PIR motion sensors are most effective when mounted:

• A. so the intruder walks across the unit
• B. facing the intruder directly
• C. mounted on the ceiling above the doorway
• D. at a corner of the room

Answer: (A)

PIR sensors rely on detecting changes in infrared energy across their sensing elements. They respond best when a warm body moves across the sensor’s field of view, because the movement causes sequential changes in different sensing zones. When someone walks across the unit, multiple elements are triggered in quick succession, producing a stronger, more reliable signal and reducing the chance of missed detections or false alarms from heat sources that don’t cross the field.

Placing the sensor so that a typical intruder would cross in front of it—along a hallway or doorway path—maximizes that cross-path motion. If a person moves directly toward the sensor, the energy changes can be less pronounced across the elements at any given moment, making detection less reliable. Mounting at corners or facing straight at a doorway often results in less effective coverage because it limits the number of sensing zones activated by normal movement.