Anti-shoplifting tags teardown

Thixotropy article tells about the technology of anti-shoplifting tags used on store merchandise. They are high-Q tank circuits which if carried between a transmitting and receiving panel at a store’s doorway, they enhance 9 MHz signal propagation between those two panels and the alarm goes off.

When a purchase is made, the store clerk deactivates the tag with a high energy 9 MHz energy burst that blows the capacitor to a short circuit.

For information on other types of tags used for the same purpose, check Electronic article surveillance article.


  1. Yvon says:

    This is neat, I did not know how these things worked before. I know that they usually rub the things on a magnet before bagging your items, is that what short circuits the chips? I have not seen those chips before so are they something new that stores are using?

  2. Tomi Engdahl says:

    The Picard Loop

    Consider this diagram of two wire loops. It is the basic sensor used in some anti-shoplifting systems. I am told that it was patented in 1938 by one Pierre Picard and that it is called the “Picard Loop” in his honor.

    The basic operating principle is that an excitation signal is applied to the red coil of wire to create an alternating magnetic field to which the blue figure-eight coil of wire does not respond because with perfectly balanced construction, the pickups of the two halves of the blue wire cancel each other out. However, if someone comes near with a magnetically active tag, something that can magnetically saturate at a pretty low flux density, that tag will disrupt the “blue” winding’s balance and cause an output signal to be generated.

    These things have been deployed as a part of anti-shoplifting systems for many years. In theory, if you carried that tag exactly along the right path, you could preserve cancellation and the balanced coupling, but unless you are one of The Flying Wallendas, the odds that you could actually manage to do that are virtually zero.


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