The Addacom is a little black box with four pushbuttons which sits next to the master station of a 2-way intercom. If an extension calls in, a light emitting diode (LED) lights up on the Addacom and then you press the corresponding button to talk to that extension.

In reality, the Addacom consists of the bank of four pushbuttons and a little circuitry to indicate which extension is calling the master station.

Before we describe how the Addacom works, we need to have a look at the circuit function of a typical 2-station intercom. There is really very little to them.

Most push-to-talk types consist of a master and one or more slave stations. The active master station unit contains an amplifier, a small loudspeaker, a battery and a switch. The slave station is much simpler, consisting of another small loudspeaker, a switch, capacitor, a diode and a light emitting diode (LED)

The small loudspeakers do double duty, acting as microphone or loudspeaker, depending on whether the stations are talking or listening. The basic intercom circuit is shown in Fig.1.

We have one pair of wires and one amplifier (IC1) so only one end can speak to the other at any given time. Switch S1 at the master station swaps the connections to the two speakers to allow for two-way communication.

The telecommunication term for this is "half-duplex" – one station can listen while the other talks and then the first station can talk while the other listens.

Telephones are actually "full duplex" devices since both parties can simultaneously talk and listen. The other type of communication is "simplex" and is one-way only, eg, broadcast TV, radio etc. You cannot talk back (swearing at the TV when the ad break comes on is not counted!)