The simple switch shown so far can be used in simple audio applications. Here is a simple “all-master” intercom circuit that steers an audio amplifier towards one of several distant receivers. All units shown are identical and are connected to the same multipair cable. One battery powers all units.
You press the button corresponding to the station to whom you want to talk. When any “TALK” switch is pressed, the horisontal diodes feed 12v DC power from the cable to the MIC amplifier 741. Here you can see how diodes are used to combine the DC to a common source. This prevents power being fed back to the other channels.
The 1K0 resistors feed the DC power to one of the four selected diodes. The DC operating point of the 741 Op Amp is about 5.5v, which is applied to the cathode of all four vertical diodes. The TALK switch and associated resistor causes one of these four diodes to be positively biased. The diode will clamp the selected station’s line to 6.2v, with 500mV RMS audio superimposed upon this. In this circuit you can see that station 2 button has been pressed.
At station 2, the DC and audio signal is passed through a chocke to remove unwanted RF. The 2K2 resistor clamps the input to ground in the absence of any input signal. When called, the 2K2 is too high a value to affect this. The receiver circuit is a DC-coupled audio amplifier. When not called the DC conditions are upset and it draws no current. When called, the correct DC bias is received from the calling station and the amplifier drives the speaker.
In operation you just press the button of the station you wish to speak to. That person presses your station button to speak back. This method means that the other stations are also free and can also have a private conversation: for example stations 1 to 2, and another conversation between stations 3 and 4.
The Op Amp MIC amplifier has a pair of back-to-back diodes to limit the audio signal to 1.5volts peak-to-peak, which prevents the diode switching being overloaded. The 220R resistor in the feedback circuit is selected for correct gain. I have assumed that an ELECTRET mic is used and has a 10mV to 20mV output. If you use a dynamic mic then you need to remove the marked 10K resistor, and perhaps select the 220 Ohm for more gain (lower the value). The speaker and MIC should ideally be placed as far apart as possible, and pointing away from each other. Avoid mechanical conduction between these two. In normal operation you will need to speak close to the MIC, and the 220 resistor should be set so that feedback does not occur.
The speaker impedance should be 36 Ohms.
