Quite a while ago, I bought some device with a laser pointer. I did lost it over time and eventually ran into it just a couple days ago. In the meantime, I bought an Arduino and started experiencing with it, especially with photo-sensitive resistors (photoresistors).
So I wondered what kind of fun could come out of a laser + a photoresistor + an Arduino. Actually, it didn't take long to figure out what to do. Remember all the Mission: Impossible-like movies and shows, where people are to move into laser-secured rooms, without interrupting any of the laser beams? Well, that's pretty-much the idea I came up with: "securing" my room door with such things.
The setup is pretty easy: the Arduino is on one side of the door, with the photoresistor and another 10k resistor that we need to have a changing voltage. The laser is on the other side of the door, directed right to the photoresistor and connected to the Arduino via a couple wires. So the general idea here is to make the Arduino power the laser and get the tension from the photoresistor (or the 10k resistor) a couple times every second. Here is a scheme:
Here is a demo video of the Arduindoor system running (first time making an alarm ring, second time playing/pausing music) :
If you're interested in the idea, you can setup your own with the additional help of: