How to Convert a Telephone Into an Intercom

by April Kohl
Telephone Intercom Cable Station image by Charlie Rosenberg from

An intercom is a modified telephone system in which users need not dial one another in order to make a connection. If you have a set of old telephones collecting dust around the house, you can put them to good use by assembling your own intercom, for use at home or in the office, with minimal fuss.

Step 1

Strip the insulating cover from a two-inch section of the telephone extension cable, around three to four inches from one end. Cut the red wire inside the cable in the center of the stripped area. Strip the insulation from the ends of the cut wire. Solder one end of the cut wire to a terminal on the battery connector. It does not matter which way around the connector is soldered, telephone lines are bidirectional.

Step 2

Strip the insulation from the ends of your two-inch piece of wire. Solder one end of this wire to the other terminal on the battery connector. Solder the other end of the wire to one end of the 680-ohm resistor. Solder the other end of the resistor to the spare end of the red wire you cut in the telephone extension cable.

Strip the insulation from both ends of the telephone extension cable and wire one end to the first telephone socket. Wire the other end to the other telephone socket. Plug a telephone into each telephone socket. Attach your battery to the battery connector, to power the circuit. Lift the receiver on both telephones to complete the circuit and activate the intercom.


  • Only cut the red wire in your telephone extension cable. It is this wire that needs to be powered for the telephone line to work, while the other line must remain intact for the signal to operate. Cutting the wrong wire will render the cable useless, as repairing it will inevitably mean signal loss or too great a resistance for the signal to pass through (or both).


  • To power the ringer on a telephone, you need to supply 90 volts AC electricity with a frequency of 20 hertz. Because no main supply in the world uses 20 Hz, you will need to either improvise your own power supply or choose an alternative means of getting the other person's attention on the intercom. One option to consider is using telephones with a speakerphone facility and just leaving that turned on.


Photo Credits

  • Telephone Intercom Cable Station image by Charlie Rosenberg from

About the Author

Based in the United Kingdom, April Kohl has been writing since 1992, specializing in science and legal topics. Her work has appeared on the Second Life News Network website and in British Mensa's "LSQ" magazine. Kohl holds a Bachelor of Science in physics from Durham University and a diploma in English law from the Open University.

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