Four easily distinguished tones in the vicinity of a kilohertz are coupled to the tip phalanx, one tone per finger. At the tip of the thumb is a probe which the user presses on the skin of the target phalanx. The probe is monitored to determine which frequency is loudest, indicating the finger, and by how much, indicating which phalanx. This assumes that the thumbed finger's tone gets softer and the other fingers' tones get louder nearer the base of the thumbed finger. This approach has the advantage of making phalanx identification relatively insensitive to the resistance between the probe and the thumbed phalanx. This is necessary because the resistance can vary widely with pressure of the thumb and dampness of the skin.
Obvious choices of tones are the low group frequencies (697, 770, 852, and 941 Hz) or the high group frequencies (1209, 1336, 1477, and 1633 Hz) of the Dual Tone MultiFrequency (DTMF or TouchTone) standard used in telephony. These two groups have the advantage of universally available circuits for their generation and detection.
This scheme requires only five wires between the hand and the encoder: four for the tones to the fingers, and one for the probe on the thumb. No separate ground wire is needed.