Passive infrared alarms are triggered by which type of sources?

• A. Cold/heat sources
• B. People movement
• C. Water leaks
• D. Electrical noise

Answer: (A)

Passive infrared sensors work by detecting infrared energy—the heat radiation—that objects emit. They don’t sense motion directly; they sense changes in infrared energy in their field of view. When a warm object, such as a person, moves across the area, the heat pattern reaching the sensor changes, and that change triggers the alarm. That’s why a heat source is the source that activates a PIR sensor.

Water leaks or electrical noise don’t produce the kind of infrared energy changes these sensors are designed to detect, so they wouldn’t reliably trigger a PIR alarm. Static warm objects might not trigger unless there’s a noticeable change, but movement of a heat source is what creates the detectable infrared variation.