A passive infrared sensor (PIR sensor) is an electronic sensor that measures infrared (IR) light radiating from objects in its field of view. They are most often used in PIR-based motion detectors.

For most of our Arduino projects that need to detect when a person has left or entered the area, or has approached, HC-SR501 PIR sensors are a great choice. They are low power and low cost, pretty rugged, have a wide lens range, easy to interface with and are insanely popular among hobbyists.

HC-SR501 PIR sensor has three output pins VCC, Output and Ground as shown in the diagram below. It has a built-in voltage regulator so it can be powered by any DC voltage from 4.5 to 12 volts, typically 5V is used. Other than this, there are a couple options you have with your PIR.