This cirucit utilizes a PIR sensor or Pyroelectric sensor as a main component. There are variety of applications for this circuit. It can be used as automatic light circuit, mainly for energy saving purposes, for security of room or home, as a light alarm, for all security purposes.
Let us see its working by a realistic application of this circuit. The PIR sensor should be connectd in entrance to a room. When anyone enters the room, the sensor senses and switches the bulb EL to ON state. The light remains ON till the sensor sense the human. After some time, the bulb goes OFF automatically. To extend the time of glowing of bulb, we have to adjust the R1 and C1 value. If the bulb is replaced by any alarm, it can be used as a burglar alarm or doorbell too.
This circuit finds its application mainly in Energy saving also. Usually people forget to switch OFF the tubes and lamps when they leave the room. But, it is no needed to take care when this circuit is installed. It automatically switches ON and OFF depending upon the presence of a person.
One more advantage of this circuit is that, it has a LDR, which senses the intensity of light. If its a day time, then there is no need to make the light to glow. So, during day hours the light will not glow. You can also disable this option when you use for security purpose.