Using the Doppler principle, ultrasonic sensors work by bouncing ultrasonic sound waves off objects in an area and measuring the time it takes for the waves to return. Movement by a person in the area causes the sound waves to return at a changed frequency, resulting in a Doppler shift and occupancy detection. These sensors utilize our advanced signal processing circuitry to automatically adjust the signal detection threshold in compensation for changing levels of activity and airflow. This helps eliminate false ON activations common in other ultrasonic sensors.