The pen is mightier than the sword, but the IBM Model M keyboard, properly applied, can knock teeth in. There are a few more IBM keyboards even better suited to blunt force trauma – the extremely vintage beam spring keyboards made for terminals and desktop publishers. Being so very old, there’s no easy way to connect these keyboards to a modern system, so when [xwhatsit] wanted to make his work, he needed to build his own controller.
The beam spring keyboards use capacitive switches, and with 122 keys, the usual method of reading capacitance – putting a capacitor in an oscillator – would be far too slow to be of any use in a keyboard. There is another method of reading capacitance: measuring the current going through the capacitive switch. This can easily be accomplished with an LM339 comparator.
[xwhatsit]‘s keyboard controller uses this capacitive sensing circuit to read the four rows of keys, with a few shift registers taking care of the columns. An ATMega32u2 is the brains of the outfit, running LUFA to translate the key presses to USB.
If you’re lucky enough to have one of these ancient keyboards, [xwhatsit] is selling a few over on the usual mechanical keyboard forums. There’s also a controller for the Model F keyboard using the same basic circuit. If you need one just drop him a line or grab the gerbers and roll your own.