voltage v 2 this is illustrated by block 14 in fig2 when any of the doors is subsequently opened the door contact 4 will be shut which will be detected by the alarm control unit 2 block 15 if other doors are subsequently opened this will be detected in a corresponding manner by not shown further door contacts when the last door is closed the door contact 4 is again opened which is detected by the alarm control unit 2 as the vehicle having been left block 16 this will cause a first time delay t 1 to be initiated in the alarm control unit 2 block 17 this time delay t 1 is set to an appropriate value for example 15 seconds and if this has expired block 18 the alarm system will be activated block 19 in this case an alarm will be triggered in the event of a subsequent reception of a signal from the sensor 1 the present invention is arranged to prevent undesired alarms which are caused by leaving a person or an animal in the vehicle for this purpose the sensor 1 is also connected to the delay unit 5 when the system is in a state where the time t 1 has not yet expired and a movement takes place in the passenger compartment of the vehicle the sensor 1 will detect this and emit a.