You will need a few components to make this project. Here is the list and the links to were i got them. Everything will cost around $40 http://www.radioshack.com/product/index.jsp?productId=2062599 ...
I have been messing around with a few components and my arduino. I figured out this project last week and just had to share it with all of you. What it is, is a laser beam hitting a photo cell. The arduino reads the photo cell and when it detects the voltage level is below a certain amount, the beam must be broken while sounding an alarm. The alarm stays on until you punch in the code you have set up to the keypad in. Once the correct code is typed in, the arduino turns of the buzzer and gives you 15 seconds ( or what ever you set the delay time to) to reset the laser beam. You are able to change the code if you like. The steps are very easy to follow and i hope everyone makes one!!!
also some spare wire, small mirrors, hot glue gun, solder, and a soldering iron.
you can get all theses at Sparkfun and Radioshack
Now if you look at the list, you'll notice there is no switch to turn this device on and off. If there were to be an off and on switch, the person that sets of the alarm would just have to simply turn the device off. That defeats the purpose of the keypad to turn of the buzzer. You can add a switch if you like, but i'm not going to. To turn this off, your going to have to unplug the adapter from the wall.