ps. A local town center carpark where I park from time to time has a chip coin system. The coin is magnetized when you stick in in the pay machine and that's how you get back out at the bottom of the ramp.
Only, its magnetized as soon as you put it in the machine, because there's only one place for it to drop out.
And there's a cancel button on the machine.
Saved me quite a few when I've left it there overnight for example
*cough*