Use old phones as an intercom

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Talking over the phones is easy. You put DC current through the phone and it transmits and receives audio. So two phones and a current source (about 25mA) all in series will give you a talking circuit. A suitable current source can be as simple as a 9V battery and a series resistor whose value is adjusted (with both phones offhook) till about 25mA flows. You can then bypass the battery and the resistor with a capacitor to couple the audio straight across and get a loud and clear connection.

What is much harder is signaling the other end. To ring the bell you need to put 90V (RMS) 20Hz AC into the phone (nominally). Lower voltages will work (down to about 40V) but different frequencies won’t. You can’t ring the phone at 60Hz. I have a ringing circuit in a PBX I built but it consists of a 20Hz sinewave generator, a push-pull power booster and a big transformer. Much too elaborate for a simple 2-phone intercom circuit, and anyway the ringing voltage could painfully zap a kid.

So forget the bell and look into other forms of signaling. This is what I have come up with:

                              +  | | - 
      +-------+------ - - --+---||||---/\/\/--+---- - -----+-------+ 
      |       |             |    | |     R    |            |       | 
      |       |             | 24V             |            |       | 
      |      ---            |                 |           ---      | 
      |     |   |           +---||------------+          |   |     | 
      |      --- Sonalert       C                Sonalert ---      | 
      |   C   |                                            |   C   | 
      +---||--+                                            +--||---+ 
      |      _|_,                                         _|_      | 
      |      / \  15V                               15V   \ /      | 
      PHONE    -+- Zener                             Zener  -+-    PHONE   
      |       |                                            |       | 
      |       |                                            |       | 
      +-------+------------------ - - - -------------------+-------+ 

As before, set R to give you a talking current (both phones offhook) of about 25mA. Start with 1K ohm. Leave it in if the phones work well enough; the current is not very critical. The capacitors C are audio bypass capacitors and should be about 0.47uF.

When the phones are onhook they present an open circuit, and the 24V battery voltage is not enough to overcome the 30V series drop of the Zeners and no current flows. When both phones are offhook they present a very low resistance and the talking current (determined by R) flows.

When only one phone is offhook it places its low DC resistance across the Zener diode on its side so that the full 24V supply is applied to the other side. This overcomes the voltage drop of the other Zener diode so the other Sonalert beeps. The wonderful thing about Sonalerts is that they make a loud noise with only a few milliamps of current so the series resistor R doesn’t matter. Especially nice is a pulsing Sonalert which goes “Beep beep beep” automatically. While the far-end Sonalert is beeping, you hear the beeping in the near-end receiver (at low volume thanks to the bypass capacitor across the far-end Sonalert) to confirm that the line is working and the other end is being signaled.

The power supply can be three 9V batteries in series but since 80% of the power is lost in series resistor R rather than in powering the phones it seems a little wasteful. A 24V wall wart with clean filtering would be better.

The signaling components can be mounted inside the phones. Only two wires are needed to go to each phone, and the power supply can be mounted centrally, out of harm’s way. If R is adequately big (12 watt) and has enough ventilation then both lines can be indefinitely shorted out without any fire hazard and there is not enough voltage anywhere to hurt anyone.

I have tested this with 500-type phones and two different types of piezo buzzers (pulsing sonalerts and non-pulsing brand X ones) and it works great. You should be able to get all the needed parts including piezo buzzers at Radio Shack. I love telephones. Too bad I don’t have any kids who want an intercom line.


  • Markus Wandel mwandel[at]



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