Hi Ron,

thieves are normally smart. They spy their objects for weeks sometimes to make sure they can do their "work" without any disturbance. If they find any alarm device they will of course cut a simple wire at several places just to take no risk.

First thing which will be kind of hard for them is the quality of the wire. It will be stainless steel of 6mm diameter. In order to cut that they will have to use a bolt cutter which is normally not isolated.

I really don't care shooting a burglar in my garden. But getting out in the dark and search for somebody around I prefer to blind fire onto the given location.

In the meanwhile I have worked out a schematic for just one section. It will take a lot of relays but that doesn't bother me as long as the desired function is achieved.

ST offers a current sensor which works with two different voltages. The high side can use up 30V input voltage and the low side can be supplied by any MCU power supply. The current it consumes is comparatively
very low at 300µA.

Have a look at the design and post your opinion please.

Thanks

Hans

 

Sorry,

upload failed, here it is

 

Let me just throw out another idea. It could detect and locate two cuts easily, but not more. Of course, the plan you have to monitor each segment could detect more cuts, but might be more complicated.

Someone earlier suggested a resistence network, such as shown below. The wire could be connected to ground at many places and from the total resistance to ground at the source, one could calculate which segment was cut.

What if instead of resistance, one looked at capacitance, and instead of using ground as the reference, simply used a twisted pair? In other words, the uninsulated live wire would be twisted with an insulated wire.

As an experiment, I took a partially used roll of telephone (CAT 3) wire. It was initially 250 ft, but I had used quite a bit of it. The capacitance between one pair was 2.16 nF. I removed 8 ft and the resulting capacitance was 2.00 nF (i.e., 0.02 nF per foot). That would allow me to estimate the remaining length at about 100 ft, which is about what it looks like.


In practice, if the current to the hot wire drops (or any other factor you may monitor) indicating a cut, one could immediately measure capacitaance from each end and determine the location of the cut or cuts in distance from the source. Of course, the capacitance measurement could be made by a variety of means; I simply used a meter for this experiment. If the high voltage is pulsed, you could get a measure of it continuously. John

Attached Images

network.png (8.6 KB, 3 views)

 

hi boncuk,
Its unusual for the intruder to cut the HV wire on the electric fence.
In South Africa, a common way is just to throw a grounding wire over the HV wire or a wet blanket/carpet.

If you dont test for a grounding link then you will not be aware the fence is grounded and its useless.

Roff's idea of detecting the presence/or lack of the HV pulse is the best way.

Is the HV fence wire a continous loop.?

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

Hi to all involved in the problem,

the entire HV part has been taken care of already. There is no way to ground the HV without detection. I just didn't mention that because it is the most simple part of the installation. It is done by measuring the voltage drop over a defined terminating resistor between the hot line and earth.

Measuring the capacitance in a twisted pair of wires burries too many 'perhaps' and estimation work. Measuring the current flow through each segment during HV pulse pauses is from my sight, the most promising solution. This kind of measurement just requires a minimum voltage drop of 910mV to indicate the segment is OK. All higher voltages will indicate an OK-status too.

If the minimum voltage drop is not present there is definitely something wrong with the fence.

I know that it takes a lot of circuitry, but as I already mentioned will the investment by far not exceed the cost for a completely new household now and then, not to speak about items which will not replaceable like personal and unique belongings.

Regards

Hans

P.S. A timing diagram is already contained in my faulty schematic. Correction to follow.