Abstract: An electronic key for a motor vehicle includes a hand held portable transmitter carried by the authorized driver of the vehicle and a receiver and associated detection circuitry maintained in the vehicle. The receiver is programmed to respond to periodic identification signals from the transmitter by applying periodic control pulses to the associated detection circuitry. The detection circuitry enables the vehicle in response to the presence of at least one control pulse during each periodic interval and disables the vehicle in response to the absence of a control pulse during any one of the periodic intervals. The electronic key provides individual vehicle control in the same manner as does standard mechanical keys while being highly resistant to tampering or duplication by unauthorized vehicle users.

Abstract: A vehicle theft prevention system includes a receiver located in the vehicle for receiving a signal from a remote transmitter. When the distance between the receiver and transmitter exceeds a predetermined amount, such that the signal strength at the receiver is less than a predetermined signal level, a control circuit is actuated which upon actuation may actuate an alarm, deactuate the vehicle ignition system so as to prevent further operation of the vehicle, or both.