There is nothing especially complicated about the device itself. Full schematic could be found here
. Attached to the step is a simple diagram that explains basic components.
It was not trivial to put everything into a small box. Since I wanted to manufacture everything at home, multi-layered circuit design did not quite work. So, I had to fake a multi-layered board with an Arduino shield-like circuit. Blue lines on the attached schematics represents bottom layer, red lines are connections via shield headers.
I used photoresist method to etch the actual board.