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

Dupont jumper variant of the 2019 Classroom Data Logger from the Cave Pearl project: This version uses solderless dupont jumpers to reduce assembly time to about 1 hour

(Last update: July 2019)

It’s only been a couple of weeks since the release of the 2019 EDU logger build, and we’re already getting feedback saying all the extra soldering that we added to that tutorial creates a resource bottleneck which could prevent some instructors from using it:

“Our classroom has just two soldering stations, and the only reason there are two is I donated my old one from home. So we simply don’t have the equipment to build the logger you described. And even if we did, some of my students have physical / visual challenges that prevent them from working with a soldering iron safely…”

Or goal with the new design was to give students their first opportunity to practice skills that are useful when doing field research. However helping people do science on a budget is also important – so that feedback sent us back to the drawing board.  After a little head scratching we came up with a version that combines the Dupont jumpers we used in 2016, with this more flexible flat-box layout. In the following video, I assemble one of these ‘minimum builds’ in approximately one hour.  To put that in perspective, the fully soldered version takes 2 – 2.5 hours for someone with experience.

Note: After you’ve seen the video to get a sense of where you are headed, it’s usually much better to work from the photos (below) when assembling your logger. Youtube videos make it look easier than it actually is actually is when you are just starting out. So the first one you build could take many hours as you figure out what you are doing, the second will take half as long, and the third one you make usually takes less than two hours.

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

This time reduction involves a few trade-offs, and the bringing the I2C bus to pins A2&A3 leaves only two analog ports readily accessible ( although A6 & A7 are still available if you solder some jumpers). Removing the regulator & battery voltage divider adds ~30% more operating lifespan, but it also forces you to deal with a changing rail voltage as the Lithium AA batteries wear down. The daily variation is usually quite small, but for quantitative comparisons on monthly scales you will need to correct for the change in rail voltage over time if your sensor circuits are not ratio-metric. (Note: Even commercial loggers suffer from battery related effects on the sensor readings, so you might as well learn to address this issue right from the beginning -> batteries are always affected by temperature)  Another proviso is that you have to change a few components compared to the soldered build:       (note: full parts list at the end of this page)

Pro Mini Prep:

First, solder the 6 UART pins & test your pro mini board with the blink sketch.

Remove 2 LED limit resistors – if you can’t find those remove the indicator LEDs.

Remove the voltage regulator with snips. This step means your system voltage will vary with the battery voltage, but you can log that in code without a voltage divider.

Add pin headers to the sides

Bridge the two I2c bus connections with the leg of a resistor. Connect A4->A2, A5->A3.

adding DIDR0 = 0x0F; in Setup disables the digital I/Os on A0-A3 so they don’t interfere with I2C coms.

Technically speaking, bridging the I2C bus over to A2/3 subjects those wires to the pin capacitance and input leakage of those analog pins (regardless of whether that channel is selected as input for the ADC p257). But in practice the pull-up resistors on the bus can handle that at the 100 kHz default.

Screw-Terminal Component Stack:

Add 3 layers of double sided tape so the tape is thicker than the solder pins.

Align RX&TX corner. The GND points on the terminal board may be interconnected via the back-plane & must match ProMini’s GND pins.

Gently rock the Pro Mini back & forth until the pins are fully inserted. ~1 in 10 of the ST shields has bad headers making this insertion v. difficult.

Remove the last three ‘unused’ pin headers to make room for the SD adapter

Remove bottom 3 resistors from the adapter – leave the top one in place!

Separate Dupont Cable wires & click them into a 6-pin shroud.

Color Pattern: Black GND, Purple MISO, Brown SCl, Orange MOSI, Grey CS, and Red Vcc

Use d-s tape to attach SD module to the Screw Terminal board. Make sure the metal tabs are visible on top.

Measure, cut & strip the 4 SPI bus wires on the SD module

Attach Grey to ProMini D10, Orange -> pmD11, Purple ->pmD12, Brown -> pmD13 (NOTE the Nano ST board labels are A0-A3 which does not match actual ProMini pins on this side)

Add three jumper wires to the power line from the SD module, one with male end pin

Strip & twist the power line wires together & add heat-shrink for strain relief

Shortest jumper to RAW input to recruit the orphan capacitor at Vin

Longer red jumper bridges power to other side of the Terminal board.

