This is a fairly simple intercom with an unusual use of a phase splitting circuit that means both sides can talk and listen without either having to press a talk button. It also has a mute button that lets either side mute their microphone so that the other side can't hear them, much like a privacy button on a phone.

Much of this circuit is from the audio amplifier project, only the phase splitter and mute circuit are new. The biggest difference is you will need to make at least two, one intercom by itself isn't a lot of use.

This circuit can be run from a six volt battery supply, such as four AA cells, or from a six volt DC power supply. The actual voltage is not critical, it should work from four volts upwards, but it is recommended not to go higher than twelve volts or the LM386 may be damaged.

The circuit is laid out on the breadboard in roughly the same arrangement as in the schematic. The speakers used, one for each intercom, are old PC speakers so a pin header is inserted into the breadboard to accept it. The connection for the microphone is also shown as a pin header in the layout diagram but the electrets actually used to have wire ends so headers weren't used.

Speakers don't much care which way round they are connected, if the connections are reversed all that changes is the phase, but they are usually marked + and - and it is good practice to always connect the + lead to the signal and the - lead to 0V.

Electret microphones are polarised and will not work connected the wrong way round so you must connect the + lead to the amplifier input and the - lead to 0V.

The connection between the intercom(s) should be made with screened wire and all connected with the shield going to 0V.

In the layout diagram the + end of all connections are marked by a red stripe or block, the 0V end being the other pin.

The pots only had short leads so tinned wires were soldered to each so that they would plug into the breadboard. If the volume pot works in the reverse sense, i.e. turning it clockwise makes it quieter, then fitting it rotated 180 degrees should correct this.

Mute switch.

When the mute switch is pressed it connects the 10uF capacitor between the microphone circuit output and 0V. This effectively shorts the microphone signal to 0V silencing the microphone. The 150K resistor across the switch provides a charge path for the 10uF capacitor so that when the switch is pressed there isn't a loud click heard at the other end (there will be a small click due to leakage current through the capacitor).

Phase splitter.

The signal from the microphone is reproduced, with almost the same amplitude and the same phase, at the emitter of the phase splitter transistor. Because the current for the emitter circuit comes from the collector circuit the signal from the microphone is also reproduced, with almost the same amplitude but the opposite phase, at the collector of the phase splitter transistor. These two opposite phase signals are summed in the null pot and at some point along the pot they will always sum to zero. If the pot is set to this point then no signal from the microphone will pass to the following amplifier.

However, the microphone signal reproduced at the collector circuit will all be passed to the other intercom so there it will be heard. Conversely the signal from the other intercom appears at one end of the null with no matching but out of phase signal at the other end pot so it will not sum to zero at any point and will pass through to the amplifier and be heard at this end.

The 4K7 resistor from the bottom of the null pot almost ballances the apparent AC impedance of the other intercom connected to the other end of the null pot, dominated by the 4K7 collector resistor of the other intercom, making it easier to null the local microphone signal.

Here the is one of two circuits laid out and working on an actual breadboard. You can see the speaker and microphone wires and the screened wire that connects to the other intercom.

This is just a small section of a much larger breadboard. The extra components are capacitors that I usually have scattered across the breadboard to eliminate noise from the power rails. They make no difference to this circuit.

The extra links are because this particular breadboard has a break in the horizontal rails at the mid point. I don't want this break so I usually keep these positions jumpered.

There is a small amount of setting up needed before the intercoms can be used.

Start with the intercoms on and connected together, the volume pots set to minimum and the null pots set to about midway.

Make some noise at the microphone, I find running one finger over the front of the microphone makes more than enough, and turn up the volume until you can hear it. Now, using the null pot, find the point where the noise is quietest. Repeat increasing the volume and nulling out the sound until you are sure you have it as best you can. It should be possible to get it good enough so that even with the microphone adjacent to the speaker there is no feedback.

Once one intercom is done return the volume on that intercom to minimum and repeat the above setting up on the other intercom(s).

Ths done the intercoms are ready for use, just set the volume to a suitable level.