The Electronic Guard Dog.

"This is the electronic device that deters intruders by making them believe a fierce guard dog lurks within. The unit's radar can detect movement from up to 30' away, even through walls and doors, allowing it to be concealed anywhere. When the device senses motion outside your door, it emits the realistic sound of an agitated dog barking; like a real canine, the frequency of barking increases the closer the subject approaches. The device can also be set to bark and then sound a siren. It has adjustable volume and sensitivity controls. For indoor use only; for best results, place three to five feet off the floor. Plugs into AC. 7 1/2"" H x 5 1/8"" W x 6"" D. (2 1/4 lbs.)"

The The Electronic Guard Dog. from the Electronics>Gadgets category can be bought online from Hammacher Schlemmer.