The circuit described here can be used as burglar alarm or can be used as an indicator that door or window is open.
Now the summer time is coming and we all are going to use AC in our home and offices. AC functions efficiently, only when doors and windows are closed properly. If any of the door or window is left open then effect of AC cannot be felt. So we have described a circuit here that will give an alarm when door or window is left open.
Another application when you can use this circuit, in your cupboard or lockers. When light fall on the sensor it will give an alarm indicating that somebody is trying to steal your valuable thing. And will also give an alarm when any unwanted person is trying to open door and window.