Originally published by Paul Stenning in Electronics in Action, April 1994

• Diagrams and Schematics

I needed a simple intercom system, so that I could keep in touch with civilisation when I was hiding in my workshop! My partner was getting rather fed up of making journeys just to ask me if I wanted a cup of tea, or to tell me that the editor of an electronics magazine was on the phone!

For this application a separate "call" function is not necessary. Instead I opted for a short tone when the talk button was pressed. This is to get the other person's attention, rather like the "bing" sound on the announcement systems in public places. I was originally hoping for a "peep" sound, although it ended up more like a "pweee"!

There is a talk button at each end, so it's impossible to eavesdrop on the other party - unlike some cheap commercial systems. However if the remote talk button is replaced with a normal switch, it can be left on so that the system may be used as a baby monitor.

I already had a twelve-core cable between the workshop and the living room, carrying audio from the hi-fi, serial computer data and a telephone extension. There were just two cores left, hence this design for a two-wire system. Any convenient two core cable can be used, thin loudspeaker cable is ideal.

What - No Chips!
I built this unit using bits from the "Junk Box", and assumed others might like to do the same. If you don't have the right chips, there's no option but to buy them. However if you don't have the right transistors, you can usually find something similar, and the same applies with most of the other components. Since the circuit doesn't have to do anything exacting, a simple transistor design is ideal.

There is sufficient gain in the system that a person can be heard clearly when talking at a normal volume, about two feet from the unit. Since the person speaking would be pressing the talk button, this is the maximum distance necessary.

The Works
Figure ** is the main circuit diagram, and shows the components mounted on the PCB. Figure ** is the interwiring diagram, which shows the switching arrangement and loudspeakers.

The requirement for a talk switch at each end on a two-wire system complicated the design slightly. Capacitor C17 is connected in series with the remote speaker, and is short-circuited when the talk switch is pressed (R22 limits the discharge current). The electronics at the local end detects the DC path when the switch is pressed.

TR3 and the associated components form the first audio amplifier stage. When a low resistance DC path is present between SK1 and SK2, TR1 will switch off, allowing TR3 to be biased by R6. The audio signal is coupled to the base of TR3 by C1 and C2.

R1 reduces the input impedance to reduce the chance of noise pickup along the long connecting lead. C3 and C5 remove any RF interference that may be present and limit the top end of the frequency response to about 4KHz. The low values of C2 and C8 limit the lower end of the frequency range to about 200Hz. This frequency response is adequate for speech, and allows a high gain to be achieved with minimal problems due to hum and noise.

Since TR3 is biased off when the unit is not in use, the gain is low and no hum or noise can be heard.

TR5 to TR8 and surrounding components form a basic class B audio output stage, with a power output of about 200mW RMS into 8R. VR2 is adjusted to give a quiescent current of 2mA, to minimise crossover distortion. Preset VR1 sets the audio gain. In use this will probably be set to maximum, although it is useful to be able to reduce this when testing to avoid feedback.