Wikipedia > Shopping cart > Theft prevention > Electronic
“Theft deterrent systems are becoming popular in many shopping centers. An electronic system works by locking one of the wheels, usually one on the front, when the cart is rolled out of a designated area. Each shopping cart is fitted with an electronic locking wheel, or ‘boot’. A transmitter with a thin wire is placed around the perimeter of the parking lot. The boot locks when the cart leaves the designated area. Often a line is painted in front of the broadcast range to warn customers that their cart will stop when rolled past the line. The locked wheel is usually unlocked with a portable electronic device carried by store staff called a “CartKey” that sends a signal to the boot, unlocking the wheel.”