accordingly. (check the wiring diagram)  These can be left without heat shrink as they are tucked under other wires when the platform is completed, so an accidental touchdown is unlikely. Clean the joins with a Q-tip after soldering if there is a great deal of flux residue.

8. Now pinch one of the GND wires from the Arduino and the ground wire from SD Card adapter together and hold them in place over the ground line for the RTC breakout. Cut them both to length, strip the ends and twist & solder both of the two ground wires together.

 

 

 

9. Do the same with the red VCC wires so that the 3.3v regulated power line from the Arduino joins the VCC line and the power wire from the SD card adapter.

Using a pair of pliers to hold the wires in place so you don’t burn your fingers, solder the combined VCC and GND wires to the matching connector pins on the RTC

 

 

10. Now bring the (white) A4 wire from the Arduino to the pin SDA pin on the RTC board. Trim this wire to length, strip, twist and tin the end of the wire. Then solder this wire to the SDA pin.
Repeat this process for the (yellow) SCL wire from A5, and connect the (blue) interrupt pin 2 wire to the RTC pin labeled SQW.

 

11. After drilling a couple of holes in the platform, secure the three Red Green and Blue wires from pins 4, 5, & 6 together with the second black ground line from the Arduino. Also secure the battery power leads from side of the Arduino board with drilled holes and cable ties.

 

12. Trim the LED wires to the same length. Strip and Tin the ends, then solder those the ends to the female side of a black WSD1241 Micro 4B plug in the order R,G,B-GND (with ground on the dimpled side of the connector).

As with the I2C connector pre load the pins on the connector with solder & thread the heat shrink over the wires first. Be sure you are using the female connectors on the Arduino side of these interconnects because they are the ones delivering power.

13. The final step on the main platform is adding connectors to the battery holders and the Arduino input leads. Start by trimming the battery leads to a position about ½ way across the battery holders. These wires are can be fairly thin so reinforce their insulation with a few layers of heat shrink before trying to solder them to the pre-tinned battery connector posts. Before soldering these beefy connectors make sure the male side has been inserted so that the nylon does not soften and let the metal contacts inside shift around when you are applying heat. They really get hot during this process!

14. Now measure the wires from the Arduino against the connector that is already in place on the battery holder side.  Use the same procedure of reinforcing the wires with 1/16” poly shrink wrap before connecting to the main power tabs and adding a final layer of 1/8” 3:1 heat shrink tubing over the connections.  The Arduino should be on the male side of the main power connector.

Before trying to run your data logger from that battery module, it’s a good idea to test the battery holder connections with some AA batteries. If your 3x AA battery module is above the promini voltage regulator 3.5v minimum, you can connect the logger to see if it is still running the blink sketch. (provided you did not remove the p13 led)

At this stage the main logging platform is ready although the RTC clock has not been set yet.

Note: In Part 1 of this assembly guide we covered the preparation of the three main components that were used here. This post covered connecting the core components of the main logger platform, and in Part 3 we will cover mounting sensors onto the housing assembly to finish the data logger build. Part 4 covers techniques for optimizing your loggers power consumption, but some of them are more advanced so its probably a good idea to build a couple of the basic three component loggers before tackling everything there.