keyboard interface

 

I went to Good Will today, and found myself a suitable USB computer keyboard.  Upon taking it apart, I was pleased to find that it contained a nice, small control circuit board (as seen above) which was easily separable from the matrix of keyswitches.

I'm using the first three number keys, '1', '2', and '3'.  Fortunately, these turned out to all share the same row scan-line in the matrix, with three different column lines.  So, a total of four wires are needed, a single "common" plus the three switch lines.  Closing any of the switch lines to common causes that corresponding character to be produced on the USB output.  I couldn't have built any smaller or simpler of an interface myself, with an Atmel MCU or similar, and all the work is already done here (other than tracing out the right contacts and soldering wires into place, a non-trivial and fairly delicate task, but now it's done).

The control panel with the buttons which engage with this interface, will actually be tethered by a 6-conductor wire (or more -- I might use some CAT-5 cable if that's convenient).  There are four switching wires, but none of them is ground, so I'll also need ground and +5V power, for the purely-for-looks purple LED illumination (gotta have that!).