For example the HC-SR501.
Passive infrared have a very wide field of view and ‘see’ infrared. In the right-most picture the ‘image sensor’ is shown. In normal use the image sensor is behind the white plastic ‘lens’. They are very good at detecting movement by objects and life forms that are warmer than their surroundings.
If something changes in their view, the output signal is set to ‘high’.
Finding the right settings for the potentiometers may require some experimentation to get them exactly right for your application.
The sensitivity can be adjusted from 3 meters (potentiometer fully clockwise) to 7 meters (fully counter-clockwise)
The time delay can be adjusted from 5 minutes (potentiometer fully clockwise) to 3 seconds (fully counter-clockwise)
Single-trigger mode (‘L’): after motion has been detected, output is set high for the duration of one time delay period only. Repeatable trigger mode (‘R’): in addition, motion detected during the time delay period starts the time delay again.