A device for controlling a security device, preferably for securing a motor
vehicle against unauthorized use, a portable key having a transmitter for
sending messages after every actuation and the security device comprising
a receiver for messages and the messages including alternating code values
which change from message to message. A lag time of equal duration is
implemented in the transmitter and receiver respectively, such that the
transmitter is switched off by the duration of the lag time. When the
transmitter which is not switched off is actuated, a message is
transmitted, and when the switched-off transmitter is actuated the
transmitter is switched on. The security device is controlled if the time
difference information included in the message corresponds to the time
difference information which is correspondingly determined in the
receiver.
