Sensor switch for DIY Taut Wire Alarm System


Old 08-13-13, 09:50 PM
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Sensor switch for DIY Taut Wire Alarm System


We are a residential complex having 500 meters of boundary wall.
It is covered with barbed wire, but as a secondary precaution, we intend to install a Taut Wire Alarm System which we will install ourselves
Basically we will be running 50m of Steel Wire in 10 sections to cover the entire boundary wall, which will be tensioned and then hooked upto sensors on both sides.
In case of any increase or decrease in the tension load, the sensor should trigger an alarm.
Any recommendation on which type of sensors we can go for which will serve our purpose

Thanks for advising
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Old 08-14-13, 02:43 AM
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This is the only switch like this that I can readily think of:
Old 08-14-13, 08:08 AM
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50 meters/150+ feet of metal line. You are talking about a heavy duty type device. Something to carry that much weight and detect stress and not "false alarm." Quite a demand.

You'll almost need to look into prison fence systems for detection methods.
Old 08-15-13, 10:42 PM
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'50 meters/150+ feet of metal line. You are talking about a heavy duty type device. Something to carry that much weight and detect stress and not "false alarm." Quite a demand.'

Just to piggyback onto what PJmax said:
I don't want to discourage you from going ahead with your DIY project, but 35 years of troubleshooting alarm problems has given me the habit of thinking ahead when an idea is proposed. Please forgive me if I sound pedantic and you may already have thought this through, but I would be remiss if I didn't point out several things to consider:

We ~don't~ live in a static world. The environment around us is active all the time, although we usually perceive only a small fraction of the activity. This is an order of magnitude more so when you move outside of protective enclosures like homes and offices.

50 meters of steel wire is going to sag in the middle, not a big problem, but you're going to have to adjust the tension so that whatever sensor you use will be tripped with only a few cm of movement at the ends. You won't be able to adjust the tension one time and be done with it.

The greatest difficulty to overcome in any alarm system is not the developing of devices that will detect an intruder. That part is easy. The real trick is to develop a system that will detect only intruders. In other words, will not trip from other environmental factors, a.k.a. False Alarms. An outside system that doesn't have false alarms is especially difficult.

Most environmental false alarm factors are intermittent, occasional, not often, every now and then. They don't necessarily happen every single night, or even every week. But in the alarm industry, even once a month is too much if it's chronic. Our customers are quite adamant on this point; and you may find that your customers (your community) will be too. Alarm equipment needs to work reliably 60/60/24/7/365, & 366 on leap years.

(1) You haven't mentioned what climate you're in, but all climates have their environmental factors: Where I am (near Washington DC, USA), we don't have weather extremes, i.e., it rarely gets over 35 degrees in mid-summer or much under -5 deg in mid-winter. But even here, 50m of wire would collect a substantial additional weight of ice once or several times a winter. That's going to play hell with your tension adjustment, and just in the kind of weather you -don't- want to be out making adjustments to 10 sensor posts in the fence.

(2)We rarely catch the edge of a hurricane here, but we get 15 meter/sec winds a few times a year, with gusts up to 20m/s not unusual. Think about the effect that will exert over 50m or wire.

(3) Our occasional ice storms aren't severe, but add ice accumulation to storm winds, and figure the effect on the wire. Also, adding rain to wind increases the force exerted on a long stretch of wire.

(4)Birds land on wires in both winter and summer here, but more of them and more often in summer. Don't rely on how often you've noticed it, unless you're a birder. Ask a birder/naturalist in your neighborhood how many birds perch on wires, and what kind. Around here, red-tail hawks are a common sight on telephone lines. One of those probably equals the weight of a several crows, which would exert a considerable tension in the middle of 50m of wire.

(5)Temperatures from winter to summer, and at different times of day are a consideration: What is the coefficient of expansion of the steel wire you're planning to use? It will be a factor in the tension between the sensor posts. I can't work out the math, not knowing the COE of your proposed wire, but I suspect that 50m of wire will expand or contract enough in different seasons and different days and different times of day, to make a substantial difference in the tension of the wire.

All of these factors might not be difficult for a 5 meter length of wire, but over 50 meters they can be daunting. The farther between posts, the thicker the wire you need to use, and the more ice, birds and wind cross-section that thicker wire offers per meter of wire, and the more tension is added or subtracted by wire shrinking or expanding with changing temperatures.

The bottom line is that I suspect planning 50 meters between sensor posts is overly optimistic. I understand you'd like to minimize the costs, but I suspect that cutting the number of sensor posts by a factor of 10 will increase the problems by at least that much, and probably more.

Those are just my thoughts for your consideration.

Incidentally, I assume you're planning to enclose the sensors in tamper-protected boxes with holes for the wires? And you've already planned how to protect the alarm cable from sensor boxes to control panel from tampering?
Old 08-16-13, 02:59 AM
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Thanks for advising.
Let me clear a few doubts for you.
We are in Bangalore where the temperatures are pretty moderate
5C to 35C
No snow or hurricanes
Moreover we already have coiled barbed wire on the fence and this wire will be just as the same height as the top most portion of the barbed wire, so birds will avoid it.

We have the liberty to set the activation range anywhere from 50 gms to 50 Kgs
So with a bit of trial and error, we should be able to determine a ideal point where the false alarms are very limited and genuine intruders are not missed

Basically I am thinking of attching one end of the wire directly to the post and the other end to a S type Load Cell and then the post. The Load cell is connected to a programmable indicator wherein we can set the parameters for the alarm trigger
Using a screw mechanism we will give tension to the wire and set the indicator to trigger an alarm in case the load changes beyond say +- 5 kgs

Any loophole I am missing in my scheme of things ?
Thanks for advising
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