How do 'library book detectors' work?
Glen K Moore
a strip of some sort in the spine. The book (strip) can be demagnetised? and
then borrowed without setting off an alarm. Some clothing stores use a much
bulkier system which looks much more expensive and complex but the book
strips are a much simpler solution.
How does this work? What is composition of the strip?
Terry Braddock
g...@cc.uow.edu.au
Robin Fairbairns
I don't know about your library, but Cambridgeshire County Council
uses a thing that appears to be a coil-of-foil. The layout (holding a
paperback up to the light) isn't entirely clear in the books I
currently have in my brief-case, but it's a square-sided loop of stuff
with something-or-other sticking out into the middle of the loop.
--
Robin (Keep Radio 3 != Classic FM) Fairbairns r...@cl.cam.ac.uk
U of Cambridge Computer Lab, Pembroke St, Cambridge CB2 3QG, UK
Mr T.G. Griffiths
The strip contains a small LC circuit that is tuned to a certain
frequency. When it goes out the door that frequency is played at it. Any
resonance from the LC circuit can be picked up by the recivers that you walk
through as you go out - which sounds an alarm. To turn off the circuit you
simply magnetise it, which stifles the resonance. This can wear off in some
circumstances. Anyone who's been caught going INTO the libary with a book to
return will know this....
Tim
Michael Moroney
There are two types (that I know of), a one-time use (can't be reactivated
if deactivated) and a re-usable one, that can be repeatedly be deactivated
and reactivated. The first is a coil/capacitor combination that works
as a tuned circuit resonant at a particular frequency. This is usually
used as an anti-shoplifting device. It is deactivated at time of sale
usually by blowing/shorting the capacitor. These appear as a metal spiral
if you take one apart carefully.
The other type is some magnetic substance that, when activated, generates
harmonics of a certain frequency when placed in a magnetic field of that
frequency. At least this is how I understand it, I'm not quite up on this
type. This type has the advantage (for libraries) that it can be repeatedly
deactivated (when you check a book out) and reactivated (when returned).
-Mike
Stewart Smith
>Libraries can detect books illegally removed from libraries by placing
>a strip of some sort in the spine. The book (strip) can be demagnetised? and
> ...
>How does this work? What is composition of the strip?
>Terry Braddock
I'm not sure about the library strips, but I once bought something from a store
that had a similar kind of thing. This particular strip was a plastic thing
about 1cm wide, 2cm long and about 1-2mm thick. Inside is a cavity with two
pieces of metal foil - one was flat, the other curved slightly so that it
touches the first piece in the middle but the not at the ends.
I imagine it works like this (don't take my word for it - I'm only guessing):
The two foil bits are probably insulated. When they are magnetised,
they stick together; when demagnetised they spring apart again.
When sprung apart the system probably resonates at a known frequency so that if
you hit it with a burst of radio-frequency energy it will ring well enough for
you to pick up the echo (hence the big antennas you have to walk between on
your way out of the library).
I have heard of similar things being used by the millitary as passive pressure
transducers. If you have a sealed cavity with conductive plates on either end,
the resonant frequency of the cavity depends on the spacing of the places,
which is in turn dependant on the applied pressure.
Using a micromanipulator you can make the thing very small. The fun bit's next.
Apparently stress in animals is related to the pressure within the eye, so if
you implant such a device in the eye of (say) a rabbit, you can measure how
stressed it is by looking at the frequency of it's radio echoes. Such things
were being done long before the library systems came into being. Which just
goes to show, that people usually come up with the sinister applications
first :)
Stewart Smith
stew...@marsh.cs.curtin.edu.au
Alan Murta
g...@wampyr.cc.uow.edu.au (Glen K Moore) writes:
>Libraries can detect books illegally removed from libraries by placing
>a strip of some sort in the spine. The book (strip) can be demagnetised? and
> then borrowed without setting off an alarm. Some clothing stores use a much
> bulkier system which looks much more expensive and complex but the book
> strips are a much simpler solution.
I have the misfortune of living in a building in which access is gained
using a tuned inductance-capacitance device, of the anti-shoplifting kind.
I assume the typical detectors used by shops aren't too selective when it
comes to differentiating devices tuned to roughly similar frequencies, as I
manage to set off around 10% of all shop alarms I pass by.
Being accosted by security staff several times in one day can become tedious
after a while.
Alan.
Lee Boyle
> g...@wampyr.cc.uow.edu.au (Glen K Moore) writes:
>
> >Libraries can detect books illegally removed from libraries by placing
> >a strip of some sort in the spine. The book (strip) can be demagnetised? and
> > then borrowed without setting off an alarm. Some clothing stores use a much
> > bulkier system which looks much more expensive and complex but the book
> > strips are a much simpler solution.
