Microwave Sensor for Car Alarms

Microwave sensors send out ultra high frequency sound waves, and detect the reflections. If someone walks near your car, they will start to reflect the microwaves. The receiver detects the change in reflections, and if the object remains within the microwave field for a length of time (depending on the alarm it is used with), it will trigger the alarm. It has an effective range of 3m. It also has a sensitivity adjustment to help avoid false alarms.