A very useful simple project of a telephone in use indicator circuit which is very easy to build. The working of the circuit is simple when some one is using the phone or phone line is busy then the LED will light in the circuit to provide the indication of line in use. Connect the circuit to the phone line in
parallel. Diode D1 to D4 are working as a bridge and provide power to the circuit by converting the phone line voltage in to DC (Direct Current). When any one use the phone the line voltage drops to 12V Due to which the Transistor Q2 gets off and trun on Transistor Q1 which makes the LED glow.