Hackaday is one of my daily websites. Always entertaining and educational.
Now and then they have some challenges with cash prizes. They have just wrapped up one such challenge…. Build something that uses a coin cell battery.
Out of all of them, I like the light level Geolocator. Very clever software.
It uses the amount of light and dark in a 24 hour cycle and figures out where in the world it is.
Anyway, if you are into electronics and squeaking out as much power as you can from a coin cell, well worth taking a look.