old phones as an intercom with ring tone


Posted on Aug 11, 2012

You may wish to use a 12 to 24V battery, like a Gel Cell, since the car power or cheap power wall wart style supply is probably too noisy. You may have to adjust the resistor value (1.2K for 24V). Whatever gets the phones working and does not use more than 30mA total loop current.



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




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