This circuit detects the 20 Hz, `` 86-V rms ring signal on telephone lines and initiates action in an electrically isolated circuit. Typical applications would include automatic answering equipment, and interconnect/ interface and key systems. The circuits illustrated are bare bones circuits designed to illustrate concepts. They might not eliminate the ac/dc ring differentiation, 60-Hz noise rejection, dial tap rejection, and other effects that must be considered in field application.
Ring-detector - schematic

The first ring detector is the simplest and provides about 1-mA signal for a 7-mA line loading for 1/10 sec after the start of the ring signal. The time delay capacitor provides a degree of dial tap and click suppression, as well as filtering out the zero crossing of the 20-Hz wave. This circuit provides the basis for a simple example, a ring extender that operates lamps and buzzers from the 120-V, 60-Hz power line, while maintaining positive isolation between the telephone line and the power line. Use of the isolated tab triac simplifies heat sinking by removing the constraint of isolating the triac heatsink from the chassis. Lower line current loading is required in many ring detector applications. This can be provided by using the HllBX522 photo-Darlington optocoupler, which is specified to provide a 1-mA output from a 0.5-mA input through the -25°C to +50°C temperature range. The next circuit allows ring detection down to a 40-V rms ring signal while providing 60-Hz rejection to about 20-V rms. Zero-crossing filtering can be accomplished either at the input bridge rectifier or at the output. Dependable ring detection demands that the circuit responds only to ring signals, rejecting spurious noise of similar amplitude, such as dialing transients. The configuration shown relies on the fact that ring signals are composed of continuous frequency bursts, whereas dialing transients are much lower in repetition rate. The de bridge-filter combination at the HllL input has a time constant; it cannot react to widely spaced dialing transients, but will detect the presence of relatively long duration bursts, causing the HllL to activate the downstream interconnect circuits at a precisely defined threshold.

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