Telephone Line Monitor. Beginner’s Trial 12

This simple circuit performs multi function. It always monitor the Telephone lines and see whether there is any misuse and gives LED indication if it occurs. It gives indications like Phone ON hook state, Off hook state, dialing etc. It is ideal as a tester to see whether the lines are available or not , modem in use etc. It needs no external power and uses the voltage from telephone lines. Two LEDs and a single resistor will make this circuit. You can fix it in the telephone junction box. Make and invite your friends to see your work.

In the On hook state, the telephone lines have around 50 volt DC, which drops to 12 volt DC in the Off  hook state. More over there will be a reversal in polarity of lines during dialing and call.

LED Status
1. Off hook – One LED lights – Telephone lines are OK.
2. Dialing – LED Blinks – Phone / Modem in use. Tapping and misuse of lines


4 thoughts on “Telephone Line Monitor. Beginner’s Trial 12

  1. vinodvinu 20 Oct 2012 / 11:03 pm

    ഈ സർക്യൂട്ട് സ്‌പ്ലിറ്ററിനു മുൻപ് ഘടിപ്പിക്കണോ അതോ സ്‌പ്ലിറ്ററിനു ശേഷം ഘടിപ്പിച്ചാൽ മതിയോ ?.

    • D.Mohankumar 21 Oct 2012 / 9:05 am

      Connect the lines going to the Phone

      • V Krishna Swamy 23 Oct 2012 / 7:56 pm

        Very simple and easy for biggners and also very useful to see if the telephone line is working or line disconnected.


  2. Vicente G.C. 20 Oct 2012 / 10:03 pm

    Much interesting, easy to assemble, well explained, I am on the lookout for when they come to me mail and a big hello Thanks

Comments are closed.