PIR it stands for passive infrared. PIR sensors detect the infrared radiation from the object. PIR does not detect any heat radiation from any objects. It only detect infrared radiation. PIR sensors mainly used for presence of any object. Mainly for human presence.
This is a simple human interfernce detector. Here we using PIR sensor and BC547 npn transistor. Normaly the PIR sensor output is approximately 1.7V. When any object presence the output of the PIR become approximately 3.8V. Here we using the switching action of npn transistor. When enough bias voltage is come to base terminal the transistor became ON. Here we using a bias resistance of 22K. Because of this bias resistance transistor Q1 is ON only when more than of 2.5V is arrived on the base terminal. So that only any object interference made the transistor Q1 to ON state. After Q1 ON it bias the Q2 transistor and it also became ON. And the indicator LED became glow in the presence of a human. Here we using transistors as Darlington pair for drive amplification purpose.
Q1,Q2- BC 547