Raspberry Pi Burglar Alarm

I recently got an LCD screen from my Uncle Mark, (which I can't seem to get to work), I lost one of the resistors. While searching for it, I saw a my 'long lost' photo-sensitive sensor thingy ( basically, I light sensor), and I thought of all of the things I could do with that. Finally, I decided on a kind of burglar alarm. I don't know why, there aren't any burglars around where I live :). I had a lot of red LED lights left over from my Pumpkin Pi for Holloween project, so I decided to use those, too. My idea what to use the light sensor to get the current light level when the program ran, then check if the light level changes by 20. It it gets too dark or too bright, the alarm will go off, including flashing lights. If a burglar comes and shines his flashlight around (I don't know if they do, just saw that in movies :P), it will most likely hit the light sensor and that will go off. Or, you could have a beam of light shining directly onto the light sensor and when that beam is broken, the light sensor will detect a change of light and go off. I might do a video if this turns out well (which it probably won't, I never did get the Pumpkin Pi project inside my pumpkin).