metal fence alarm project

[lerameur]

Hi,

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,

ken

 

[Bob Masta]

Hi,

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,

ken

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):
http://en.wikipedia.org/wiki/Time-domain_reflectometer
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
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
Science with your sound card!

 

[John Larkin]

Hi,

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,

ken

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

 

[lerameur]

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

 

[John Larkin]

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

 

[me]

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

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

 

[Ross Herbert]

even an

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

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

 

[[email protected]]

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

- Show quoted text -

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

 

[Bob Masta]

Just saw this TDR article referenced in EDN:
http://www.edn.com/article/CA6470825

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


Best regards,


Bob Masta

DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
Science with your sound card!

 

[sparky]

Hi,

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,

ken

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.

 

[John Fields]

Hi,

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.