Short-term data storage in between wake cycles
I made a temperature and humidity logger a while ago. It used an Atmega328P on a custom board. Logging is done to an SD card with a level-shifting interface board.



That runs from 3 x AA batteries. I think I've only changed the batteries over once in the last 3.5 years.

The LED display is only occasionally used, to tell you the temperature right now if you press the red switch (plus other stuff like the date/time, the count of writes to the SD card, and the number of errors).

According to my calculations on that page when asleep it uses 6 µA, and averaging out the time taken to write to the SD card that adds 36 µA extra. Still, 42 µA is below the self-discharge rate of the batteries.

From time to time I pop the SD card out of its holder and download the resulting data.

The overall schematic is here:



Code and other details are available on the page I mentioned above.