It is a resonant circuit, which is triggered by a transmitter in the
doorway. By passing a strong magnetic field over the device, it can be
deactivated - I am not quite sure how this works but I think what
happens is that part of the strip is ferromagnetic. If it is magnetised,
it affects the resonant circuit, either by changing the frequency or
introducing harmonic distortion.

Some variant devices use (I think) a small low-signal diode which is
destroyed by an electromagnetic pulse - until this is done passing the
circuit through the door will generate a strong second harmonic signal
which will trigger the alarm.

> ----------
> From:         Jarek Pawlega[SMTP:jpawlega@PERLE.COM]
> Reply To:     pic microcontroller discussion list
> Sent:         Tuesday, December 02, 1997 8:46 AM
> 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.
>