>
> >How does this work? What is composition of the strip?
>
> harmonics of a certain frequency when placed in a magnetic field of that
> frequency. At least this is how I understand it, I'm not quite up on this
> type. This type has the advantage (for libraries) that it can be repeatedly
> deactivated (when you check a book out) and reactivated (when returned).
>
In school, a practical joker "friend" of mine managed to grab a
small box of these strips and entertained himself for weeks by
slipping them into his victim's backpack, jacket seam, etc.
He was dissapointed to find that the smallest he could cut them
and still have them function was about 6 inches.
--
Lee Boyle;boy...@agcs.com;standard disclaimers apply;;
Q8Z9000
>In article <C7M8o...@world.std.com>, mor...@world.std.com (Michael Moroney) writes:
>> g...@wampyr.cc.uow.edu.au (Glen K Moore) writes:
>>
>> >Libraries can detect books illegally removed from libraries by placing
>> >a strip of some sort in the spine. The book (strip) can be demagnetised? and
>> > then borrowed without setting off an alarm. Some clothing stores use a much
>> > bulkier system which looks much more expensive and complex but the book
>> > strips are a much simpler solution.
>>
>> >How does this work? What is composition of the strip?
>>
>> The other type is some magnetic substance that, when activated, generates
>> harmonics of a certain frequency when placed in a magnetic field of that
>> frequency. At least this is how I understand it, I'm not quite up on this
>> type. This type has the advantage (for libraries) that it can be repeatedly
>> deactivated (when you check a book out) and reactivated (when returned).
>>
>
an extremely uniform magnetic field. The strip perturbs the uniformity
of the field, this is detected and the appropriate buzzers/spotlights/
automatic weapons are activated. Anyone else know more about this?
M.A. MacKenzie
B. C. Webb
Helmholtz coil sets up a radio frequency field.
The saturated (all magnetized one direction) magneticly soft
material responds non-linearly to the applied field, since
if the field tries to magnetize it more in the saturated direction it
fails since it's all lined up, but it can move away from saturation
for the other half of the sine wave. Thus the material acts like a
diode. A rectifying response like this creates a harmonic with
twice the frequency - this is detected and the alarm goes off.
If the material is demagnetized no harmonic is generated - thus the
strip can be turned on and off.
The limits to detectability are either due to just needing enough material
or else the strip has to be a reasonable fraction of the rf wavelength.
Some of the bulky things in stores just contain an actual diode itself
with some antenna wires, but use the same detection scheme.
Ranger Mau'Dib
>As I understand it, it's a simple Helmholtz coil set up, which produces
>an extremely uniform magnetic field. The strip perturbs the uniformity
>of the field, this is detected and the appropriate buzzers/spotlights/
>automatic weapons are activated. Anyone else know more about this?
I don't know how the detector works, but I do know how the metal works:
The metal was originally designed to be a new super alloy. Part of it's
construction is to take the still hot (sometimes molten) metal, pour it into
whatever mold you want and then quickly dunk it into a bucket of extremely cold
water. It has been a while since I saw the show explaining how they were made,
but I think it might not have been water.
Anyways, the metal doesn't crystalize at all, but it still didn't have any
strength. Just the neat property that it could be magnetized and demagnetized
at will. The field the detectors create can be interfered with, but the
detectors can get a pattern of the disturbance. The neat thing is it detects
the magnetic strip in all that interference. Sometimes the demagnetized strip
is still magentized enough to set off other library's detectors (Like the
public library books I bring into the S.F.U. library will set off their
detectors but won't even bother another library's detectors).
The clothes thing is evidentally customizable. It can generate a specific
pattern so other types of protectors go unnoticed but the store's one will get
a BEEP-BEEP-BEEP "Step away from the underwear, sir". This whole field is quite
amazing. I mean the subject of study, that is. :-)
Travers "I was the inspiration for Beakman" Naran
Have a _NICE_ day! :-)
--
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Travers Naran | "We fear
Mail address: na...@fraser.sfu.ca or na...@sfu.ca | change. WHACK!
Simon Fraser University, British Columbia, Canada | WHACK! WHACK!"
Cmpt. Science student wanna-be | -Garth
Trekker, Leaper, Red Dwarf'er, Prober, etc. | "Wayne's World"
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James Gibbs
Another thing about these detectors is that I can tell if they are on
because my Walkman humms as I go through them.