What The Hell Is That Thing?
Why Am I Hearing Those Beeps?

The object in the illustration is called an Inventory Control Device (ICD). It's placed on merchandise in stores, and in library books, in an attempt to keep the stuff from walking out the door.

Inventory Control Device

I became interested in these things in the early 1980s, when I first saw the exit devices and the tags appearing on items, and the cashiers were very strict about my having to get those damned things neutralized.

Most people imagine that these things are magnetic 1. Well, I wondered about that and quickly realized that they were not likely to be simple magnets. My reasoning was: 1) too many people are walking around with iron objects, they would set off a magnetic detector all the time, and 2) the ICD is very light and would you would need for the magnetic detectors to be extremely sensitive, so sensitive that they would be detecting every truck and bus that came within 50 feet of the detectors, there is obviously no magnetic shielding around the detectors. So I figured that a magnetometer was out for this arrangement.

I wondered about these things for a little bit. They worked, and they were not giving a lot of false positives. So this had to be something electromagnetic.

And then it hit me. This was radio technology, and rather ancient radio technology at that! I figured that those extremely light ICDs were some sort of a radio circuit. Now obviously they can't have a power source to allow them to be broadcasting 24 hours a day. So they must be very simple circuits.

Well, there are such circuits. They are called Tuned Radio Frequency (TRF) circuits. Basically, these TRF circuits constitute tiny radio receivers which will resonate at a very specific and, you hope, narrow frequency.

Those two towers you see on exiting the store or library are radio transmitters. They put out a very feeble signal on the exact same radio frequency at which the TRF circuit in the ICD is set to resonate. In the presence of this signal the ICD becomes a Parasitic Oscillator. In other words it is receiving the signal from the two towers, it's oscillating. This is exactly how the front end of your radio and TV works, they receive a certain frequency and no others, if everything's working properly.

Now the signal those two towers are putting out is very weak and there is a property of the Parasitic Oscillator that comes to the fore here: it's absorbing the radio frequency energy they put out, just as your radio does. The broadcasting towers are set up to try and maintain a set field strength. When that field strength is lowered, as when the ICD absorbs some of their energy, this can be readily detected. I don't have any inside knowledge about how this detection is accomplished, a quick and dirty way would be to simply monitor the amount of current that the tower transmitters draw, but it's pretty easily done.

When a change in the field density is detected, through whatever means they're using, another circuit is triggered and that sets off those alarms which are supposed to draw the attention of the security guards standing around by the door.

All right, now we know how the ICD system works, but how do you get it to stop working? After all, you need to have people walk out the door with stuff after they've purchased it without setting off the alarm and being harassed by the guards.

There are two ways of accomplishing this, you can either remove the ICD or you can detune it, destroy its ability to oscillate at the exact frequency being broadcast by the two towers.

Most people have seen the older ICDs that are still used on clothing. They're big, white plastic things that are in two parts which are held together by pins that go through the fabric of the clothing, and sometimes tear the weave a little. A special device is used to bend the plastic knobs on those ICDs and unlock the two halves from each other.

The other type of ICD, the one illustrated here, is much cheaper and smaller than the older one and so they are simply deformed and allowed to walk out of the store, effectively neutralized.

To show how this is done we have to discuss the ICD itself a little bit.

The ICD Dissected

The TRF consists of a set inductance and capacitance in series; in the physical world this requires a coil and a capacitor. Simply out, this is how a radio receiver works. In the illustrated ICD we have a capacitor at the center of the whole thing. That solid, square thing in the middle of the ICD is one plate of the capacitor. A capacitor will consist of two "plates," each of which is oppositely charged from the other "plate." That is, one plate will hold more or fewer electrons than the other. The coil in our illustration is that bunch of almost rectangles with rounded corners surrounding the central capacitor. It may not look like what you expect a coil to look like but it is one, it's just flattened out. The two ends of the coil are connected to the two plates of the capacitor.

The coil and capacitor are joined at the upper left in our illustration by what appears to be a crimp. That colorful thing going from the capacitor plate to the crimp is basically a wire. It's flat, and the coil's shape is different around it, so that it will not interfere with the coil and change the resonant frequency of the ICD. That rectangular thing with the rows of tiny circles in it is a structural member that keeps the center of the ICD all lined up correctly.

So to deactivate this ICD all that's needed is to change the value of the capacitor or the coil. The manufacturers of this ICD have chosen the method of either altering or destroying the capacitor. See those two white circles in the middle of the capacitor plate? Those are there to destroy the capacitor. When it's time for the cashier to check you out the ICD is pressed against a specially designed surface that presses those two metal circles into the capacitor plate. This will either press the two plates of the capacitor together, shorting out the TRF circuit, or else will lessen the distance between the two plates, which will tune it to a different frequency all together.

After this treatment the ICD no longer resonates at the exact frequency that the towers are broadcasting at and it, and the merchandise it's attached to, can pass without setting off the alarm.

I've seen some systems where pressing the ICD against the neutralizing device drives a very tiny razor blade through the coil, thereby cutting it and opening the TRF circuit accomplishing the same thing as the above description.

In an effort to keep people from guessing what's going on the manufacturers of these ICD systems like to let people think that they are in fact magnetic. To this end they have even installed magnets on the neutralizing devices to mislead the cashiers. All these magnets accomplish is to erase the ATM cards of the cashiers when they accidently pass a wallet or purse across the magnet!

Postscript - This discussion is about how the ICD works. It's not a tutorial on how to defeat these devices. Enough information is left out of this discussion so that anyone attempting to use this information to steal things would probably get caught.

1There is another system! An article in the back of the May 1997, issue of Scientific American, p. 120, says that there is a magnetic element to these devices. Is the author, Joseph Ryan, Jr., who is a V.P. for global source tagging at a firm which makes these things, describing a different system, or is he perpetuating a myth? Or am I just completely wrong? This determination is left up to the reader. I do note that this particular “Working Knowledge” section of Scientific American is not reproduced on their Web site. Resume reading

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