qz thoughtsTaking Apart a Locking Shopping Cart Wheel
These are old photos from something I did about ten years ago, but I thought I'd share them for people interested.
Some supermarkets around here, in order to combat people walking off with their shopping carts, have wheels that lock when they roll over a line with some sort of short range signal coming out of it. I don't really know the specifics of how the signal is sent.
This is how (but not where) I found this item. Snapped off, probably deliberately by someone who didn't like the locking wheel. "Gatekeeper" name and Pat. 5598144
I've been curious about the anti-theft shopping cart wheels for a while. It's clear that they are radio operated by a signal with a very short broadcast range. Less clear was how they actually stop the wheel from turning. My original hypothesis that the wheel contained a motor which would actively stop the wheel from moving. After finding a wheel that had broken off a cart (honest, I had been watching out for one for several years before finding one; this one was in a Target parking lot), I took it home and opened it up.
Afer removing the axle bolt, just a philips head screwdriver needed.
You can't see much yet, but the basic operation is revealed.
It turns out my hypothesis was far off. The center part of the wheel is fixed in place, well anchored to the axle which does not spin. Put another way: the outer part of the wheel moves, but the axle and the inner part do not. When the system receives a signal, it expands a plastic drum brake against the outer rim of the wheel, to prevent the wheel from spinng. When it receives an unlock signal, the plastic drum brake is retracted. During normal operation the only power draw is a small circuit listening for those radio(?) signals. At those times, it then switches on a motor to expand or retract the brake.
The ridges on the inner and outer part interact when the wheel is locked.
Now you can see the levers that activate the drum brake, expanding or contracting the ridged inner plastic.
These CR123 lithium cells are a common long life / high power non-rechargable battery. Checking prices today, a twelve pack costs about $45 from Home Depot.
You can see the motor and gears, the driver board with an antenna near the rim, and the waterproofing orange gasket.
Heavy sheet metal here to hold this inner part still.
I saved some of the metal bits, but tossed all of the wheel and brake.
This post prompted by learing of begaydocrime.com which has more technical detail and audio files you can play to lock or unlock this sort of wheel.