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