Variable impedance detecting circuit for a telephone line...

Details Variable impedance detecting circuit for a telephone line... Variable impedance detecting circuit for a telephone line...

H - Electricity

04

M

379/17, 379/67,

H04M 1/65 (2006.01) H04M 1/654 (2006.01)

Patent

CA 1056528

ABSTRACT OF THE DISCLOSURE 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 transformer. The breakdown voltage of the Zener diode is selected to be above the voltage level produced by sound (conversation) signals and busy tone signals, whereby the device presents a high impedance to such signals and is not actuated thereby.

263041

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