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.

Circuit diagram with Parts list. 



  • 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.




  1. What additional components would I need to attach this to a 7 line analog phone block to indicate that any of the lines went off hook?

  2. this is awesome!please i need a circuit which could block a any incoming or outgoing phone calls.i need it for a school project. thank you very much