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Barcode sticker with chip on reverse side

Discussion in 'Electronic Basics' started by [email protected], Jan 18, 2004.

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  1. Guest

    I peeled off a barcode label that was covering some information on a
    package. The reverse side contains what looks like a microchip
    surrounded ny a spiral track.
    I'm guessing this has something to do with setting of a shop alarm if
    something leaves the premises that hasn't been zapped at the checkout.
    If I'm right, how does it work?
    If I'm wrong what's it for?
  2. Don Bruder

    Don Bruder Guest

    It is, as you've guessed, an anti-shoplifting device. Search google for
    "Grid dip meter" to get more information than you're ever likely to be
    interested in on the "how it works" part.

    The nutshell version:
    The spiral is a tuned antenna. The sensors at the entrance of the store
    are a combination transmitter/receiver. When there's no "antenna" in
    their field, they stay silent. When there is an antenna tuned to their
    frequency in their field, they do the equivalent of screaming
    "THIEF!!!". The zapper in the shop burns out or shorts together
    (depending on the exact system - details vary slightly from one to the
    next - the important part is that the antenna gets de-tuned, not the
    exact "how" of it happening) a segment of the spiral, which de-tunes the
    antenna - The sensors "can't see it" anymore, so they stay silent.

    The spiral on the label (or wherever else it's located) is
    electronically inert - The whole system relies on the tuned antenna that
    is the printed spiral causing a "grid dip" in the field strength, and
    that "dip" being detected by the sensors. You can, if you work at it,
    put together a sort of "jammer" for the sensors that will cause them to
    go off every time. It only takes the right length of wire, properly
    formed. Some items, like purses, belt buckles, boot shanks, and quite a
    few others, can be accidentally resonant, causing their owners no end of
    headaches when they try passing the sensors - They may not be carrying
    anything "stolen", but the sensors still wake up and scream "THIEF!!!!"
    because they're carrying/wearing something that unintentionally behaves
    the same way as the un-burned spiral that you saw on the label.
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