Many stores have installed locking wheels on their shopping carts that are supposed to stop shopping cart theft. The cart gets a signal when it leaves the parking lot and one of the wheels locks up. This instructable will show you how to forge that signal and lock up anybodies shopping cart where you want (works inside and outside the store).
This will work on any cart that has the solid wheel like shown in the picture below. These are the wheels that walmart uses. This method works the same way as the EMP Shopping cart locker but is way easier to make, however the range is far less, about 2 to 5 feet.
I made this a couple weeks ago and I saw the high power one today and decided to post mine. It consists of an mp3 player playing the lock signal attached to a coil of magnet wire. When the mp3 player and the coil are placed in my shoe I can step about 6 inches from someones cart wheel and their wheel will lock.
Note: The picture is using the old coil, which had far less range, the coil I am now using is about 30 turns around a 1 foot diameter, with nothing in the middle. This increased the range to about 4 feet.
Step 1: The signal
I only have the lock signal, if you get the unlock signal you could probably play it the same way.
Step 2: How to make it
Magnet Wire ( available at radio shack )
Right Angle Headphone Plug
Something round with a 1 foot diameter
Take your magnet wire (if you got it from radio shack use the green one) and wrap the whole spool around your round thing, making sure to have both ends of wire sticking out enough to solder, you probably want at least a foot sticking out. Should have about 30 turns. When the whole coil is rolled, wrap it in electrical tape to keep it together, but keep the end wires sticking out. You might want to take it off your round thing
Take a knife and scratch away the coating on the outside of each end wire about and inch down.
Take your right end headphone plug and cut off the casing. You should come out with something like the one in the photo. Solder either alligator clips or your actual coil wires onto the ground and left channel, as shown in the picture.