Abstract: A security system for a vehicle such as an automobile, which provides an audible alarm and radio transmission to a pager when an attempt is made to enter the vehicle. A passive system which automatically sets itself when the ignition is turned off, with an exit time delay permitting the operator to leave the vehicle after turning the ignition off and with an entrance time delay permitting the operator to enter the vehicle and turn the alarm off. A sound detection system which provides an alarm when vehicle intrusion is attempted as by breaking glass or use of a pry bar, while discriminating against ordinary street noises such as operation of other vehicles, sirens and the like. An alarm operated by battery voltage changes such as are produced by opening of a door and energizing a courtesy light, while inhibiting alarm when there are other battery voltage changes such as those produced by operation of a cooling fan.