I would like to install an alarm system on a metal fence (it is made of 15 metal wires horizontal spaced about 5inches with vertical wires attached every 5 inches. So it makes a grid) It is approximately 1 mile in length. I thought of measuring the resistance with a micro controller and when some one cuts the fence at one place it will increase the resistance and sound a horn. But after some test even if I cut 1 or 14 wires, the resistance do not change. Anybody has an idea on how to install an alarm system so when some cuts the wire the alarm will go off? I cannot electrify the fence by the way. thanks,
Hmm, if this is like many/most fences, it has good electrical contact with the ground, which will appear to be in parallel with the fence resistance. You will probably need to use insulating standoffs to the fence posts, like electric fences use.
A simple resistance measurement may still have problems if the fence is not completely cut away, since you will be measuring a fairly small resistance change. (The resistance will increase with every horizontal wire cut, but until it goes open the increase will probably not be a big percent of the total in a mile of fence.)
Another thing you might want to explore is Time Domain Reflectometery (TDR):
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This is really intended for use on uniform conductors like phone lines, transmission cables, etc, that are well-characterized. But it might work out here as well. In typical applicaitons, TDR can determine not only the presence of a fault, but can tell whether it is a short or an open, and its location along the length of the conductor. That last might be especially useful in your case.
Best regards,
Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis
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I don't think that can be made to work. The change in resistance will be too small to measure reliably. Besides, it isn't one continuous section of fencing, is it?
You could weave in a few smaller wires, such that they'd be broken if the fence were compromised. Then you could measure their resistance.
The fence is basically like a net, so even if you cut a small portion, or even most of it, the resistance do not change. But the TDR sounds like it could work. k
There is a special type of co-axial cable manufactured for fence security. It is woven through the mesh of the fence and uses a measures a change in capacitance when it is moved, such as someone climbing the fence. I have never seen it used in the type of fence you have but it is quite common in chain link type fences.
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