By Christie, Jim, John, and Terry Noe
Our dog, Maggie, loves to run around in our big yard. But how do we fence her in? Building a 500-foot fence is expensive and unsightly, so we use an “invisible fence.” This is a buried wire that runs around the edge of our yard. Maggie wears a radio collar that beeps when she gets close to the wire, or gives her a mild shock if she strays over the edge (it doesn’t hurt).
One of Maggie’s favorite sports is digging up the gophers that are endemic to our neighborhood. Unfortunately for us, the gophers have a hobby of their own, which is biting through the buried wire of our invisible fence.
How do you find a break in a 500-foot buried wire? Digging up the entire 500 feet would not be fun. You can buy commercial cable finders that will find buried wires and wire breaks, like the Armada Technologies Pro 700, but these cost from $500 to $2,000! We decided to make our own and use the money we saved for something useful, like building a big trebuchet.
Steps
Step #1: Here's how it works.
Next
- Our WireFinder 9000 has 2 components: a transmitter that puts a radio signal on the buried wire, and a radio receiver. You connect the transmitter to one end of the buried wire. This causes a current to flow through the wire, which creates a radio signal that radiates from the wire. Then you can find the wire by walking around with the radio receiver.
- When the receiver is close to the buried wire, the radio signal is strong and you can hear it over the receiver’s speaker. The strength drops rapidly as you move away from the cable, causing the signal to drop in volume until it becomes inaudible.
- If there’s a break in your wire, the current can’t reach the isolated segment. So when you move past the break, the signal level drops off rapidly. Using the WireFinder, we were able to locate the break in our wire quite accurately: the place we started digging was only 2 feet from the break.
- This technique works for finding both simple wires — like our invisible fence — and buried coaxial cables as well.
- To keep things simple and cheap, we chose a regular AM radio as the receiver. Then we designed our transmitter to generate a signal of the right frequency and modulation type so that an AM radio can pick it up. Fortunately, the AM radio band is very low-frequency, which makes construction and wiring very simple. The total cost of the transmitter is about $35.
Conclusion
This project first appeared in MAKE Volume 20, page 141.
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