Wall Switch Insertable Node is a board which you can insert into basically every wall switch.
Small size - only 31x49mm with 8.5mm height! See drawing.
CR2450 battery holder. I haven't completed all tests but hope this board will work about 5 years from battery.
Very clever, cheap and micropower switch hardware controller. You can use up to 4 switches and buttons (both tacts and ordinary ON/OFF switches). Every channel has hardware debouncer (so you do not need software debouncing). Every state change of every switch triggers interrupt 1 (pin D3). Switch controller circuit has very low power consumption (less than 1uA). See schematic.
Onboard divider to measure battery level.
Onboard ATSHA204A (optional)
Standard UART header
JST (Japan Solder less Terminals) to connect switches.