This intercom was built specifically for the school hall where I teach. in the past, walkie talkie's with headsets just didn't work. They were unreliable and not very good quality thus causing problems when needing to communicate backstage or from the lighting desk (where I sit) to the sound desk, spot operators, or video cameras. So a good quality, reliable system that was hard wired was needed. I had some idea of what we needed since we take part in The Global Rock Challenge every year and on some occasions I had directed the lighting for the performances. We had also hired the local theatre to put on 3 day performances where again I had the opportunity to use professional lighting desks and understand how communication occured in the theatre. There is loads of dialogue you never hear!

The intercoms were designed so that they can be added at will to any part of the link line. The Link is a 2 core 14 strand speaker cable of pretty standard quality but it provides almost a noise free connection.
At first batteries were used but since the current consumption can reach 35mA or more with the LED's, a low voltage power ring was installed alongside the audio Link cable. This was powered by a Science lab 4A supply - no more power supply worries!

Currently there are 7 intercoms built that can be plugged in to any of the points shown. Plugs on the Link and Power cables are all in parallel with 2 sets of wires in each plug - bit of a squeeze!

The circuit diagram below shows the two distinct parts of the circuit. They are based upon standard amplifier designs. The top section is the low signal pre-amplifier for the microphone and the lower section is the power amplifier for the headset. Due to its simplicity, an intercom complete costs less £10 to build.

Note there are many components not mounted on the PCB. Connections to the PCB are denoted by SIL or TB.

PRE-AMP
This amplifier is a standard inverting amplifier based around a 741 op-amp with decoupling capacitors. The gain is set by

-R5/R4 = 1M/10K = 100.

This provides an output around 500-800mV which is fed to the level preset on the PCB to adjust the level of signal placed on the Link. Connection to the Link is via SW2 to enable muting, but audio audio receptions will still be heard from other intercoms.
To enable the amplifier to work on a single voltage supply, a psuedo 0v was created by R2,R3,C1 which gives the amplifier 9 - 4.5 - 0v to operate from.

POWER AMP
This is based around the LM386 IC which provides about 500mW with just 4 components. The volume is set by VR2 which is mounted on the lid.

POWER SUPPLY
This uses a 9v/7.2v PP3 which powers the intercom via a switched 2.1mm DC power socket. The switched contacts inside allow for the battery to be charged through R9 when a supply of 10v or more is pluged in. If you are using a power supply, which is advised, then remove the battery unless it is rechargeable.

Diagram with lid open showing routing of wires. Thick grey wires are screend cable.

Unfortunately the wiring is a little complex inside but reduced a little by mounting components between switches and LED's. The case used was cheap but not bad looking but since I needed a few of them, cost was a factor. The battery compartment was made from a piece of vacuum forming polystyrene and a bit of foam from the textiles department to stop the battery knocking about. Adhesive PCB posts were used to mount the PCB - quick and simple.

• Circuit Diagram - Livewire
• PCB layout
• Front Panel - Draw
• PCB overlay - JPG (resize to 61x45mm)