excellent find... thanks ripster, and talis
i'm thinking of an alternate technique, which i believe model f uses:
when a hammer is too far away from its pads, there's too little capacitive coupling between a pin generating a test pulse and an input pin (both on the microcontroller)... so the test pulse is too weak to be detected.
but when a hammer is close enough, coupling is strong and a pulse comes through.
where's talis, to debunk this theory?