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 ...

So the TX and RX leds are driven by a couple of pins on the ATmega32U4 (see a comment on the hardware connection below) to make it look like they are on the serial bus. They were not intended to be ...