Just three keys may be useful, but seems like a very simple thing, so I decided to try and also make a more advanced version. This time it's a full keyboard, using the smallest switches I have in my drawer.
I'm not sure if I will have the stamina to actually assemble it, but since I was ordering the PCB for the 3-key version anyways, and the main cost is shipping, I decided to throw this design in.
On the back there is a single atmega to do the scanning of the matrix. Since there is no way I could fit diodes for every key in there, they keyboard will have ghosting effects. I added a separate column just for the special keys, like shift, function and so on, to mitigate that a bit. It's going to talk to the parent board over serial.