Using my favorite CAD software, I drew up the initial schematic. As you can see, the circuit itself is pretty small, but this is good. Fewer components means smaller package and lower battery usage. There are also still pins left over on the chosen micro controller and if I want to add a USB functionality later it can be done with minimal changes. In a future revision I may also add a recharge circuit for a standard LiPo battery.

One of the ideas that I had was to build the circuit itself using the smallest components possible and building them into the actual battery enclosure.