A PIR sensor detects changes in the amount of infrared radiation sensed. This varies depending on the temperature and surface characteristics of the object in front of the sensor.
When a person passes between the sensor and the background, the sensor detects the change from room temperature to body temperature, and then back again. The sensor converts the resulting change in the incoming infrared radiation into a change in the output voltage. Other objects with the same temperature as the background, but have different surface characteristics, will cause the sensor to detect a different emission pattern.
The PIR sensor used in this demo is the HC-SR505 mini, but any PIR sensor with a digital output above 3V can be used.
More information about how the PIR sensors work can be found at the following link: Passive Infrared Sensors.
