Quote Originally Posted by kenandere View Post
Do you have any idea, what kind of sensors I can use to detect unexpected visitors?
Regards,
Most (inexpensive) motion sensors use an infra red sensor with a fresnel lense on it. The heat of the human/animal is above background, so as it moves, the fresnel lense makes the heat hit and miss the sensor. This gives quick varying temperatures, where normal heating and cooling of the room, or outdoors, occurs slowly.

You can go to the hardware store and buy a cheap outdoor light and steal the parts out of it like this:
http://www.acehardware.com/product/i...LAID=109386813

Or buy the parts:
http://www.futurlec.com/PIR_Sensors.shtml