Modular keypad/keyboardThe basic unit is a 4x4 matrix with 16 keys. Up to 4 of these can be connected to each other side by side.
4x4, 4x8, 4x12 and 4x16 are possible.
There are pads for header pins on each side that complete the circuits from board to board. These can be permanently connected with solder bridges or with pin headers and shunt jumpers.
The PCB is a perfect square. Exactly 3 inches square. Standard switch spacing is 0.75 inches, 19.05mm. The header pins are the common 0.1 inch spacing (2.54mm).
The Gerber files are on github.
It is running TMK. The 4x16 layout is here on github. For 4x4, 4x8 and 4x12 you can either modify the matrix or just ignore the columns on the right that are unused.
Arduino Micro on the right. It is right side up so that the reset button can be pressed.
Arduino Micro (not a Pro Micro). The Arduino Micro has an odd number of pins and I had to cut down two different sockets to fit it. If I had a 40 pin socket I could have just cut off pins from one end to make a 34 pin socket.