Inside is Arduino Pro Micro and a PMW 3389 optical sensor on a breakout board. [Ben] was going to use flexible 3D printed panels as mouse buttons, but then had an epiphany — why not use keyboard ...

Arduino Micro has some extra leds that it is tricky to programme. Like almost all Arduinos, has a built-in led – the green one on the left in the photo. And it has a pair of yellow leds – called TX ...