Dial-pulse-indicator

  
This indicator senses the switching on and off of the 48-V de line voltage and transmits the pulses to logic circuitry. An Hl1A10 threshold coupler, with capacitorfiltering, gives a simple circuit which can provide dial pulse indication, and yet reject high levels of induced 60-Hz noise. The DHD805 provides reverse bias protection for the LED during transient over-voltage situations.
Dial-pulse-indicator - schematic

The capacitive filtering removes less than 10 ms of the leading edge of a 40-V dial pulse, while providing rejection of up to 25-V rms at 60Hz.




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