Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site ut-sally.UUCP Path: utzoo!watmath!clyde!bonnie!akgua!sdcsvax!sdcrdcf!hplabs!hao!cires!nbires!ut-sally!crandell From: crandell@ut-sally.UUCP (Jim Crandell) Newsgroups: net.physics Subject: Re: anti-shoplifting devices Message-ID: <326@ut-sally.UUCP> Date: Tue, 27-Nov-84 12:37:48 EST Article-I.D.: ut-sally.326 Posted: Tue Nov 27 12:37:48 1984 Date-Received: Fri, 30-Nov-84 07:45:13 EST References: <179@ihnet.UUCP> Organization: U. Texas CS Dept., Austin, Texas Lines: 19 > 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