Here is a simple circuit that can be used as a telephone status indicator. When the telephone is in use (off hook) the transistor Q1 switches ON making the red LED D2 glow. When the telephone is not in use (on hook) the Q1 turns OFF and Q2 turns ON. This makes the red LED D2 off and green LED D3 ON. The circuit is powered from the phone line itself and no external power supply is required.

Circuit diagram with Parts list.



  • The circuit can be assembled on a general purpose PCB.
  • IF 2A ampere bridge is not available, make one using diodes like 1N4007.
  • Note that some countries prohibit people from connecting other devices to the phone line.


  1. Bruno Salas

    This circuit worked very well. I only changed the 2n3392 transistor by 3904 and the value of three resistors, and worked for me. Thanks! 😀

  2. J.P.Cochran

    how Q1 get biased when the voltage gets low?
    and also Id like to know how the circuit works if I connect end of R4 between R3 and R5 instead of Q2 collector ?

  3. That is a nice circuit. Also a telephone line checker. In North America telephone system, red wire is negative and green wire is positive. ( Use a multimeter ) In-hook ( no phone is pick up ) The tel line content a DC AND AC. DC about -48 to -60, AC is about 90 to 120. On-hook, the DC is -10 V and no AC, just a phone signal ( talk, dial-tone, modem signal ). The circuit simply use the DC part of the phone line. Useful to check the phone line at your home. I don’t know about European system or Asia system.

  4. an red LED should show that user is engaged on phone! and green for free…..thanks in advance

  5. hello sir,
    i need to make a project on similar line but very complex sir….please help…..suppose there are three or more telephone lines for a user, and user wants to attend only one phone at a time (for incoming). As soon as he picks up one line other lines should be engaged for callers(incoming)…..but user can do an outgoing from any line while on conversation from 1st line….is this project possible???
    plz do reply!

  6. seetharaman

    Hi Frank When telephone is on hook (not in use) the terminal voltage will be high (around 50 volts) hence Q2 will conduct green LED will glow. When it is in use the terminal voltage will be less ( around 12 volt), Q2 cannot conduct, Q1 will get the bias, RED LED will glow indicating line is engaged. the terminal voltage is DC, without test instruments we donot know which lead is positive and which lead is negative hence a bridge rectifier is added. Please note R2 also should be 3.3K.

  7. Uhm..
    Could you tell how it’s really works
    I’d like to know why the resistor and transistor put in that path and also what is it for ??

    Thank’s before
    hope you’d give some light to this newbie