Add two extra wires to the black GND line from SD, one with male Dupont end pin. Bundling wires like this is easier if you make the wires a bit longer

The GND bundle completes Pro Mini / ST board / SD module stack. The Red & Black jumpers shown here are about 1 inch too short

Note: You could connect the battery holder lead wires directly into the multi-wire Vcc & GND bundles: skipping the 2 jumpers crossing the ST shield.  But those jumpers provide extra Vcc/GND points & the ability to change the battery holder later if you have a battery leak.

I always try to make my Dupont connectors so that the metal & plastic retainer clips are accessible (in this case facing upwards) after the logger is assembled. That way you can diagnose bad wire connection with the sharp tip of a meter probe, and if necessary, pull out & replace a single bad wire in the Dupont connector without taking everything apart.

RTC module:

Remove two SMD resistors from the RTC board with the tip of your soldering iron.

The DS3231 modules often have flux residue – clean this off with 90% isopropyl

 

Blk-Red-Wht-Yel-Blu Cable shroud retainer clips face up & no wire on 32K output.

First tape layer

Next two tape layers

Write the install date on the coincell with a black marker

Optional: After removing the SMD resistors, you can clip the Vcc leg on the RTC chip to force the clock to run from the backup coin-cell battery. This reduces sleep current by about 40%, usually bringing a “no-reg & noRTCvcc” build well below 0.1mA between sensor readings. But the risk is that if you bump the RTC backup battery loose it resets the clock time to Jan 1st, 2000.  A CR2032 can power the RTC about four years but you have to set bit 6 of the DS3231_CONTROL_REG to 1 to enable alarms when running from the coin-cell. This modification also disables the 32.768 kHz output.  Check our RTC page for more information on this clock module.

Final Assembly:

Attach main stack & RTC to housing.

Trim wht & yellow I2C wires  & add an extra wires to jump lines to the BBoard

Attach yellow beside the red Vcc connection, then white next.

The extra jumpers on Vcc, GND, & both I2C lines…

…get patched over to the breadboard so you can add I2C sensors there. Add a 2nd layer of foam tape to the bottom of the board.

RTC power line joins the short red jumper on Vin

Some the box bottoms have slight bowing. If something doesn’t stick, add another layer of tape.

GND & blue RTC alarm line to D2

Tape 2xAA battery holder down with 2 layers of tape. Trim wires to length. Use black 30lb Mounting Tape for extra bond strength.

Battery wires join the black & red jumpers from other side of the terminal board. All 6  of the ‘unused’ screw terminals can be used to make very secure ‘dry’ wire connections like this.

Connections now complete except for the indicator LED. 5050 modules come with pre-installed limit resistors which make them safer for the classroom. But you can also use a raw 5mm LED with INPUT_PULLUP mode.

5050 common cathode RGB LED module attached to pins D3=GND, D4-Bl, D5-Gr, D6-Red. Default code requires ledGND to be D3 – D7

Your Logger is ready!

Now you can test your new logger to confirm all the connections are working:

1. Test the LED – Edit the default blink sketch, setting the digital pin connected to the ground line of the LED to “OUTPUT” and “LOW” in setup.  Since we removed the ‘default” indicator led on the Pro Mini board, you will need to change that in the code to one of the pins connected to the color channels on your led module.

2. Scan the I2C bus – with the scanner from the Arduino playgound. The eeprom on the RTC module is at address 0x56 or 57 and the DS3231 should show up at address 0x68.

If you don’t see those two devices listed in the serial monitor when you run the scan, there is something wrong with your RTC module or the way it’s connected: It’s very common for a beginner to get at least one set of wire connections switched around during the build. With the screw terminal shield this takes only a few moments to fix.

NOTE: Some sensors really need the stability provided by the on-board voltage regulator (most do not). Here is an alternative arrangement of parts for the classroom logger that leaves the 3.3v regulator in place on the ProMini and powers the logger from 4xAAA batteries      (NOTE: this build also leaves out the short red jumper that was used to recruit the capacitor on the raw vin line – since all devices now get connected ONLY to the regulated 3.3v output of the regulator)   The whole point of the way we’ve built the Cave Pearl Logger is flexibility, so you can change components and positions like this to suit the needs of your experiment.

