A detection system for use by a checkout clerk in a checkout line to detect a load left on the lower tray of a shopping cart by the customer. The shopping cart has a lower tray pivotably mounted on a frame. The tray is movable between a first and second position when a load is placed on the tray. A field...http://www.google.de/patents/US4723118?utm_source=gb-gplus-sharePatent US4723118 - Magnetic detection system for items on the lower tray of a shopping cart
