I’ve taken apart an old keyboard to scavenge the analogue to digital converter and the USB interface (the green board in the picture below). I followed parts of this excellent tutorial to get me started on this project.
Keyboards are rather simple circuits. There two groups of pins (top of the green board above) and every key is connected to one pin from each group. When a key is pressed, a circuit is closed between a unique combination of pins and the chip interprets that pin selection and sends the appropriate signals via the USB interface.
So far, I’ve soldered connector wire from both pin sets to a project board. I selected four from one side and seven from the other, for a total of 28 unique combinations, which should be more than sufficient. I’m thinking of having 12 buttons on the guitar neck and a few on the guitar body (including the strum button). I’ll connect buttons to the project board in columns, according to their pin assignments.
Next step: finding switches!