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From: "Michael Morrow"
Subject: Re: wire loop proximity alarm schematic
Date: Tue, 26 Nov 2002 13:26:51 -0800
Organization: Posted via Supernews, http://www.supernews.com
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
What event happens after a human or critter is detected?
Really old fashion? How bout a fence?
"Smile, it can only get WORSE" wrote in message
> 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.
> Thanks for any suggestions.
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