An occupancy sensor includes an infrared sensor, a variable bandpass filter operably coupled to the infrared sensor and a controller operably coupled to the infrared sensor and the variable bandpass filter. The bandpass filter includes digital potentiometers adapted to control a gain of the bandpass filter, the tuning of the bandpass filter, and the ratio of a center frequency of the bandpass filter to a bandwidth of the bandpass filter. The controller includes a bandpass filter engine to control the variable bandpass filter, a Doppler shift engine to characterize the signals filtered by the variable bandpass filter, and an occupancy sensing engine to determine the presence or absence of the occupant within the defined region. The controller is adapted to filter the signals generated by the infrared sensor and process the filtered signals to determine the presence or absence of an occupant within a defined region.