Telephone off-hook indicator

Posted on Feb 7, 2014

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.

Telephone off-hook indicator
Click here to download the full size of the above Circuit.

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