The original note, as written at the time
A wireless keyboard, and possibly the click functions of a mouse, that would operate this way: when a user presses a key a unique barely audible sound is generated and is picked up by a high precision microphone at the computer. The information indicated by the keyboard’s input is then read and interpreted by the computer. The advantage is that you could have a wireless keyboard that does not need any batteries.
Ongoing development
A wireless keyboard, and possibly the click functions of a mouse, that works by sound.
When a key is pressed it generates a unique, barely audible sound, which a precision microphone at the computer picks up and interprets. The advantage is a wireless keyboard with no batteries to change.
The battery is the objection everyone has to wireless peripherals, and it is a legitimate one that nobody solves because the solution is uneconomic. A keyboard that harvests its own energy — from the keystroke itself — is a keyboard that never needs charging, so it never needs a charging port, so it can be sealed, so it can survive a workshop.
Acoustic signalling is the cheap version. It needs no radio, no pairing, no licence, and the microphone is already in the laptop. It also has a lovely property: the keyboard is not transmitting, it is being listened to, which means the receiving device can be told to listen or not.
The weaknesses are real and worth naming. Every keystroke must be distinguishable from the ambient sound of a room, which means either a narrow frequency band or a very quiet environment; and the moment two keyboards are in the same room, the machine hears both. There are answers — a per-key chirp with error correction, a mat under the keyboard, a pairing handshake — but they all cost more than a battery.
The version I would actually build is hybrid, and it is the most interesting part of the idea: harvest the energy from the keypress and send the signal over radio only on press. No battery, no listening problem, no ambient noise. The acoustic idea is the elegant one; the harvesting idea is the one that ships.
Still — a keyboard that power itself from being used is correct in principle. It is a machine that pays for itself by working.