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Simple telephone ring detector Circuit

2015-01-08 01:07  
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A telephone ring detector circuit is provided which finds particular utility in telephone answering systems. The detector circuit responds only to ring signals on the telephone line to activate the system, and it is unresponsive to dialing pulses, or other signals, that may be present on the telephone line.This circuit detects the 20-Hz approximately 86-Vrms ring signal on telephone lines and initiates action in an electrically isolated circuit. Typical applications include automatic answering equipment, interconnect/interface and key systems. The detector is the simplest and provides about a 1-mA signal for a 7-mA line, which loads for 0.1 s after the start of the ring signal. The time-delay capacitor provides a degree of dial-tap and click suppression, and filters out the zero crossing of the 20-Hz wave.The detector circuit of the invention distinguishes the ring signals from the other signals on the telephone line by counting the number of pulses present on the telephone line for the various signals and providing a ring detect output only when the number of pulses is in excess, for example, of the pulses which form the trains of the dialing signal.
 

Simple Ring Detector