The functionality of the PIR sensor is based on radiation emitted by human bodies. Objects generate heat as infrared radiation and those objects include animals and the human body whose radiation is strongest at a wavelength of 9.4 μm.
When a human passes the front of the sensor, the temperature from the PIR sensor point of view changes from the background value to the human value. The sensor detects this change in the infrared radiation and changes its output voltage, signaling the detection.
To increase the sensitivity of the PIR sensor, a Fresnel lens is mounted in front of it. A sensor is in fact a FET transistor with a source pin connected with a pull down to ground. We can see this setup in the following figure:
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