Electronic Puzzle Board

For a recent geocache, which is a tribute to the well-known Tetris game, I wanted people to solve a simple physical puzzle. As a reward they get a code that they can use to open a box. So, what do you have to do to make things work? And how do you make sure that you do not have to change batteries for the next 20 years? Here is the result:

The short story is: Use Neodymium magnets in the puzzle pieces and Hall sensors in the board. Connect these to an Arduino and sense whether the pieces are in place. Well, when you then want to make sure that all this only requires minimal supply current, then things become tricky. I reduced the quiescent current to 800 nA. When the board is active, i.e., it has been moved or somebody is solving the puzzle, then the electronics need on average 12 µA. Using these numbers (and a Li-SOCl2 battery), I calculated that the puzzle should live for 20 years and the puzzle can be solved more than 40, 000 times. However, you need to flash the board with a new bootloader and you have to deactivate the BOD fuse. Here is a picture of the final box before I closed it:

If you want to read the long story, then you should read my blog post.