The circuit depicted here can be used as an indicator when the telephone receiver is off-hook. The circuit can be in corporated with old telephones that does not have such an indicator.
The circuit uses a complementary darling to pair using Q1 (2N3904)and Q2 (2n3906) to sense whether the receiver is off hook and glows a LED to show the condition.
Notes.
* The LED must glow when the circuit is not connected to the telephone line as well as when the telephone is off-hook.
* If the LED does not go OFF when the receiver is restored on hook change the value of R2 so that about 3.5 V comes across it.
* The circuit can be powered from a 3V battery.
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