eklhad@ihnet.UUCP (K. A. Dahlke) (11/25/84)
Anybody know how those anti-shoplifting devices work? You know, those little plastic bars they stick on clothes and things, and terrible alarms go off if you sneak one past the store exit. When you buy the stuff, they remove the plastic device. Is it magnetic, radio, or what? They are quite small and light (and hopefully cheap). -- Karl Dahlke ihnp4!ihnet!eklhad
herbie@watdcsu.UUCP (Herb Chong, Computing Services) (11/26/84)
If they are the same ones that I have heard of, the have a magnetized
wire inside them.
Herb Chong...
I'm user-friendly -- I don't byte, I nybble....
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N2L 3G1 (519)885-1211 x3524mike@amdcad.UUCP (Mike Parker) (11/27/84)
> Anybody know how those anti-shoplifting devices work? > You know, those little plastic bars they stick on clothes and things, > and terrible alarms go off if you sneak one past the store exit. > When you buy the stuff, they remove the plastic device. > Is it magnetic, radio, or what? > They are quite small and light (and hopefully cheap). > -- > > Karl Dahlke ihnp4!ihnet!eklhad When I went to school in Philadelphia, I got a look inside one of those white things, nothing more complicated than a magnet in there. We often laughed about the possibilities of buying a couple dozen magnets from Radio Shack. We could quietly drop them into those big shopping bags with handles on them that so many people carry around in malls. Or, we could just stand in front of the mall giving away free magnets. For some unknown reason, we spared mall security the pleasure of this experience. Mike @ AMD
crandell@ut-sally.UUCP (Jim Crandell) (11/27/84)
> Anybody know how those anti-shoplifting devices work? > You know, those little plastic bars they stick on clothes and things, > and terrible alarms go off if you sneak one past the store exit. There may be numerous systems, but the one I've read about uses VHF radio waves, and the ``tag'' itself is fairly passive, usually containing just a resonant circuit, a diode and a small antenna. The alarm unit radiates a rather low-powered field in the vicinity of the store exits, and the tag receives this (usually) dead carrier, but the diode serves to distort the originally sinusoidal waveform so that it contains numerous nonfundamental frequency components, which the antenna then radiates, albeit weakly. The alarm unit's receiver is tuned to two or three times the transmitter's frequency, and so when a tag is brought near it, it senses the radiated harmonic, and the alarm goes off. -- Jim Crandell, C. S. Dept., The University of Texas at Austin {ihnp4,seismo,ctvax}!ut-sally!crandell