When your phone is on-hook then it is disconnected from the phone line. The phone line can be cut and there is not much to measure except the 50VDC. The source resistance is about 1k ohms. Just connect a circuit with a very high input resistance that indicates if the 50VDC is there or not. The voltage drops down to 6VDC to 12VDC when the phone is used. The 50VDC has 90VAC on top of it during ringing.
cut phone line detector
Posted - Nov 15 2007 : 05:05:22 AM
