How do the security tabs on store merchandise and library books work?
People seem to wave a magnet or something near them to deactivate them,
or else they set off an alarm when passed through the exit.
 
  I took one apart the other day (a white tab on a CD from Best Buy), and
inside was a trapezoidal piece of metal that dented, and a rectangular
piece of thinner metal that doesn't dent easily (it makes an interesting
twangy sound), reflective on one side, not on the other.  Both metals are
attracted by magnets, and may be magnetic themselves (or I may have caused
that).  The metals are separated by a thin clear plastic.  Neat device!  I
heard it's an "RF reflector," but I just don't get it.  Does someone else?

18 responses total.

Something to do with whether it is magnatized or not.  I once read a
description of the process, but have forgotten most of it.  The gist was that
the tab would display different RF reflecting or resonating properties
depending on whether or not the ferrous parts were magnetized.  The gate at
the store entrance sends out a signal at a specific freqencie[s] and looks
for a certain reflection back.

Thanks for entering this! I have wondered idly how they work, but
never gave it enough thought to disassemble one. Magnetization would
change the permittivity of the metal, and could change the resonant
frequency of a resonant circuit made from the metal. Were the two pieces
of metal formed into a "U" - that is, connected anywhere? I guess the
next thing is to go into the store with my RF counter, to try to
measure the frequency.

  I'm pretty sure the metals were completely separated by a thin sheet of
adhesive clear plastic.  I wonder if one piece being trapezoidal is 
significant, or if they just did that to make them easy to tell apart
in the factory (the latter seems unlikely).

I've taken one of these things apart too.  Mine was similar except it didn't
have an insulator / the thin sheets of metal.  Also at least one of the pieces
was arched (I bent up the other while taking it apart.)  I figured that they
were originaly magnitized in such a way that the were attracted to one another
and had one resonant freq.  When demagnitized, they pulled apart and were reso
nent at a different freq.  The same would be true if they were seperated by an
insulator... Which, come to think or it, mine may have been...  It's been a 
while since I pulled it apart.

The strips of metal act as an antenna and inductors.  The gap between them is
the capacitve element.  In this case the L is a constant while C can be varied
by magnitism, changing the resonant freq. of the circuit.  Detect that change 
and you have a working system.

Wouldn't that mean they operate at microwave frequencies? 

That was my guess.  Microwave would be a cheaper way to go to.

  So if you've got this inductive antenna device, whose resonant
frequency can be changed, what do the microwaves they transmit do
that allows them to detect the difference from afar?  Why wouldn't
the same frequency waves bounce off the device (and your keys or
other reflective materials) regardless of the device's freq?  (Go
easy on me, I'm unfamiliar with this stuff :-).

  Oh wait, maybe I get it...it absorbs energy from the microwaves,
through inductance, somehow, and generates its *own* freq waves as
a result?  Hmm.  That would work?

It's a "harmonic resonator" of sorts. The pieces of metal are tuned to the
same frequency as a transmitting source in detector at the store entrance.
When enabled, they absorb the radiated energy and retransmit some of it
at a different frequency. A reciever in the detector picks this up and sets off
the alarm. When disabled, the device is no longer tuned to the detector's
transmission frequency.

It could not absorb energy at one frequency and transmit it another
unless or had a lot of electronics *or* it is a nonlinear device and
a harmonic is detected. 

What I was suggesting, was that the "other freqency" was a harmonic of
the base frequency.

Hmmm, what if the 2 pieces of metal are each responsive to 2 different
frequencys that are transmitted by the detector. This could result in the
unit producing a beat frequency possibly?

Hey, the harmonic bit sounds *very* familiar.  Plus which, by coding the RF
you send out, you can look for the same coding coming back to filter out
background RF.

So how does the magnetization (or demagnetization) turn on (off) the
echo of the harmonic (coded or uncoded)?

Good question.  Wish I knew the answer!  Any hams out there know this one?

I doubt there is any "code" in these things. I'll bet the security tags
come "armed" and are demagnitized at the stores purchase counter before
you leave, "disarming" them.  Wonder if they can be put on cassette tapes
or non CD based software.  The demagnization proccess would also erase
the tapes or disks...

Well, the library does *not* use this method on tapes, which is a good clue.
BTW, when I mentioned coding I meant that the transmitter would put out a
specific pulse train and then look for the same coming back.

Also, the library reuses the tabs (hidden somewhere far inside the book), so
that it must be possible to re-magnetize the tags in order to re-arm them.

I suspect that the transmitter puts out a lot of FR noise and the resonant
tag will then cause it to have more RF at the resonant freq.  Same concept
as playing white noise inside a tube.  Those freqs. at which the tube is
resonant will be reinforced.

I guess the tabs could be re-magnitized.  The ends of the thin metal 
strips would repell oneanother when magnitized.  When de-magnitized
they would be closer together.

I often work the checkouts at Best Buy.  We call the thingys stuck to the
products "soft tags" and there are also "hard tags" which are attached to 
camcorders, keys, and stuff.  Hard tags are not supposed to leave ths
store, so I don't know if the cashier's magnet can disable them.  The
magnet has to come in contact with a soft tag.  It doesn't work at any 
distance, so maybe that's why it's no problem to soft tag things like
floppy disks.  Music cassettes don't need soft tags because they are 
displayed in those long plastic holders which cashiers have a key to
remove.  The holder, not the cassette has a soft tag on it. 

If you've got a credit card which has expired, it might be fun to hit it
as if it were a soft tag and see if there's enough there to disable the
stripe on the back.

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