Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!decwrl!reid
From: reid@decwrl.dec.com (Brian Reid)
Newsgroups: sci.electronics
Subject: Re: Library book detectors.
Message-ID: <341@bacchus.DEC.COM>
Date: 30 Mar 88 01:08:21 GMT
References: <5398@swan.ulowell.edu| <1261@uop.edu| <2521@ihuxv.ATT.COM| <2530@ihuxv.ATT.COM> <2258@c3pe.UUCP>
Reply-To: reid@decwrl.UUCP (Brian Reid)
Distribution: na
Organization: DEC Western Research
Lines: 34


I have explored the inside of one library anti-theft system; I expect that
others might work the same way.

As several posters have noted, the anti-theft device itself is a collection
of high-Q tuned circuits. A band-sweep ping is sent out a few times a second,
and the response frequency signature is recorded. This system used devices
with  4 different tuned circuits in them, and each tuned circuit could be one
of 100 different center frequencies. This gives a large number of possible
combinations. The frequencies are in the hundreds of megahertz; I don't
recall the exact band. Something like 320 MHz comes to mind.

When a book is checked out, the librarian runs it over a "deactivator".
In fact, that deactivator is another sweep pinger, and it records the
frequency signature in a short-term database. For the next K minutes, the
alarm will not ring when the matching anti-theft tag passes through the
doorway detector. A few hours later, the entry expires from the short-term
memory.

This scheme does not even TRY to prevent duplicates. They just take the
detectors out of the box and stick them into books. Because there are a
lot of different combinations, the probability of duplicates is very low,
but if by some remote chance I check out a book and then you try to steal
one, and your anti-theft code is the same as mine, then you can steal the
book undetected.

At the library whose operation I am familiar with, the expiration period was
about 2 hours. This allowed people to make a trip or two back into the
library with the recently-checked-out books without ringing any bells.

In other words, the anti-theft device wasn't deactivated, but the detector
was trained not to buzz on its number.

Brian Reid