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From: francis@cs.ua.oz.au (Francis Vaughan)
Newsgroups: sci.electronics
Subject: Re: Library anti-theft walk-thru device...????
Message-ID: <3206@sirius.ucs.adelaide.edu.au>
Date: 7 May 91 09:34:53 GMT
References: <050691.224256WDBURNS%MTUS5.BITNET>
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In article <050691.224256WDBURNS%MTUS5.BITNET>, WDBURNS%MTUS5.BITNET
(William D. Burns) writes:
|> Greetings netlanders!
|> 
|> Quick question:
|>   Our university library installed one of those anti-theft walk-thru
|>   devices and I was wondering how the little bugger works.
|>   
|>   Observations:
|>   1) When walking thru with Walkman (tm sony) on TAPE-playback, I heard
|>      4 or 5 very short, high frequence "clicks" thru my headphones.  (Of
|>      course I had to go back thru again to hear 'em again! :)  They swear
|>      that this device is safe for magnetic media (i.e. disks).
|> 
|>    2) In the binding of the library's books was a strip of paper (dimensions
|>        of 5 X .25 inches) that had evenly-spaced metal strips in it.
|> 
|>    3) To allow a book to pass thru without setting off all the alarms, the
|>       library person runs the book binding over a bulk-eraser type thing.
|> 
|> So, how does this little thing work?  Is it sending a low energy magnetic
|> pulses out and picking up and strange resonances from the little strip?  Or
|> is the wave RF in nature?

The former. As I understand it the little chunks of metal are a very 
interesting magnetic material. It has a different hysterysis curve depending 
upon its state of magnetization.

In one state of magnetisation the curve has very pointy edges, so if you drive 
it around the end of the curve with an alternating field, as it is driven over 
the point it emits a bit of RF. The metal strip that the little chunks are 
bonded to may act as an antenna helping to propogate the signal.

So if you are trying to steal a book, you walk through the alternating field, 
and a detector is looking for little pulses of RF that are synchronous with the
field. If they are there it squarks.

When you borrow the book the librarian degausses (or maybe magnetises,
intuitivly
I suspect degausing) the chunks in the book so changing the shape of the curve 
to have more rounded corners and hence no RF pulsing out. Returned books get 
the reverse treatment.

The field strength of the detector can be quite low, it only needs to drive
around one corner of the hysterysis curve. Magnetic media should be safe.
Pacemakers however may be a different story.

Maybe someone else can provide some more specific info, and/or corrections.

						Francis Vaughan.