3. Set the RTC time, and check that the time was set – There are dozens of good Arduino libraries you could use to control the DS3231, and there is a script over at TronixLabs.com that lets you set the clock without a library. [ in 24-hour time, & year with two digits eg: setDS3231time(30,42,21,4,26,11,14);  ] The trick with Tronix’s “manual” method is to change the parameters in setDS3231time(second, minute, hour, dayOfWeek, dayOfMonth, month, year);  to about 2-3 minutes in the future, and then wait to upload that code until about 10-15 seconds before your computers clock reaches that time (to compensate for the compilers processing delay). Open the serial window immediately after the upload finishes, and when you see the time being displayed, check that it’s not too far off, then upload the examples>blink sketch to remove the clock setting program from memory – otherwise it will reset the RTC to to that OLD hard coded time whenever the Arduino re-starts -> and the Arduino restarts EVERY TIME you open the serial window.  An alternative method would be to use the SetTime / Gettime scripts from our Github, but you need to download & install a library before you can use them.

4. Check the SD card is working with CardinfoChanging chipSelect = 4; in that code to chipSelect = 10;  Note that this logger requires the SD card to be formatted as fat16, so most 4GB or larger High Density cards will not work. Most loggers only generate 5 Kb of CSV format data per year when they are running.

Adhesive Mounting Bases are an easy way to add attachment points to the body of your logger so you can zip-tie sensor cables, or dessicant packs into place. Cable tie mounts come in many varieties, and go for about 10¢ each at most hardware stores.

5. Optional – If you are running your logger from batteries with no regulator: Calibrate your internal voltage reference with CalVref from OpenEnergyMonitor.  This logger uses an advanced code trick to read the positive rail voltage by comparing it to the internal 1.1v reference inside the processor. That internal ref. can vary by ±10% from one chip to another, and CalVref gives you a constant which will make the rail/battery voltage calculation more accurate. Load the program into your Arduino, and – while the logger is running from USB power – measure the voltage between GND & Vcc terminals with a good quality voltmeter. Then type that number into the serial monitor window and write down the reference voltage & reference constant output on the serial monitor window. Write these build specific #’s inside your logger with a black sharpie, because you will need to add that info to the core data logger code later on.

6. Find a script to run your on logger. For test runs on a USB tether, the simplest bare-bones code ever written is probably Tom Igoe’s 1-pager at the Arduino playground. It’s not really deploy-able because it never sleeps the logger, but it is still useful for teaching exercises and testing your sensors after you set chipSelect = 10;.  In 2016 we posted an extended version of Tom’s code for UNO based loggers that included sleeping the logger with RTC wakeup alarms. Our latest logging “Starter Script” has grown in complexity to ~750 lines, but it should still be understandable once you have a few basic Arduino programming concepts under your belt.

After ~20-30 seconds of kneading to mix the ingredients, you have another 30 seconds to work the putty into place. (it will be rock-hard within ~10 minutes). Be sure to leave yourself enough extra wire/space so that you can open and close the lid easily without bumping or disconnecting anything. This seal is not waterproof to continuous submersion, but it should easily withstand exposure to rain-storm events. This putty is also a quick way to make custom mounting brackets, or even threaded fittings if you wrap it around a bolt (which you carefully remove before it fully hardens)

In the previous tutorial we attach external sensors with a cable gland passing through the housing and epoxying them into a pvc cup. But for an indoor classroom project you could simply drill small a hole through the lid and stick the sensor/module on top of the housing with double-sided tape. That pass-through hole can be sealed reasonably well with plumbers epoxy putty from the hardware store and this putty adheres quite well to both metal & plastic surfaces.  Remember that breadboard connections are very easy to bump loose, so once you have your prototype circuit tested working, its usually best to re-connect the sensors directly to the screw terminals before deploying your logger in the outside world where it could get bumped around. But in a pinch you can secure breadboard pins with a little drop of hot glue to keep them from wiggling around.

Using the logger for experiments:

Logger mounted on a south-facing window and held in place with double sided tape. Here the top surface of the housing was covered with two layers of white label-maker tape to act as a light diffuser. PTFE is an excellent light diffusing material available in different sheet thicknesses. The ‘divot’ on the top of the Plano box is just a bit larger than 55mm x 130mm x 3mm (depth). The “teflon” tape that plumbers use to seal threaded joints can also be used in a pinch. PTFE introduces fewer absorbance artifacts than other diy diffusers like ping-pong balls, or hot melt glue.

