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Date: Tue, 26 Nov 2002 18:25:11 +0100
From: Rene Tschaggelar
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Subject: Re: wire loop proximity alarm schematic
Smile, it can only get WORSE wrote:
> Hi Everyone,
> I'd like to build a large loop proximity alarm to detect humans
> entering our yard. PIR motion detectors are impractical as it would
> take too many of them to cover the area around our house. I know the
> PIR is popular and cheap today, but I really want an old fashioned
> simple proximity loop based alarm.
> Years ago, they had wire loops strung at 8 to 10 feet above ground.
> The loop was the inductor in a free running oscillator circuit.
> Whenever a human (or critter) came near the loop, the resonant
> frequency would shift due to the added stray capacitance...and the
> loop would change frequency. The short term (rapid) change in
> frequency was detected, and it set off the alarm.
> My guess is that they would use a slow responding phase locked loop
> and detect the time that it took for the loop to relock in order to
> control the sensitivity.
> I don't see these being used today and a web search seems to turn up
> Anyone have plans for these??? I'd like to have a battery powered
> unit, but anything that runs is OK....the battery powered unit would
> be for keeping smaller critters out of our garden.
Why would such a loop detect humans ?
Because they influence the field with the rifles and guns ?
A low frequency wire has primarily a magnetic field that
is influenced by magnetic conductors, such as iron. At a certain degree
other conductors can be detected as they induce stray fields.
You may not go much higher in frequency as when it really begins
to radiate, the FCC will come to knok on your door.
Then consider wind to change the configuration and produce false
alarms. And finally, it will act as lightning protecting and be
susceptible to wheather again.
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
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