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It’s our weapon against the commercial machines:

Ever noticed a painted yellow line in the parking lot around many supermarkets and retail stores? The magic yellow line emits a signal that causes carts to stop dead in their tracks, preventing carts from leaving the parking lot.

Now you can build your own portable yellow line — with up to a 20 foot range. Need I say more? Hint: it works inside the store.

Somebody going by the handle “Orthonormal Basis of Evil” has posted instructions for building this device to the Web site Instructables. There are eleven steps between the first act of assembly and the assault upon retail America. Most interesting from a math-and-physics standpoint are isolating the locking signal, which requires a Fast Fourier Transform (FFT), and recreating the locking signal, which is a nice exercise in electromagnetism. Essentially, the lesson is that a current in a wire creates a magnetic field, varying the current creates a fluctuating field, and these fluctuations can propagate through space to carry energy and influence electric charges elsewhere, possibly inducing new electric currents following the lead of the first. Here, the transmitting wire is either buried beneath the parking lot or carried on the person, and the “elsewhere” being influenced is inside the locking apparatus of the shopping cart’s front wheel.

(I may be mistaken here, but I think this is actually a case of near-field induction, rather than the typical far-field plane waves one often studies.)

Installing the device on its human user is an exercise in cybernetics, which “Orthonormal Basis of Evil” has chosen to illustrate in a way a little different from the electromagnetism textbooks I grew up with:

UPDATE (1 July 2007): The cart-locker has hit Slashdot. They’re already arguing whether or not the Creative Commons license under which it was release is “Open Source” or not. Ah, Slashdot.

UPDATE (3 July 2007): This is interesting. The story has reached the Wired blogs (“This is no beginner project, but the payoff is probably worth it”) and Engadget. Commenters at the latter site have shared some interesting anecdotes, like “Scott” who worked at Wal-Mart thirteen years ago:

They had just installed the anti-theft sensors at the doors and had sheets of the RF stickers to put on valuable items and other stuff that had a habit of walking out…

Anyway, I worked nights there and had some time on my hands. So I took several sheets of those RF stickers and put one on the bottom of every shopping cart in the store. They spent about 2 weeks trying to figure out why the alarm went off every time somebody left. It was beautiful.

“Ceejay” says he and his Target co-workers used to do the same thing with the store’s own remote controls.