It’s only been a couple of weeks since the 2019 classroom logger was released, and we’re already getting feedback from instructors saying the extra soldering that we added to that build creates a resource bottleneck that might prevent some of them 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 vital for 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 soldered version takes 2 – 2.5 hours.
Note: After you’ve seen the video to get a sense of where you are headed, its usually much better to use the photos (below) when building your logger. Videos make it look easier than it actually is when you are just starting out.
This variation of the 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. Wire connections are made simply by twisting stripped ends together and clamping them under screw terminals.
This time reduction involves a few trade-offs, and the bringing the I2C bus to A2&A3 leaves only two analog ports readily accessible ( although A6 & A7 are still available). 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 age. That daily variation is quite small, but for quantitative comparisons on monthly scales you may need to correct for the change in Vcc (included in the data file automatically) over time. Another proviso is that you have to add a few more components to the part list from the soldered build:
- ½ x Dupont 2.54mm F2F 40wire ribbon cable Without Housing $1
- 2x 6pin connector shrouds $0.10
- 5050 LED module $0.75 (though you can still use regular LEDs with a 4K7 limit resistor)
- 2xAA battery holder (replaces 3xAAA in the earlier tutorial)
- Pre-assembled screw terminal boards cost $1 more
This pushes the cost for a complete unit close to $20 (before adding sensors) and the required lithium AA batteries are also more expensive.
Pro Mini Prep:
Screw-Terminal Component Stack:
|Note: You could connect the battery holder lead wires directly into the multi-wire Vcc & GND bundles: skipping the 2 jumpers to the other side of 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.|
Since this build is a variation of the soldered version, so you can test your new unit using the procedures at the end of that post. Also see that tutorial for a description of how we attach external sensors with a cable gland passing through the housing. Of course, 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. Remember that breadboard connections are very easy to bump loose, so once you have your prototype circuit working, re-connect the components directly to the screw terminals before real-world deployments.
Using the logger for experiments:
Many types of sensors can be added to this logger and the RTC has a temperature register which automatically gets saved with the starter script. The transparent enclosure also makes it easy to do light-based experiments. Grounding the LED through pin D3 allows it to be used as both a status indicator, and as a very capable 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) has shown the readings from red LED’s alone are a good proxy for total PAR. Photoperiod measurements also have important implications for plant productivity.
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. That method was used to generate this light exposure graph with a typical 5mm RGB LED, with a 4k7Ω limiter on the common ground connected to pin D3:
Characterizing light absorption and re-emission is one of the primary climate science techniques. 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 collumnated readings, and on bright sunny days you usually have to place the LED-sensor beneath a good thickness of white diffusing material to prevent it from being over-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. Chlorophyll fluorescence is another interesting application, and the response of plants to UV is fascinating.
One thing to watch out for is that full sun exposure can cook your logger, so consider adding a bit of reflective film to protect the electronics. Add a couple of desiccant pouches to control condensation. You might need to add a few holes to the housing as tie-down points: