Which of the following is not a primary component of a Passive Infrared Sensor (PIR)?

A Passive Infrared Sensor (PIR) is primarily designed to detect infrared radiation emitted by objects, typically human bodies, through its key components. The pyroelectric sensor is critical as it generates an electrical signal in response to changes in infrared radiation, directly correlating to temperature variations in the environment. The optical lens focuses the infrared light onto the sensor, enhancing sensitivity to the changes in heat signatures.

The Fresnel lens is designed to increase the effective area of the sensor by allowing it to detect motion across a broader field of view. It plays a crucial role in the functionality of PIR sensors by helping to shape the infrared signals that reach the pyroelectric sensor.

In contrast, an acoustic sensor is not part of the primary structure or function of a PIR sensor. Acoustic sensors operate based on sound waves and are not utilized in the detection mechanisms of passive infrared sensing systems, thus correctly identifying it as not a primary component of a PIR.