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 ...

My daughter likes buttons. She's learning letters. I was bored over winter break and had these parts laying around... so I built her a "Button Box" based around an Arduino. Open source, hackable ...

Except, on the Micro, there is no actual serial bus involved in programming the Arduino, as everything is done over USB. So the TX and RX leds are driven by a couple of pins on the ATmega32U4 (see a ...

There’s also an Arduino Pro Micro for the keyboard matrix, which is really two 4×6 matrices — one for each half. There’s a 6cm fan to keep things cool, and one panel is devoted to a grille ...