When we design lighting circuit, we would like to light up the LEDs only when there is somebody around. The PIR sensor is a motion sensor based on passive infrared sensor which senses infrared emitted by human body. I got the PIR Module "KC7783R" from local store, which includes the PIR sesnor and also the KC7783 motion detection IC.
The PIR module is very easy to use, it contains only 3 pins (1-Signal, 2-Vcc, 3-Gnd), where it detects somebody moving, the signal line goes high for a period of time (3 secs for example).
Here is my test circuit to drive one auto-flashing RGB Led with PIR module:
My another demo of using PIR module, combining the use of AVRtiny13 as a timer. When it detects motion, it will light up the light bulb for 10 seconds, afterward it will go into deep sleep to save power and wait for the next interrupt send by PIR module.