In many systems, like at BlockBuster, the "bar code" is just
a fake label to disquise the fact that it contains the device.

In some systems there is a diode between the metal foil
patterns which act as an antenna to pick up 2 GHz
radiation, and reradiate back a lot of harmonics, which
in turn are picked up by the alarm receiver.  The device
is defanged by applying a pulse that destroys the
diode.

But I have also disected some tags that are slipped into
the binding of library books, that seem to be only a
couple of parallel foil strips, no diode.  I can't imagine
how they work, but would also be interested if anyone
has the answer.

John
-----Original Message-----
From:   Jarek Pawlega [SMTP:jpawlega@PERLE.COM]
Sent:   Monday, December 01, 1997 2:46 PM
To:     PICLIST@MITVMA.MIT.EDU
Subject:        anti-theft sticker

I went to the bookstore, during my lunch break today
and I bought a book. There was a plastic sticker on the
book, with a bar-code on it and two isolated strips
of metal inside the sticker.

 As soon as the bar-code is scanned the anti-theft
gate doesn't 'care' the sticker, assuming, the good was
paid. Otherwise, carrying the book through the gate will cause
activating the alarm system.

 We all used to see them, almost everywhere.

 Does anyone of you, gentlemen, know, how this
system is called, how the sticker 'works' ?

 I guess, it might be some kind of resonant / absorbing
device, those two strips of metal are acting like
but I can't believe it is addressable 'by sticker'.
It should require very high accuracy.


Regards,
Jarek


p.s.
Of course, I'm not going to still anything. Except some knowledge.