car alarm

A car alarm is an electronic device installed in a vehicle in an attempt to discourage theft. Car alarms work by emitting high-volume sound when triggered or when circuit is breached. Car alarms can be designed to be triggered by vibrations, tilting of the car (to prevent unauthorized towing), touching the car, the opening or closing of special switches (e.g. door contacts), sensing small but rapid changes in battery voltage (which might indicate an interior light going on, or the ignition circuit being activated), or using volumetric sensors such as ultrasound or infrared. The vast majority of car alarms which sound are accidental. They are caused by the passing of large trucks, the vibration of thunder or people coming into contact with vehicle. With the rapidly develop of modern techs in China, 87,000,000 US dollars of car alarm products were exported in 2004 and a 25％ increase (109,000,000 US dollars) in 2005 while the total value of auto security market reached to 30 billion US dollars.