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From: rambler@pnet51.orb.mn.org (Dan Meyer)
Newsgroups: sci.electronics
Subject: Re: Library anti-theft walk-thru device...????
Message-ID: <4863@orbit.cts.com>
Date: 9 May 91 03:25:04 GMT
Sender: news@orbit.cts.com
Organization: People-Net [pnet51], Minneapolis, MN.
Lines: 37

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?
>
>Just an enquiring mind wanting to know. :)
>
>Bill

As I understand the system in use here in the Twin Cities Metro area, *any*
book that passes through the detectors will sound the alarm. the reason the
alarm does not go off when you check out books is the librarian passes the
books *around* the detector. If you don't believe me, check out a book, and
then go back in and the back out with the book.

-- Dan
Remember: " Buffalo never Oink " Seen on a South Dakota travel brocure.
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