Capacitive sensing with ancient keyboards



The Model M keyboard is a designation for a group of computer keyboards manufactured by IBM, Lexmark, Unicomp and MaxiSwitch, starting in 1984.

According to Wikipedia, the many variations of the keyboard have their own distinct characteristics, with the vast majority boasting a buckling spring key design and many having fully swappable keycaps.

As the venerable M keyboards are understandably ancient, there really is no easy method of connecting the device to a modern system. This unfortunate fact prompted a modder by the name of xwhatsit to ultimately build his own controller.

According to Hackaday’s Brian Benchoff, the beam spring keyboards use capacitive switches.

“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,” he explained.

“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. Atmel’s ATMega32u2 MCU is the brains of the outfit, running LUFA to translate the key presses to USB.”

Interested in learning more? Well, you’re in luck, because xwhatsit is selling Atmel based controllers for the Model M as well as the Model F using the same basic circuit.

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