Here’s a cheaper and easier method of making a telephone headset adapter. All that’s required is a cheap headset ($5 at Harvey Norman), a DPDT switch and a few connectors. Each transducer in the headset measures about 40O. There is also an inline volume control measuring about 500O per leg, across which each earphone is connected. This means that each earpiece has a minimum resistance (at maximum volume) of about 37O. 128O is the desirable impedance. This can be achieved by wiring the earphones in series and adding a 56O resistor. Although there is a reduction in maximum volume due to the resistor, this was easily accommodated by the author’s telephone, which has an amplified audio output.
Circuit diagram:Cheapskate's Headset Adapter Circuit Diagram
The headset does not have a connection between the earphones and the microphone, so no other modifications were required. Series connection of the earphones is achieved by not picking up the sleeve connection at the socket and connecting only across tip and ring. As the author’s telephone uses an electret microphone in its hand-piece, no additional biasing circuitry is needed for the headset’s microphone.
Author: Brian Critchley
Copyright: Silicon Chip Electronics