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?

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Fail of the Week: CAN-Bus Attached HUD for Ford Mustang

This edition of Fail of the Week is nothing short of remarkable, and your help could really get the failed project back on track. [Snipor Bob] wanted to replace all of the dashboard readouts on his Mustang and got the idea of making the hacked hardware into a Heads-Up Display. What you see above is simply the early hardware proof of concept for tapping into the vehicle’s data system. But there’s also an interesting test rig for getting the windshield glass working as a reflector for the readout.

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IP-based engine remote enable switch


[Mariano] owns a late 90’s Jeep Wrangler, and had no idea just how easy it was to steal. Unfortunately for him, the guy who made off with his Jeep was well aware of the car’s vulnerabilities. The problem lies in the ignition – it can be broken out with a screwdriver, after which, the car can be started with a single finger. How’s that for security?

[Mariano] decided that he would take matters into his own hands and add a remote-controlled switch to his car in order to encourage the next would-be thief to move on to an easier target. He describes his creation as a “remote kill” switch, though it’s more of a “remote enable” switch, enabling the engine when he wants to start the car rather than killing it on command.

The switch system is made up of two pieces – a server inside the car’s engine bay, and a remote key fob. The server and the fob speak to one another using IPv6 over 802.15.4 (the same standard used by ZigBee modules). Once the server receives a GET request from the key fob, it authenticates the user with a 128-bit AES challenge/response session, allowing the car to be started.

It is not the simplest way of adding a remote-kill switch to a car, but we like it. Unless the next potential car thief digs under the hood for a while, we’re pretty sure [Mariano’s] car will be safe for quite some time.