144 keys on an Arduino MicroThe Arduino Micro (not the Pro Micro) has 24 usable data pins. With this you can make a 12x12 matrix.
Two of the pins on the Arduino Micro are connected to LEDs (PB0 and PC7 used as indicators when used with the Arduino IDE). This may cause problems if using them as inputs, as outputs they should behave normally in a switch matrix.
The PCB is very basic, I used the reversible switch footprints so that the board can be flipped and the Arduino Micro installed right side up or upside down depending on which way the board is oriented. Having the Arduino Micro on the left allows access to the reset button.
The largest matrix you can do with a Pro Micro with its 18 pins is 9x9, 81 keys. I made a handwired one here.
unquadquad gerber files on github. Firmware.
DSA keycaps. Not really sure of what the layout will end up being.
Bottom of the populated PCB. Lots of standoffs.
Using a clone Arduino Micro. This one is a total redesign. The pinout is identical, but it has components on only one side and is much more low profile.
144 Gateron clear switches.
6mm M2 spacers and screws. A genuine Arduino Micro is much taller and would need taller spacers.
Bottom blank PCB with 3M bumpon silicone feet.