The HC-SR501 PIR sensor
The HC-SR501 is a device that connects to a BrainBuilder digital input.
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)