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!!!
Arduino laser detector with keypad – [Link]