metal fence alarm project

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):

Best regards,

Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis

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.

John

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

As a practical matter, it probably won't.

But the fencing didn't come on a mile-long spool. It must have many section gaps, spliced together somehow. It'll be an electrical mess.

Fiberoptics might work.

John

John Larkin wrote in news: snipped-for-privacy@4ax.com:

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A body capacitance sensor would probably work the best...

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Since the fence is fully grounded, how will a body (human or wild animal)produce a change in capacitance?

How about vibration sensors every few meters. false alarms could be a problem though. Cheers Rob

Just saw this TDR article referenced in EDN:

Talks about history, theory, and modern instruments. Of course, it doesn't get into homebrew fence measurements!

Best regards,

Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis

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.