How Store Anti-Theft Alarms Work: Magnetostriction

Now that’s uncanny. Two days before [Ben Krasnow] of the Applied Science YouTube blog posted this video on anti-theft tags that use magnetostriction, we wrote a blog post about a firm that’s using inverse-magnetostriction to generate electricity. Strange synchronicity!

[Ben] takes apart those rectangular plastic security tags that end up embarrassing everyone when the sales people forget to demagnetize them before you leave the store. Inside are two metal strips. One strip gets magnetized and demagnetized, and the other is magnetostrictive — meaning it changes length ever so slightly in the presence of a magnetic field.

A sender coil hits the magnetostrictive strip with a pulsed signal at the strip’s resonant frequency, around 58kHz. The strip expands and contracts along with the sender’s magnetic field. When the sender’s pulse stops, the strip keeps vibrating for a tiny bit of time, emitting an AC magnetic field that’s picked up by the detector. You’re busted.

The final wrinkle is the magnetizable metal strip inside the tag. When it’s not magnetized at all, or magnetized too strongly, the magnetostrictive strip doesn’t respond as much to the sender’s field. When the bias magnet is magnetized just right, the other strip rings like it’s supposed to. Which is why they “demagnetize” the strips at checkout.

We haven’t even spoiled [Ben]’s explanation. He does an amazing job of investigating all of this. He even measures these small strips changing their length by ten parts per million. It’s a great bit of low-tech measurement that ends up being right on the money and deserves the top spot in your “to watch” list.

And now that magenetostriction is in our collective unconscious, what’s the next place we’ll see it pop up?

44 thoughts on “How Store Anti-Theft Alarms Work: Magnetostriction

    1. An old co-worker of mine used to work in a movie rental store when he was younger. He had way too much fun with those tags. Being annoyed with customers walking off with his pens, he started putting these tags on his pens – much to the chagrin of (accidental?) thieving customers. Or if bored on the graveyard shift, he’d tag items he knew the day shift wouldn’t think to demagnetize – like chocolate bars, etc. He was a fun loving putz of a guy to work with.

    2. You should come and see how it is here in Berkeley. I don’t know how we got to this point, if it is just a cultural expectation, or a general dopeyness of residents around here, but it is absolutely standard for people to set off their car alarms as they are getting into their car. All day long, you will hear car alarms that got for 3-4 beats, and then get shut off. It is one of the most astonishing things that I have seen in my life.

      If I try to work from home, I will hear 30+ of these, every day.

        1. When I was a kid I lived at a place that had B-1Bs. This was when car alarms seemed to start getting popular.

          Each time one came into Land, it would set them off for half the city. (at least) Though it was amusing to hear. I wondered at the time why, because someone could just wait for one to come in, and no one would hear it. After a few years, the alarms got rarer. I later wondered if it was because it was a fad, or if it was because of the owners getting first the B-1Bs rattling the windows and doors, then having an annoying car alarm go off at least twenty times a day.

      1. It’s not the car alarms in Berkeley, it’s the car alarm owners in Berkeley. Move to a nice town in the Midwest. You won’t be bothered with these car alarms. It’s not that there aren’t car alarms in the Midwest; it’s the people.

  1. So, if I make a small 56kHz coil-cap LC resonator to fit in my coat pocket it will drive the system nuts as it would have a lot of signal compared to a single strip and be detected at a much larger distance?

    1. Some of the permanent security tag such as those inside the spider wrap uses coil and cap to provide resonance when exposed to the field. It cannot be turned off short of destroying it and risk tripping other alarm. Some of the newer security devices has 3 alarm systems, one that goes off at the security pedestal, one that loudly beeps when it’s disconnected with a key to discourage those with stolen key or homemade key, and one that goes off if someone cuts it.

      Once I left Walmart with the clear security case that cashier didn’t remove (new probably) and no one came to check my bag after a few minutes. When home I cut it apart and found such permanent security tag, About half inch long and 1/4 inch diameter coil wrapped around core with a cap on the side connecting both end of the coil wire. (and sent the smashed empty case back to Walmart with a strongly worded letter about how idiot forgot to remove it and how no one checked my bag when I waited a few minutes. I swear some Walmart isn’t doing enough to stop real shoplifter if they don’t even try to handle accidental alarms.

      1. Go to Hawaii! They mustn’t count as Americans though, they generally don’t fit ANY of the mainland stereotypes. Seriously, I was amazed at how pleasant and actively helpful staff were pretty much everywhere in Hawaii, including Walmart.

        1. Well, Hawaii’s only part of the USA on paper. Historically it’s people came from a completely different group to the whites who invaded North America. So it’s natural their culture’s gonna be different, even if businesses don’t see the place as significantly different.

