Monday, February 14, 2011

Invisible Fence Troubleshooting

Our invisible fence for the dog went on the fritz a month or so ago. Specifically, somewhere on the property the wire broke and the loop indicator buzzer went off signalling a break. Until this past week, our dog didn't notice and it wasn't a problem. Sometime over the past week Sophie noticed the fence wasn't there anymore and started wandering the neighborhood- a big problem. Luckily the weather was great this weekend allowing me to try and fix the fence.

An invisible fence consists of a single strand of wire that is buried around the perimeter of your property. A box uses this wire to set up an electric field around the property at a certain radius from this wire. Your dog then wears a collar, and when the collar enters the field it first warns the dog with a audible tone, then if the dog continues it gives a nice jolt. Sophie was only shocked once- after that the audible tone was plenty. The fundimental problem with fixing these wire loops is that you don't know where the break is. In our case the fence encloses about a half acre and because of all the snow isn't really accessible.

There are some commercial products you can buy to do this, but the cheapest is over $50 and requires ordering online. There is another method using an RF choke placed across the output terminals of the box, but this method doesn't appear to work with the Petsafe Sportdog invisible fence setup we have.

My first step was to dig up the wire in four or five spots, and I tried to use a non-contact voltage probe to see if the wire at that point was energized or not. This didn't seem to work that well- it seemed to be very inconsistant and was potentially giving both false positives and false negatives. The probe indicated one section of fence was bad, and after cutting it out and laying new wire on top of the snow the fence circuit was still broken.

My next step was to purchase a continuity tester. I thought I could cut the wire in several locations, then use this device to detect if there was a break in each individual wire segment.

Unfortunately after I got home I realized that you needed two wires for this device to work. I wasn't about to lay out a huge wire segment just to let this tool work. So back to Lowes' went the tool.

So, finally I decided to make my own continuity tester. I thought I could use the fence wire and the ground to complete the circuit. I thought since a battery's negative pole is the electron source, and the electrons flow back to the positive pole, I'd design the circuit using a positive ground. Afterwards I did some internet searching and it looks like a negative ground might be a better design. However, it worked.

I used a two AA battery holder to generate 3V, then attached an allegator clip to the negative lead and a long nail to the positive lead. I then pressed the nail into the ground (after first thawing it with a heat gun) and attached the negative lead to one end of a fence segment.

For the continuity indicator, I soldered a 100k ohm resistor to the LED cathode to limit current. The aligator clip attached to the resistor. I then attached the grounding nail to the anode. When I pressed the nail into the ground and attached the aligator clip to the fence wire the LED lit if the fence wire was unbroken. This method seemed to work. After some trial-and-error due to poor grounds in some locations I was able to locate the break and string new wire on top of the snow.

Invisible Fence Continuity Tester Schematic

   The home made tester worked OK- but the LED was rather weak when used for the longer wire runs. (probably 150 feet or so). Perhaps it'd be better if I switched the positive ground for a negative ground. I think another option to try would be a slightly more complex circuit using a transistor switched LED, where the fence lead would only deliver a "high" or "low" signal, and the transistor would be powered directly from an attached battery.


  1. I don't mind re-laying the back yard, but I really don't want to re-trench the wire through the yard.