Telephone off-hook indicator

Description.

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. 

telephone-in-use-indicator-circuit.JPG

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.

 

Show Comments

2 thoughts on “Telephone off-hook indicator

  1. AJ

    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?

    Reply
  2. samuel

    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

    Reply

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