Transmission line signal sensing circuit employing means for conserving power, especially for use with a telephone disconnect circuit, and associated method

Abstract From the Canadian patent

An apparatus and method for sensing a predetermined tone signal transmitted on a transmission line such as a, telephone transmission line and for generating a signal, indicating the presence of the tone signal. The apparatus comprises a tone detector circuit including an amplifier bandpass filter and comparator, a power supply for generating power for the detector circuit from a voltage present on the transmission line and logic circuitry for gating pulses of power from the power supply to the detector circuit on a periodic basis thereby to enable the detector circuit to check for the presence of the tone signal during the durations of the power pulses.