          I think Hawaii’s adoption into the USA was more of a strategic thing, due to it’s location, rather than a great need for pineapples and ukuleles by the Americans. Or more burgers from Hawaii.

    2. I thought he said 58Khz. And yes inferring from his YouTube video a remote oscillator would make for much fun at Walmart watching NO ONE actually responding. I wonder what Sam Walton expects his EAS system to do – physiological operations? The Loss Prevention (LP) people (i.e. store detectives) are not allowed to break their cover to respond. They can only act on theft they actually witness and must only stop and detain shoplifters PAST all points of payment – which usually means outside in front. Also would be fun if you put a EAS tag on the electric wheelchairs. They are NEVER to be removed from the store lobby as no one brings them back and local college kids actually steal them too to use in their dorms. Quinnipiac University in CT (USA) is loaded with them.

      Using your SDR listen on the Walmart RF channel to the Walmart chatter (MURS ch 1-5). If you could talk too then imagine the “fun” you could cause. LP doesn’t use those frequencies for obvious reasons (i.e. they bust employees & mgrs too). They use FRS/GMRS for tag team approach to surveillance (or their personal cell phones).

      I must say this video was very revealing. I had no idea how the EAS tags were built. Who thought up this stuff? Can you imagine the Eureka! moment at some science lab? “Now we can help Sam Walton recoup his billions in shrinkage! Or not…”


        1. David Maher – Sorry Dave I guess I should have been more explicit: Samuel Robson “Rob” Walton is NOT dead! His is only 71. Albeit. his son-in-law Greg Penner runs WALMART today Sam still has a lot of “say so” in day to day operations. Greg married into the family fortune I guess… He married Carrie Walton… she’s rich (BILLIONS) too BEFORE marrying Greg.

      1. Yep, on a nail gun that I have, they hid a tag within a rubber label. Only found that when the darn bottle of oil that came with the nail gun decided to leak all over the gun and resulted in the label coming off.

  2. I took some tags in once to investigate the radiation pattern they emit. We just hed the tags in various orientations at different places in the detection region and watched the response. In the dead center they would trigger, and it radiates like a dipole, roughly.

    No one ever came over to check.

  3. Gee, and I’ve been getting nothing but troll crap from people on that video, about my theory that they act more like RF antennas than purely mechanical devices. Oh well.

    1. Perry Harrington – That’s because you are hanging out in Troll City (YouTube). The collective IQ out there is well below Idiot Savants (with more emphasis on the 1st word). Here at HaD the IQ level is much higher and your words where not wasted on morons. I enjoyed the video and learned a lot of what I THOUGHT I knew about EAS tags. Turns out I did not know much. I love your lab too. That OScope is so fricking rad! All my stuff is old school.

      I am sort of an amateur Q from James Bond 007 genre (i.e. wannabe?). Hence the Q in SQTB below which is supposed to be SOTB for my biblical-based moniker sonofthunderboanerges (English/Aramaic reference to an observed personality trait attributed to hot heads). You should be able to psych-profile me from that? :-)


  4. Great video. I always wondered about these. Back in High School this friend of mine decided to put an active strip in my backpack. So everyone always knew when I entered or exited the library. I never found it, just eventually got a new backpack.

    1. I used to trigger the alarm everytime I entered the Post Office. The counter staff came to recognize me and we’d joke about it. It turned out there was a tag in a hidden compartment of my billfold that I discovered maybe a year after I’d purchased it.

  5. I was wondering how those worked, although I’ve been seeing fewer and fewer of them recently, being replaced by RFID tags. Bit of a shame though, the fact that they are razor thin, yet a quite strong and stiff, make the useful for all sorts of little uses and with harvesting them being so trivial (soaking in ‘dirty’ solvent overnight works pretty well to dissolve the glue)

  6. The big square anti-theft stickers use 8.xMhz and are encoded. Those are more common now days and are disabled by a transmitter under the counter only after the POSi pings it during a sale. The same procedure for the old strips though just electromagnet.

    The little plastic cover with two strips inside can be defeated with an electro-magnet. Both can be defeated with aluminum foil or copper mesh where psychical measurements allow it.

    Physical commerce facilities know these are weak that’s why they all place items with a sale price above a certain amount in glass cases or more intelligent electronic protection like those ping-timeout ones with strings around boxes or cord-connected like with camera and laptop displays.

    1. If they’re stuck onto something flexible which won’t be destroyed by bending, or doesn’t matter if bending it wrecks it, folding the tag in half defeats it.

      Poking a straight pin through the tag then wigging it around to distort and short the metal strips probably will defeat it.

