Sensor device having an ultrasonic receiving circuit

A sensor device configured to be installed in a space, the sensor device comprising:

• an ultrasonic receiving element configured to receive ultrasonic waves and generate an ultrasonic reception voltage signal, wherein the ultrasonic waves comprise non-Doppler-shifted ultrasonic waves when the space is vacant and a combination of non-Doppler-shifted ultrasonic waves and Doppler shifted waves when the space is occupied, the non-Doppler-shifted ultrasonic waves characterized by an ultrasonic frequency;
• a control circuit configured to control generation of a supplemental signal that is characterized by the ultrasonic frequency and is combined with the ultrasonic reception voltage signal;
• an ultrasonic receiving circuit configured to receive the combination of the ultrasonic reception voltage signal and the supplemental signal, the ultrasonic receiving circuit configured to generate a detection signal that indicates when the space is occupied;
• and
• a control circuit configured to receive the detection signal and detect an occupancy condition in the space in response to the detection signal.