Have you ever been using the modem or fax and someone else picks up the phone, breaking the connection? Well, this simple circuit should put an end to that. It signals that the phone is in use by lighting a red LED. When the phone is not in use, a green LED is lit. It needs no external power and can be connected anywhere on the phone line, even mounted inside the phone. Note: This circuit may cause problems for some when used.
3.3K 1/4 W Resistor
33K 1/4 W Resistor
56K 1/4 W Resistor
22K 1/4 W Resistor
4.7K 1/4 W Resistor
2N3392 NPN Transistor
1.5 Amp 250 PIV Bridge Rectifier
Wire, Case, Phone Cord
1. This is a very simple circuit and is easily made on a perf board and mounted inside the phone.
2. LED1 and LED2 flash on and off while the phone is ringing.
3. Do not worry about mixing up the Tip and Ring connections.
4. The ring voltage on a phone line is anywhere from 90 to 130 volts. Make sure no one calls while you are making the line connections or you'll know it. :-)
5. In some countries or states you will have to ask the phone company before you connect this to the line. It might even require an inspection.
6. If the circuit causes distortion on the phone line, connect a 680 ohm resistor in between one of the incoming line wires and the bridge rectifier.
Source: Electronics Lab