    2. The square 8MHz ones, you mean the stickers with the metal foil coil attached? So they’re similar to RFID? I did wonder how easy it would be to defeat just by cutting the coil. Should be 100% effective, but then RF stuff is weird, RF uses open circuits all the time anyway.

    3. Yeah the big stickers with the coil and COS in the middle. Never tried cutting them, but they use NPC and other numbers in their response unless disabled. You could easily make a PIC tool to defeat any in any store.

      The problem with cutting and bending of all the types is often manufacturers put them inside packaging and they are pre-enabled. Sometimes they are even embedded in packaging.

      You can’t go be just the technical defeat though. You have to think of the thief’s logistics. Anything worth stealing is under CCTV monitored by a security team and almost always strategically positioned and enclosed in a locked display case.

      Pretty much anything concealable worth more than $50.00 is going to be enclosed in any store. A PS4 has the tag embedded in the box and is in a locked display box under CCTV, for example. Even a $80.00 watch is like this.

  7. Hate to say it, but this is, sortof, begging for some experimentation! Would a neoydmium magnet, next to a strip, overwhelm the ‘strictive material, put it in the insensitive, further-right portion of the graph? Would an AC electromagnet, powered from lithium cells over an H-bridge for high power, be enough to demagnetise the magnetic part? There’s a good few avenues open for investigating this.

    Of course it’s not everyone who has access to a shop where you could test it out, without being arrested. There’s laws against burglary tools and the like, carrying equipment meant to bypass security. You could maybe do it as an employee, if you were gonna quit anyway…

    It’s something that might be easy enough to defeat, but then that must be figured in when the shop makes their decision to install it. Compared to the immense cheapness of the system, and the fairly good accuracy, losses due to a technical hole or two aren’t going to cost them more money than they’d have to spend otherwise on a better system. And it’s so versatile, with being able to activate / deactivate easily and quickly, and the tags cheap enough to throw away.

    In reality, most places won’t look twice if you set the alarm off. Unless they’ve had their eye on your for other suspicious behaviour. If you just walk through like nothing’s happened, almost always nobody will care. Of course how you dress, body language, not LOOKING like a shoplifter helps. As ever, the human element is the weakest. I know of people who’ve strolled into a place, picked up a microwave (the oven type, not the photon), and walked straight out with a straight face. The key is to look the part. Think “I’m the man who’s come to take the microwave”. Give off that impression. As a bonus, in many Western countries, people are terrified of strangers, and won’t make a fuss.

    But enough of Shoplifting School. It would be interesting to find out more about these just for fun. Going equipped to steal, using any technological widgets, would be more trouble than it’s worth if you’re caught, particularly doing weird-looking stuff to the product labels. So a technological attack isn’t going to be a huge amount of practical use. Probably works best as a warning to would-be thieves.

    1. Greenaum – Shoplifting School? Maybe we could ask Michael Vincent Kostiw the 1982 BACON GUY to help out? What’s he doing these days? Still shoplifting at supermarkets? Or maybe Kenneth H. Senser who’s job it is to make sure Walmart Loss Prevention people are up to snuff on shoplifters worldwide. Both Kenneth and Michael share a common back story… No not shoplifting but close enuf’ 8-D

    2. It appears at WALMART all you need is a very powerful magnet to thwart ALL of their gadgets from Spider Webs to EAS tags. And they sell them in the Arts and Craft section! Neodymium magnets in a pack for around a couple of bucks! But your best coupe is at the self-service POS terminals. I won’t connect the dots there as that might make too many Michael Kostiw’s who steal $2.13 packs of bacon. However, he got caught! Can you imagine Mike saying: “But do you know who I am!!!” :-)

  8. Just so you know Michael Kostiw was a CIA case officer that shoplifted bacon at a Virginia supermarket while he was a regular employee. You’d think a spy was better at sneaky stuff like this. He then was almost appointed executive director by Porter Goss but did not make it. The point is he has being paid well and he did not need to shoplift. Not sure how he was caught. It may have been a EAS tag or a really attentive store detective.

    Kenneth Senser is the Walmart VP in charge of Global Security. He is the boss over all Walmart Asset Protection (i.e. store detectives etc) and sets the policy for Walmart anti-shoplifting technology worldwide. He was employed by the same people Kostiw was. That’s the “inside joke” I was making to Greenaum. Sorry if it was over a lot of heads. Kinda’ what I wanted though…


  9. This article was so interesting to me; I never knew how the process worked to demagnetize the sensor tags put on clothes at a retail store. I especially thought that it was cool how the magnetostrictive strips change lengths slightly when magnetized. It’s so awesome how technology can be used to easily reduce stealing. I will remember this info the next time I see a clothing tag at a store.

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