@terryip I haven't figured out how the response would work exactly yet, but I'm thinking some kind of simple RF transmitter that (if the key is pressed) "reflects" back a signal at a specific frequency, to be detected by a base station.
@[email protected]@terryip Hmm, if we had a grid of magnets with coils around them, and each key-unit had a magnet and a coil connected to a key that's normally open, I think the base-unit could detect key presses purely by how power usage of individual coils would spike.