On May 3 we made the official third visit to the vernal pool we are monitoring. During this visit we are supposed to collect the frog call recorder and do the final surveys for amphibian eggs and aquatic invertebrates. At the second visit we failed to find any fairy shrimp, so I was looking forward to adding them to my crustacean life list. I was also looking forward to collecting the microSD card from the data logger we had installed on April 17 and examining two more weeks worth of data. And at the last minute I decided to bring a 24 foot-long pole and get some “aerial” photos from above the pool. By the time we left the pool to hike back to the road, I was dismayed at my lack of success at most of these goals.
When I extracted the data logger from under the jug and inside the peanut butter jar, the LED on the Adafruit Feather was blinking. I knew that was a bad sign and probably meant that no data were being saved. I swapped out the microSD card and restarted the logger and it seemed to be operating normally. I wouldn’t know until I got home whether any data had been saved in the time since we deployed the logger two weeks earlier.
At home I found that the data logger had indeed stopped working, but not until the morning of this visit. The logger had flawlessly saved data every 30 minutes for 16 days and then stopped a few hours before we arrived (as usual, my skills are generally indistinguishable from luck). I guessed that the problem was battery power. The battery pack of three rechargeable AA batteries supplies a nominal 3.6 volts, but as the batteries run down the voltage drops. When the microcontroller (Adafruit Feather) gets insufficient voltage it resets itself. I think it was constantly starting and stopping which would explain the blinking LED. A possible fix was to replace the rechargeable batteries (1.2 volts apiece) with alkaline batteries (1.5 volts apiece). So an emergency mission back to the pool was planned.
One possible success I had on May 3 was to deploy the third version of the data logger. This version was a test to see if I could reduce the cost of materials from about $50 to $35. It does the same thing as the first two loggers but is controlled by an Adafruit ItsyBitsy M0 Express instead of the Adafruit Feathers that I happened to have lying around the house. Having two loggers running side-by-side provides some information about the precision of a single logger, and also provides backup in case of (apparently highly probable) failure.
So the first failure of the day, caused by insufficient voltage in the data logger, did not result in as much data loss as I had feared. The second failure also happened because of low battery voltage when I tried to photograph the pool with a camera at the top of a 24 foot pole. The pan-tilt rig which was pointing and shooting the camera needed higher voltage to prevent brownout and resetting of the microcontroller (yeah, they are everywhere). On May 6 when I returned to replace the batteries in the data loggers, I had better batteries in the pan-tilt rig and overcame that failure too (stay tuned for the stitched aerial panorama)[here it is].
The third failure might stay with me for a while — I still have never seen a fairy shrimp.
The Arduino sketch running on the ItsyBitsy is here.