Advantages of PIR Sensor

In order for Jack to recognize when he had to greet visitors, he had to be given awareness of his surroundings. This awareness was granted to him by using a Passive Infrared Radiation (PIR) Sensor. A PIR sensor works by detecting changes in the amount of infrared radiation (IR) in the direction it is positioned. There are two adjacent slots within the sensor that detect the same amount of IR when no one is passing by it. However, when a person (who emits a large amount of IR) walks by the sensor, he passes by one of the slots before walking past the other one. In the moment that he walks in front of one of the slots, a positive differential change in IR is detected by the sensor, since the left and right slots no longer sense the same amount of IR. A negative differential change between the two slots is detected after the person leaves the detecting area of the first slot and passes in front of the detecting area of the second slot. These changes in the amount of IR detected are what tell Jack to begin greeting visitors.


Sources
[1] Fried, Limor (n.d.). How PIRs Work. In PIR Motion Sensor. Retrieved from   https://learn.adafruit.com/pir-passive-infrared-proximity-motion-sensor?view=all