Showing posts with label hook. Show all posts
Showing posts with label hook. Show all posts
Tuesday, May 14, 2013
Telephone off hook indicator
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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