Cable finder seeks out wall conduits

-May 11, 2000

When you debug home electrical wiring, a tool that determines the location of cable conduits comes in handy, especially when problems arise. Some tools, such as nylon or glass fiber guides, contain small magnets and compasses that act as sensor elements. These short-range systems are useful for brick walls, but not for other wall-mounting systems that include iron elements. Some electronic aid would be helpful.

The circuit in Figure 1 uses an antenna in the wall conduit and a receiver to locate a cable conduit. The combination can accurately locate the antenna placement. The simplest and cheapest approach is to use a portable AM radio, which includes a ferrite antenna with excellent directivity and sufficient range of a few centimeters for good resolution.

The circuit uses a relaxation oscillator with an adjustable frequency to apply current pulses to a twisted-pair cable. This oscillator has the AM-broadcast range of 530 to 1600 kHz. A lower frequency oscillator switches these pulses on and off. The envelope detectors of AM radios can easily detect the pulses. However, the circuit requires an additional refinement because pure tones are unpleasant. Thus, some modulation is necessary. This circuit allows for dual tones, such as a police siren, or sweeping tones, such as an ambulance siren, using a switch. The circuit provides dual-tone operation using a bipolar transistor that shorts a capacitor on and off in the capacitive section of the oscillator path. The sweeping tone is a mixed AM/FM format that the circuit produces by biasing the relaxation oscillator with a triangular signal from a lower frequency oscillator. This bias changes the threshold level of the modulating tone, which produces modulation of the fundamental frequency and the aspect ratio of the output.

A regulated power source biases the oscillator section to minimize the high-frequency oscillator's voltage sensitivity. A current source, which is insensitive to supply voltage, controls the final stage. This control, instead of a direct connection to the supply voltage, provides some protection in the event of a short circuit and provides similar output independently of the antenna resistance. The LED, which the circuit uses as a voltage reference, also indicates circuit activity. The supply voltage can vary from 7 to 12V or higher if you take care of the BD140 transistor's dissipation requirements.

To use the circuit, switch on an AM receiver and find a position without broadcast stations. Place the receiver close to the generator, switch on the circuit, and adjust the variable resistor until the tone is audible. Move the receiver to find the direction of better sensitivity. Then, use the radio receiver to find where the twisted-pair cable goes.

Beware that this circuit produces EMI not only in the AM-broadcast range, but also over a wide frequency range due to the high harmonic content of the generated signals. The use of twisted-pair cable reduces the amount of interference in the far-field region. Therefore, use the circuit cautiously, and disconnect the circuit as soon as you locate the cable. (DI #2523)

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