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The one I have has a little magnetic contact that is moved back and forth by the magnetic field of a live a/c wire, hitting another contact that sends power to a light and beeper. Any jarring movement will make the two contacts touch and set it off. Mine has an off switch, a good thing since it usually rides in my tool bag in the truck.
I have had a few of them and they seem to all react differently. It might just be due to static electricity from movement. My daughter has taken then and rubbed it against her hair to make them beep.
Rezvani's Latest Post-Apocalytic Monster Is a Ford F-150 Raptor Underneath
Slideshow: Called the Fortress, the 850-horsepower pickup combines Raptor underpinnings with military-inspired features, survival equipment, and a starting price of $285,000.