The bill validator device is a small piece of hardware that can be placed in a slot machine and used to cheat at slots. It works by allowing players to play without putting money in, which is illegal. The device can be placed at the bottom of the machine or on top, depending on its configuration.

Also known as validators or acceptors, paper currency detectors scan paper currency using optical and magnetic sensors. Upon validation, the validator will inform the vending machine controller (VMC) or other host device of a credit via a parallel or serial interface.