What triggers an alarm in a passive infrared detector?

In a passive infrared detector, the primary mechanism for triggering the alarm is based on the detection of movement. These detectors work by sensing changes in infrared radiation, which is emitted by warm objects, primarily humans. When a person moves within the range of the detector, the infrared energy levels change, and the alarm is activated as a response to this movement.

The other options are not relevant to the functioning of passive infrared detectors. For instance, sound detection does not play a role in triggering alarms with these devices, as they are not equipped to process audio signals. Climate change is unrelated, as the detector's operation is consistent regardless of broader environmental changes. Similarly, light changes do not influence the functioning of passive infrared detectors, which focus on heat, not light levels. Understanding the specific functionality of passive infrared detectors is crucial for effectively utilizing them in alarm systems.