Spy Review

In a recent article, I discussed the difference between parallel and series telephone taps. In this article, I present a very simple and reliable parallel phone tap circuit useful for recording telephone conversations onto a tape recorder.

When installing a telephone tap, its advisable not to cut the telephone line in anyway. Cutting the telephone line for any time is detectable, and therefore series devices can be detected in this way. Parallel devices can be installed without cutting any telephone wires, and is thus harder to detect.

The diagram shows a simple circuit using just 6 very cheap and common components. To build the circuit, you’ll need some Veroboard or prototype board (whatever you favourite is), some wire, and a 3.5mm plug that will connect to your tape recorder’s microphone input socket. I’ve left it to the reader to work this bit out, as its very simple to do but quite awkward to explain!

To test the circuit, connect the telephone line wires to pins 2 and 5 of the telephone line. Then plug the microphone input into your tape recorder. Pick up the telephone and press record on your tape recorder. You should be able to hear a dialtone on the tape when you play it back.

If you’re curious to what goes on in the circuit, I’ll explain that now. The resistors act as a potential divider, and limit the voltage from the phone line that reaches the recording device. The diodes ensure the incoming voltage is rectified from an alternating current (AC) to a direct current (DC), which might otherwise damage your tape recorder.

The capacitors should be a ceramic type, rated at 50V or more. The emphasis is that the capacitors should not be polarised, which would stop the circuit from working properly. The resistors can be 1/4 Watt carbon or metal film types. As you can see, the components are very cheap.

The circuit is designed to work on UK telephone lines. However it should work in other countries too. You may need to adjust the resistor values to get the best volume. To increase the volume to the tape recorder, increase the value of R2. If the sound on the tape recorder is distorted, then try decreasing the value of R2 to decrease the volume.

If your tape recorder has a voice activated system (VAS), then you can set your tape recorder to record whenever the phone is used. You just need to adjust the recording to trigger when the telephone handset is lifted.