Hook detector

The hook detector is an circuit, which detects and pulls down the “/INT1” of the controller when phone is off-hook. The circuit here uses a bridge rectifier to convert the available AC at the telephone terminals into DC. The OP of this bridge is around ~50V when on-hook and ~10V when off-hook and is ~80V when ringing. The D7 is an 20V zener diode, which breakdowns when there is more than 20V across the diode. D7 and R6 forms and acts as an switching circuit when off-hook and on-hook. Because, while on-hook , the voltage available aoss D7 and R6 is ~50V and the Optocoupler PC817’s LED glows with the voltage across R6 through D8, R7 and R8. the capacitor C5 used here is to hold the charge for a while. because , when the telephone rings, the voltage suddenly raise to ~70V and falls below 10V while which the optocoupler should not on and off and the “INT1” is triggered for each ring and as a result, malfunction of the circuit occures. To avoid this, C5 gets charges through D8 and R7 when diode conducts and discharges through R8 and the LED of the optocoupler.

Fig 2.1 Hook Detector

Diode D8 is 1N4148, whih is an switching diode, here provided to prevent the discharge of capacitor in reverse direction . When the phone is off-hook, OP voltage at bridge goes below 10. Thus the optocoupler goes off and pin 13 goes low and thus the INT1 is pulled down to indicate off-hook condition.

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End of Part II

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