Many types of sensors can be added to this logger and the RTC has a built-in temperature register which automatically gets saved with the starter script. The transparent enclosure makes it easy to do light-based experiments. Grounding the indicator LED through a digital pin allows it to be used as both a status light, and as a frequency selective light sensor.   The human eye is maximally sensitive to green light so readings made with that LED channel approximate a persons impression of overall light levels.  Photosynthesis depends on blue and red light, so measurements using those two color channels can be combined for readings that compare well to the photosynthetically active radiation measurements made with “professional grade” sensors. In fact Forest Mimms (the man who discovered the light sensing capability of LEDs in the first place) has shown the readings from red LED’s alone can be used as a reasonable proxy for total PAR (a measurement of all light frequencies from 400-700 nm with a quantum sensor that runs about $400 each)Photoperiod measurements have important implications for plant productivity, as do photo-biologist measurements transmittance through the plant canopy. Chlorophyll fluorescence is another potential application, and the response of plants to UV is fascinating.

The code for using LEDS sensors is from the Arduino playground. This polarity reversal technique does not require the op-amps that people typically use to amplify the light sensing response but it does rely on the very tiny parasitic capacitance inside the LED. (~50-300pF) This means that the technique works better when the LED is connected directly to the logger input pins rather that through the protoboard (because breadboards add stray capacitance) . We have integrated this into the starter script which you can download from GITHUB. I’ve tweaked the playground script with port commands so the loop execution takes about 100 clock cycles instead of the default of about 400 clock cycles.  The faster version was used to generate this light exposure graph with a typical 5mm RGB LED, with a 4k7Ω limiter on the common ground which was connected to pin D3:

Red, Green & Blue channel readings from the indicator LED in the logger photo above.  The yellow line is from an LDR sensor the same unit, that was over-sampled to 16-bit resolution. The sensor has a logarithmic response and the left axis on the graph is a time- based measurement where more light hitting the LED sensor results in a lower number. Note how the RED signal changes more quickly than Blue & Green at sunrise & sunset.  LED’s work well with natural full-spectrum light, but their limited frequency sensing bands can give you trouble with the spectral distribution of  indoor light sources. The peak spectral response is usually around 30–50nm lower than their peak emission wavelength. You may be able to use a floating point mapping function like fscale to linearize your data – depending on the range of output from your particular LED.

Characterizing light absorption and re-emission is also a primary technique in climate science . For example, measuring light intensity just after sunset with LEDs inside a heat-shrink tube pointed straight up can provide a measure of suspended particles in the stratosphere. An “ultra bright” LED has more than enough sensitivity to make these collumnated readings, in fact on bright sunny days you usually have to place the LED/sensor beneath a fair thickness of white diffusing material (sometimes refereed to as ‘Opal acrylic’) to prevent it from being completely saturated.  Older LEDs that emit less light can sometimes be easier to work with because they are less sensitive, so the readings do not go to zero in high-light situations. Other sensor experiments are possible with LED’s in the IR spectrum which can be used to detect total atmospheric water vapor.

One thing to watch out for is that full sun exposure can cook your entire logger: reaching temps above 80°C can cause the batteries to pop or fry the SD card. If you have to leave the logger in full sun, consider adding a bit of reflective film or a layer of aluminum foil around the outside to protect the electronics. Though if you have a light sensor you’ll need to leave a little window somewhere for it to take a reading. 

You might also find it handy to add a few holes as tie-down points,

and it’s always a good idea to add a couple of desiccant packs inside the box to prevent condensation. If you use a desiccant with color indicator beads, you can check whether they are still good simply by looking through the transparent housing.

If you clipped the RTC leg, your logger should pull less than 0.1mA while sleeping. Back of the envelope for Lithium AA’s is about 7 million milliamp-seconds of power, with your logger burning about 8600 mAs/day at 0.1mA.  So clipping the regulator and adding the RTC mod should get you out to about a year before you fall off the “upper plateau” of lithium’s burn-down curve, and out to two years if your sensors don’t use much juice (…and you have a well behaved SD card).  With the RTC power leg still attached you’ll see sleep currents in the 0.16mA range, so at least 6 months of operation before you come close to low voltage shutdown. I’m being conservative here because it all depends on your sensors and other additions to the base configuration. While the LED sensor idea is fun to work with, it’s a very slow method that keeps the logger running for many seconds per reading when light levels are low ->so reading all three color channels will probably cut your operating life in half again. Figuring out how to only take those light readings during the day is a good coding exercise for students that also saves quite a bit of power.


