How do super market anti theft trolleys work ?

The wheels don't seem to have any sophisticated electronic system ... or am i wrong ?

Answer:
Yes, there is a electronic system.

there is a radio operated brake on one (or more) wheel(s), this is monitoring for the presence of the proximity signal (which is broadcast throughout the supermarket and car park) when the signal fades the brake applies making the trolley harder to move.
Similar to the invisible fences for dogs. There is a buried wire. Once the cart crosses that line, the solenoid in the wheel activates the brake.
They just scream out "Why are you taking me?" as soon as you go past the hidden laser beams lol.
I am not sure but they probably have one of those sensors on them that will send a signal off after they are a certain distance from the store. Very much like the ankle sensor Paris Hilton thought she was going to get to wear when the Sheriff went against the judge's order and sent her home from jail after only 3 days.
I don't really know. A set of fine aluminium cattle grids seem to act as a suitable deterrent though.

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