Look inside
a sensor.

Maya fits a passive infrared sensor to a connected hallway light. She walks across the hall and the light turns on. Then she sits quietly on a bench to find out whether the sensor keeps noticing her. Inside this kind of detector, paired pyroelectric sensing elements sit behind a segmented lens. The lens focuses infrared radiation from different parts of the hall onto the elements. As Maya crosses those zones, her body’s infrared radiation changes how the elements heat up. A temperature change alters the material’s internal electric polarization, shifting charge at its electrodes. That produces an electrical signal. The paired elements help cancel changes they both receive together. The module’s electronics turn the changing signal into a high or low motion output for the controller. Before testing, Maya uses the course’s startup delay: sixty thousand milliseconds. Divide by one thousand and that is sixty seconds of warm-up before judging normal operation. Now her seated test matters. It is tempting to treat no motion output as an empty hall. But a still person may produce no changing infrared pattern, so the output can fall quiet while Maya remains there. The takeaway: passive infrared detects changes caused by movement, so silence alone cannot tell you that everyone has left.