Patent US 5,684,338

A security device which is particularly suitable for motor vehicles is proposed. It comprises an electronic control unit (10) for controlling the functions of an apparatus to be protected, at least two sensors (U.sub.1, U.sub.2) for detecting specific operating states of the apparatus to be protected and a circuit (42) to which the output signals of the sensors (U.sub.1, U.sub.2) are fed. The circuit (42) carries out an exchange of data with the electronic control unit (10) in order to control its operational capability, the output signals of the sensors (U.sub.1, U.sub.2) influencing the exchange of data according to the detected operating state such that the electronic control unit (10) is only operative in a prescribed operating state of the apparatus to be protected. For this purpose, the circuit (42) has a memory (44) in which at least one predetermined code is stored. The electronic control unit (10) is only operative if, on requesting at least one predetermined code (A) from the memory (44) of the circuit (42), it also receives the said code (A) from the circuit (42).