![]() ![]() Despite dire manufacturer warnings that reverse currents beyond 1♚ will heat manganese-dioxide/lithium cells until they explode, the ones I’ve used so far survive the abuse without issue. But I’ve yet to have a single problem (or even any detectable warming) after many days with loggers connected during development. Bridging VCC to Vbat also means a 3.3v UART will push some sub-milliamp reverse currents through an older cell. RTC current can spike as high as 650♚ every 64 seconds when new temperature readings occur. However the chip will still consume an average of 3♚ through VBat to keep the oscillator, temperature compensation & comparator logic working. Try to use ‘-N’ or ‘-SN’ chips if you can get them.Ĭutting the VCC leg depowers most of the logic inside the DS3231. We’ve had several batches of those over the years where the temperature register was off by 5☌ or more. Watch out for the ‘-M’ variant of the DS3231. Three mods to the RTC module: Running from the Vbat also disables 32KHz output so I usually clip that header pin. So let’s just build one and see how it goes… In addition, it’s not unusual to see a 50mv delta at the battery terminals for every 5☌ change in ambient so a standard lithium coin cell will not power the logger below 0☌.īut if theres one thing I’ve learned on this project it’s that datasheets only tell you so much about system behavior in the real world – especially with stuff constructed from cheap modules carrying half a dozen unspecified bits. That voltage drop increases over time because the internal resistance of a coin cell is only 10 ohms when new, but approaches 100 ohms by end of life. ![]() And most garden-variety memory chips have a lower limit of 2.7v – so a nominal 3v CR2032 can only be allowed to fall about 250mv under load before we run into trouble. But a power budget that small will necessarily add complexity to the base code, which must minimize run-time even though EEproms are notoriously slow devices. Getting rid of power hungry cards also opened up the possibility of running the entire unit from the coin cell on the RTC module. Groundwork for this change was already in place with our use of an EEprom to buffer data so that high-drain SD saves only occurred once per day. With just three core components as our starting point, the only hardware option was to remove the SD card. And the loggers run fine under mineral oil for deeper deployments. We pressure tested the centrifuge tubes: 50mL tubes can be deployed to 10m depth, 30mL tubes can go to 20m. That new baby is now ready for release with data download & control managed through the IDE’s serial monitor window. So with all that in mind, we’ve continued development of a ‘lite’ version of our logger with the lowest possible prep time. And thanks to COVID chip shortages, modules that were only 99¢ at the beginning of this project could now set you back $5 each. While that’s not unusual for university-level lab based subjects it is something of a stretch for high school teachers. (only 1/2 that is needed for in-person courses where the students pin & test the parts themselves). However an instructor still needs to invest about five days ordering parts, testing components, and preparing kits for a 15-20 seat course being run remotely. The EDU build we released in 2020 provides remarkable flexibility for courses in environmental monitoring. intervals) and that’s easily extended with $1 memory chips or modules. ![]() The 4K EEprom on the RTC board will hold 4096 1-byte RTC temperature readings (~ 40 days worth 15 min. ![]() With a bit of practice, soldering the Pro Mini & RTC together takes ~30 minutes. This ‘two-part’ logger fits nicely inside a 50mL Falcon tube. ![]()
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