This instructable will show you how to construct a laser tripwire that can twitter and grab an image from a webcam, as well as execute any command you can put in a bash script.
This instructable is actually quite simple and is even suitable as a beginner arduino project. It requires a GNU/linux (or possibly Mac) operating system with the arduino IDE and Processing IDE working properly. This project could also be implemented in Windows if you created a more complex processing application.



Hardware Requirements
Arduino Board
Laser pointer/pen
Light Dependant Resistor
100Kohm Resistor

Optional:
  Webcam
  Breadboard


Software Requirements
Arduino IDE
Processing IDE
Linux OS (I'm using Arch Linux for this project)
Curl
Webcam (part of xawtv)


Other Stuff:
Mirror
Solder and Soldering Iron
Wire
Hot Glue Gun
Hacksaw/Knife
Straw or Ballpoint Pen

 
I needed the LDR and the laser to be in a position where they would not move around so I connected my LDR and resistor directly to the Arduino without a breadboard, but you can do it however you like, below are the photos for soldering and bending them into shape I also included a simple diagram.

I cut the end off of a ballpoint pen so that I could slip it over the LDR to prevent ambient light from affecting the reading, you could also use a straw.