Abstract: Supermarket point of purchase advertising typically consists of advertisements, brochures, coupons and the like disposed on shelving units. To attract prospective purchasers to such advertising and to constitute point of purchase advertising itself, there is disclosed a shelf-edge mounted device which senses an approaching shopper and then delivers an audio message. The device has an optical sensor which is pointed underneath a shelf and towards the interior of the shelving unit, and detects the shadow of the approaching shopper cast on the back of the shelving unit. A background plate in the field of view of the sensor improves its sensitivity.