Sensormatic Resonant system

This document describes how the Sensormatic resonant tag system may work (record and clothing stores). The following data is provided for informational purposes only, and no guarantee is made to the accuracy of the contained information.

Sensormatic produces a system that appears to rely on resonance. A small rectangular piece of springy metal with magnetic properties somewhat similar to that of the Knogo system strips is biased with a permanent magnet along the long axis. Tags that will activate the alarm system show a sharp resonance around 58.2 khz. A theory as to how the system works is as follows.

A periodic pulsed signal of 58.150 (or therabouts) khz energy is sent to one of the coils. After the pulse period one (or both) coils "listens" for the ringing signal generated by the strip. Since the springy strip is very resonant, it will continue to oscillate after the signal is applied, much as a bell rings after you hit it. If the circuit detects a decaying signal of about the right frequency after the energizing pulse, it sounds the alarm.

Two experiments tend to confirm this idea. Two special boxes were built - one that was a small oscillator attached to a coil, and set to about the same frequency as the frequency that the coils generate. The other device was a tank circuit/amplifier combination, set so that it would be adjustably resonant over 58050 - 58350 hz. The amplifier was set so that the circuit would not oscillate, but would exibit an extremely high Q at the resonant frequency.

The oscillator did not trip the alarm, however the amplified tank circuit did.

There appear to be two main types of tag - the white tags with the fake bar code, and a beige removable tag. The white tags have two strips in them - a springy strip, and a strip of easily magnetizable metal. The beige tags have a plastic magnetized strip in them, and a couple of springy strips. The net effect of this is that it is possible to deactivate the white strips, where the beige tags are not easily permanently deactivated. This is because the metal strips have much lower retention than the plastic magnets, i.e. more easily influenced by outside magnetic fields.

The white strips can be activated by being placed in a magnetic field along the longitudinal axis, and then "shaken up" using a deguassing coil - this makes sure that the strip is uniformly magnetized at the right level. Deactivation is much simpler. If a fairly strong permanent magnet is placed against the face of the strip so that the magnetic field is perpendicular to the face of the strip, the strip will no longer activate the alarm. The same trick will work for the beige tags, but the magnet has to be left against the strip.


Coils can be placed relatively far apart, causing less of a problem than Checkpoint or Knogo Magnetic systems. Extremely resistant to false triggering. Disposable and reusable tags available.


Magnetism used to deactivate disposables, which may affect magnetic media such as tapes and diskettes. (Some testing here reveals that this is NOT the case, and if a person is buying tapes they can't be too concerned about sound quality anyway). System is susceptable to jamming, although this is detectable.