Telephone in use indicator.
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.