Variable impedance detecting circuit for a telephone line auxiliary device

Abstract From the Canadian patent

An automatic telephone answering device includes a diode bridge rectifier having its input terminals connected across a pair of telephone lines through a first switch and, its output terminals connected across a relay coil through a voltage controlled impedance, such as a Zener diode. A relatively high frequency, high voltage call signal is rectified by the bridge and breaks down the diode to energize the relay coil This closes a relay switch to actuate a control circuit whose, outputs a) actuates a tape recorder unit to play back a prerecroded message to the caller, and b) simultaneously transfer the first switch to connect the tape recorder unit across the telephone lines through a line …