Sughrue, Rothwell, Mion, Zinn and Macpeak
Variable impedance detecting circuit for a telephone line auxiliary device
 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) actuate a tape recorder unit to play back a prerecorded 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. 
Pioneer Electronic Corporation
H
H04M
inventional
Hiroshi Shizuo
Mizuno Ando