Telephone line monitor

Telephone Recorder

This nifty little circuit lets you record your phone conversations automatically. The device connects to the phone line, your tape recorder's microphone input, and the recorder's remote control jack. It senses the voltage in the phone line and begins recording when the line drops to 5 volts or less.

Circuit diagram

Telephone line monitor-Circuit diagram


  • R1 270K 1/4 W Resistor
  • R2 1.5K 1/4 W Resistor
  • R3 68K 1/4 W Resistor
  • R4 33K 1/4 W Resistor
  • C1 0.22uF 150 Volt Capacitor
  • Q1, Q2 2N4954 NPN Transistor
  • D1 1N645 Diode
  • MISC Wire, Plugs To Match Jacks On Recorder, Board, Phone Plug


  • The circuit can be placed anywhere on the phone line, even inside a phone.
  • Some countries or states require you to notify anyone you are talking to that the conversation is being recorded. Most recoders do this with a beep-beep. Also, you may have to get permission from the phone company before you connect anything to their lines.