[SERIOUS] How to detect if a door is opened or not ?

[akhileshbc]

Tape a piece of string somewhere not very visible (top corner of the door), between the door and the wall, so when it's opened the string will be pulled out. It only need be very small, 1" of string (sewing thread) and clear adhesive tape. You need to do this once you're out of course.

Thanks...

But even if I'm opening the door, the method is executed. I mean, how to check if the string is not pulled out (before I open the door)
Because, when anyone opens the door, the string is pulled out !

Also, if you have a webcam you can leave your computer running and it turned on, facing the door. There's software that detects motion in the picutre and starts recording, so you can catch him red-handed.

That's a good idea. But I don't have a webcam (only planning to buy)and I can't trust the electric power supply. Because, when a small wind blows, the power will be cut-off And the UPS won't allow more than 10 min backup