Telephone In-Use Relay Controller
By Bill Bowden
Telephone In-Use Relay Controller circuit
Voltage at the gate of the MOSFET should be negative (1-3 V) with respect to the source when phones are on-hook. Voltage at the gate should be positive (8-10 V) with respect to the source (ground) when any phone extension is off-hook. A high impedance meter is needed to measure the on-hook voltages accurately. Circuit should draw less than 5 uA from the phone line. Relay used is a 12 V DC / 120ohm coil, but most any small 12 V DC relay should work. Power supply regulation should be +/- 2 V.
1346 22 August 2007
The circuit presented here can be used for connecting two telephones in parallel and also as a 2-line intercom.
Audio from a telephone line can be obtained using a transformer and capacitor to isolate the line from external equipment.