TransparentSinglePixl
Bill of Materials: $20.50
Plano 3440-10 Waterproof Stowaway Box
Sometimes cheaper at Amazon as an “add-on” item.  $4.96 at Walmart and there are a selection of larger size boxes in the series. 6″ Husky storage bins are an alternate option.
$5.00
Pro Mini Style clone 3.3v 8mHz
Get the ones with A6 & A7 broken out at the back edge of the board.
$2.20
‘Pre-assembled’ Nano V1.O Screw Terminal Expansion Board
by Deek Robot, Keyes, & Gravitech (CHECK: some of them have the GND terminals interconnected)  You will also need to have a few small flat head screw drivers to tighten those terminals down.  Since this shield is was originally designed for an Arduino Nano many of the labels on ST board will not agree with the pins on the ‘analog side’ of the ProMini.
$1.85
DS3231 IIC RTC with 4K AT24C32 EEprom (zs-042)
Some ship with CR2032 batteries already installed.  These will pop if you don’t disable the charging circuit!  
$1.25
CR2032 lithium battery  $0.40
SPI Mini SD card Module for Arduino AVR
Buy the ones with four ‘separate’ pull-up resistors so that you can remove three of them.
$0.50
Sandisk or Nokia Micro SD card 256mb-512mb 
 Test used cards from eBay before putting them in service. Older Nokia 256 & 512mb cards have lower write currents in the 50-75mA range. This is less than half the current draw on most cards 1gb or larger.
$2.00
Small White 170 Tie-Points Prototype Breadboard
These mini breadboards for inside the logger are also available in other colors.
$0.60
Dupont 2.54mm F2F 40wire ribbon cable Without Housing
Cheaper if you get the ones with the black plastic shrouds on the ends, but removing those shrouds by hand is slow  work. Each $2.70 cable will let you make 2 loggers, and you”l need a couple of  6-pin connector shrouds.
$1.55
3×1.5V AA Battery Batteries Holder w Wire Leads
If you are using alkaline  batteries, changing to a 4xAA battery holder doubles the run time.
If you are running an unregulated system on 2 lithium batteries, then you can use a 2x AA battery holder.
$0.50
5050 LED module (with built-in limit resistors) 
(Alternatively, you can also use cheaper 5mm diffused LEDs with a  4K7 limit resistor on the GND line that you add yourself)  
$0.75
3.3V 5V FT232 Module
  *Be sure to set the UART module to 3.3v before using it!* and you will also need a USB 2.0 A Male to Mini B cable.
$2.75
3M Dside Mounting Tape, 22awg silicone wireheader pins, etc… $1.00
Comment:   You might need some extra tools to get started:                (not included in the total above)
2in1 862D+ Soldering Iron & Hot Air station Combination
a combination unit which you can sometimes find as low as $40 on eBay.
Or you can get the Yihua 936 soldering iron alone for about $25. While the Yihua is a so-so iron, replacement handles and soldering tips cost very little, and that’s very important in a classroom situation where you can count on replacing 1-2 tips per student, per course, because they let them run hot & dry till they oxidize and won’t hold solder any more.  Smaller hand-held heat guns are also available for ~$15, but they have no temperature control so you need to be a bit more careful with them.
$50.00
SYB-46 270 breadboards (used ONLY for soldering pins on Pro-Mini )
Soldering the header pins on the pro-mini is MUCH easier if you use a scrap breadboard to hold everything in place while you work. I use white plastic breadboards  that they only have one power rail on the side since they do not look like my regular breadboards &  I write ‘for soldering only’ on them with a black sharpie.  
$1.30
SN-01BM Crimp Plier Tool 2.0mm 2.54mm 28-20 AWG Crimper Dupont JST
I use my crimping pliers almost as often as my soldering iron –  usually to add male pins to components with lead wires too thin for safe connection on a breadboard. But making good crimp ends takes some practice.   Jumper wires that you make yourself are always better quality than the premade ones.
$16.00
Micro SD TF Flash Memory Card Reader
Get several, as these things are lost easily. My preferred model at the moment is the SanDisk MobileMate SD+ SDDR-103 or 104 which can usually be found on the ‘bay for ~$6.
$1.00
Donation to Arduino.cc
If you don’t use a ‘real’ Pro Mini from Sparkfun to build your logger, you should at least consider sending a buck or two back to the mother-ship to keep the open source hardware movement going…so more cool stuff like this can happen!
$1.00

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

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

  1. Brian Davis

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

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

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

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

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

    Reply
    1. edmallon Post author

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

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

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

      Reply
      1. Brian Davis

        “…a light activated switch…”

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

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

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

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

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

        Reply
        1. edmallon Post author

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

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

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

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

          Reply

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