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.
Light Dependant Resistor
Linux OS (I'm using Arch Linux for this project)
Webcam (part of xawtv)
Solder and Soldering Iron
Hot Glue Gun
Straw or Ballpoint Pen
Step 1: Connecting The LDR
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.