Fox and Hound circuit diagram

   © by Tony van Roon

The "Fox" and "Hound," made by Triplett Corporation (, are a transmitter and receiver for tracing telephone(and others) wires through walls. You connect the transmitter to the wire, and the receiver helps you trace the wire from several inches distance away. Triplett doesn't say much about how they work, but th Fox and Hound apparently use inductive coupling of a low-frequency signal, maybe around 50 to 100KHz.

Fig. 1. shows a cheaper transmitter that uses an AM transistor radio as the detector. It uses a type 4001 (CD4001B) cmos integrated circuit to generate a pulsing signal a 1MHz (100KHz), which is heard as a growling sound in the radio.
Connect the transmitter to the wire to be traced, tune the signal in, and set R4 potentiometer to give the weakest signal that does the job. At maximum setting you can probably pick it up several feet away; lower settings will enable you to locate wires within an inch or two. Note that you are using the AM loop antenna inside the ratio, not the FM whip, whip should be fully retracted, or if it is a screw-in type, unscrew it.
alternateAs shown, the circuit transmits on 1000KHz and is controlled by a 1-MHz microprocessor crystal. Fig. 2. shows how to use an LC oscillator to get other frequencies or to save having to order a crystal. Although build with a CMOS gate, this is simply a Colpitts oscillator. Note that the two capacitors are effectively in series, so only half of their capacitance is present in the tuned circuit.

Now for the bad news. As you may have discovered, this type of device probably will not detect a small break in a phone line. Radio waves that can go several inches through wood and plaster can also jump across a tiny gap in metal wire. Nonetheless, with care, you may be able to find the break by looking for a place where the signal strength drops dramatically.
Another approach would be to use a much lower-frequency signal, such as 5 KHz from an audio-signal generator, and pick it up inductively with a microphone-level audio amplifier. Try using a coil from a relay as the detector, in place of a microphone. Low-frequency signals are harder to pick up but also less able to jump across breaks in the wiring.

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Article reposted from Poptronics Magazine, June 2000. Poptronics and Gernsback Publishing no longer exist.
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