Another Case of a Buried Electrical Box

I read "The Case of the Buried Electrical Box" with some interest, since I ran into a similar problem once.

I spent some time servicing hard-wired alarm systems in a number of different types of buildings. I had the same problem trying to find the run of the four-wire cabling to the sensors on doors, windows, and other vulnerable entry points.

To solve the problem, I built a simple oscillator using a 555 timer chip. The output was a square wave of about 1KHz. This 1KHz square wave was rich in harmonics way up to the MW broadcast band and was boosted by a beefy transistor. The transistor's output was fed into a step-down transformer out of an old transistor radio.

The low-impedance output of the transformer was fed into the run of line from the main control panel. A small transistor radio tuned to a blank space on the dial easily found the run of the cable and showed not only where the cable ran behind the wallboard or ceiling, but it also found any break in the cable running to the sensor. It was not complicated, not fancy, but it certainly worked.

This entry was submitted by Clive Hollins and edited by Rob Spiegel.

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