"Graham." wrote:

Inductance loop buried in the ground at red line.Puts the brakes on.
Assuming you`re talking about shopping trolleys but just forgot to
mention it?

Assuming the trolley side is passive, it can't have too much
range can it?
Couldn't one just lift the trolley over the sub-surface coil, or
are trolley thieves not that bright?


Plenty of trolleys escape from out local supermarket, which is equipped
with that system. The brakes only work on one or two wheels, and since
those wheels are at the back, it is a trivial task to lift the rear of
the trolley while pushing it over the loop.

Lots of people seem to do it, and I regularly go to the supermarket
pushing a trolley that I found on the way. ;-)