Showing posts with label hook. Show all posts
Showing posts with label hook. Show all posts
Monday, August 11, 2014
Telephone off hook indicator
The schema depicted here can be used as an indicator when the telephone receiver is off-hook. The schema can be in corporated with old telephones that does not have such an indicator.
The schema 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 schema 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 schema can be powered from a 3V battery.
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