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Use old phones as an intercom

Use old phones as an intercom

Talking over the phones is easy but it is much harder is signaling the other end. To ring the bell you need to put 90V (RMS) 20Hz AC into the phone. Lower voltages will work (down to about 40V) but different frequencies will not. You cannot ring the phone at 60Hz.
So forget the bell and look into other forms of signaling.
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.

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