This cirucit utilizes a PIR sensor or Pyroelectric sensor as a main component. The operation of the circuit is as follows: When the signal from PIR sensor is received, the relay ‘K’ switches ON. If it is connected to a fan or a light or alarm or any electrical/electronic appliance, it will make the particular device to operate. So, there are variety of applications for this circuit. It can be used as monitoring circuit, automatic light circuit, mainly for energy saving purposes, for security of room or home,can be used as motion sensor based home automation , as a alarm circuit, as a voice circuit, 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 relay K to ON state. The relay remains ON till the sensor sense the human. After some time, the relay K goes OFF automatically. To extend the time of glowing of relay, we have to adjust the R1 and C1 value. If the relay 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. with a bulb or light. 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.