A sensor assembly for detecting open or closed state in windows and doors includes a passive infrared sensor configured to produce a change in an electrical signal based on a change in infrared radiation incident on the passive infrared sensor. A lens is operatively connected to the passive infrared sensor. At least a portion of the lens is configured to be mounted in an interface between a frame and a door or window mounted to open and close within the frame. The lens is configured to guide ambient infrared radiation to the passive infrared sensor at a first level when the door or window is closed and at a second level when the door or window is open or ajar.