Isolated Telephone Interface

Phone Tap Schematic

(C) 2002 G. Forrest Cook


This circuit allows you to record audio from a telephone line into a tape recorder or a computer soundcard without any hum. Most of the parts for this circuit can be scrounged from an old modem. With some rewiring work, it should possible to build this project into the old modem's case and re-use the built-in phone connectors.

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.


This is a fairly simple circuit. The two RJ-11 jacks are set up to feed the telephone circuit through from the wall to the phone. A single phone normally uses the red and green wire pair. The Yellow and black wires 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 the input of your recording device. The 10K potentiometer is used to adjust the signal level that feeds the input of the recording device.


Using two RJ-11 phone wires, connect one side of the phone tap to the phone line and the other side to a telephone. Connect the audio out to a tape recorder or PC sound card's auxilliary input. Call someone, adjust the recording level for proper levels and record the conversation. Listen to the recording to make sure that the levels are not too quiet or too high and distorded.

This circuit can also be used for injecting an audio signal into the phone line. A speaker-level audio output can be sent to the RCA jack, the level control should be all the way up and the volume should be adjusted by setting the sound source volume control for maximum volume and minimum distortion in the local phone's earpiece.


2X RJ-11 phone Jacks
1X 0.22uF 200V capacitor (not electrolytic)
1X 4.7K 1/2W resistor
1X 600 ohm to 600 ohm modem transformer
2X 5V 1W zener diodes (1N4733 or equivalent)
1X 10K audio taper potentiometer
1X RCA audio jack

Suitable transformers are available from Digi-Key, Mouser, and Jameco.

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