Isolated Telephone Interface

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This circuit allows you to record audio from a telephone line into a tape recorder or computer soundcard. Most of the parts for this circuit can be scrounged from an old modem, with some work, it is possible to rewire the modem circuitry and use the old modem case. Note that some countries have laws that require the user of a phone recording device to notify the party on the other end of the line that they are being recorded.


There’s not much to this circuit. The two RJ-11 jacks are set up to feed the telephone circuit through from the wall to the phone. The active signal for a single phone is on the red and green wires. Yellow and black are usually used for a second phone line. The 0.22uF capacitor blocks any DC current from flowing through the transformer. The 4.7K resistor limits the current of the 90V ringing signal. The transformer isolates the telephone side of the circuit from the tape recorder side. The zener diodes clamp the 90 volt ringing signal and other transient spikes to protect your recorder. The 10K potentiometer is used to adjust the level to the tape recorder.


Using two RJ-11 phone line jumpers, connect one side of the interface to the wall plate and the other side to a telephone. Connect the audio out to a tape recorder or PC sound card’s auxilliary input. Set the recording level and start recording. It should also be possible to inject an audio signal into the audio output jack and have it appear on the phone line, the level control should be all the way up, drive the circuit from an amplifier that is capable of running a small speaker.


  • 2X RJ-11 phone Jacks
  • 1X 0.22uF 200V capacitor
  • 1X 4.7K 1/2W resistor
  • 1X 600 ohm to 600 ohm modem transformer
  • 2X 5V 1W zener diodes
  • 1X 10K audio taper potentiometer
  • 1X RCA audio jack




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