Kitchen Counter Invisible Fence

Having lost a just-out-of-the-oven Quiche and a meal of freshly delivered Chinese food to an otherwise very well behaved dog, one of the guys that works here used a salvaged industrial emitter / detector pair, relay and car horn to provide a bit of doggy defense for his kitchen counter. The industrial emitter is a tad expensive, but you could probably use a laser pointer and photo transistor just as well.