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Simple intercom

     This is a simple homemade intercom (also called intercommunication device, talkback or doorphone). It is enough loud, you can reliably hear the voice several meters away from it and therefore you can use speaker instead of handset. Loudspeakers are used also as microphones for simplicity. The aplifier and the power supply is only on one side, switching is done using a DPDT relay and a SPST switch. Of course you can omit the relay and use a DPDT switch if you have one. I've used relay because a DPDT switch is harder to come by. Normally, the relay is switched to receive position, but on the remote side the switch is closed (on), as it is a Normally Closed (NC) type of switch. Thus the low resistance of the speaker coil grounds the base of the first transistor (NPN). It is therefore off, thus the second transistor is also off and no current is flowing. Only a very small current flows through the 1M resistor, so the device can be permanently connected to a battery. A 9V battery should last at least one year. After pressing the NC button on the remote side, the speaker is connected through a capacitor, so the base is no longer grounded, both transistors are biased and they act as an audio amplifier. By pressing the NO button, the relay swaps the speaker functions. The speakers should have at least 32 Ohms impedance. Remote side has no semiconductors and other polarized components, and therefore you don't have to observe the polarity of the cable. If the cable should be long (more than about 20 to 30 meters) it would be appropriate to use thin coaxial instead of an ordinary wire pair to prevent the mains hum getting into the amplifier. If no switch is pressed, both transistors are off and the speakers are completely silent.

The schematic of my simple homemade intercom.

My prototype of intercom built in 2004 (the power supply is 12V). It serves to this day without a failure.

Inside the homemade intercom.

Added: 2005