PIR stands for Passive InfraRed sensor. Detects the minimum of infrared heat emanating from a person or an object. The PIR stores as normal all that does not change around him. The wall of a room in front of him does not change temperature (only very slowly from day to night) and then records it as normal. A person who passes in front emits heat, the PIR registers this immediate change.
We build an alarm, an audible beep and an LED that flashes when someone approaches