Edit: We now have a laser tripwire kickstarter campaign! Check it out here:
In the past, I've done several laser and light sensor tutorials that were op-amp (Operational amplifier) based. I've always been fond of op-amp based circuits as they are to easy to customize. This circuit is much simpler. All you need is an NPN transistor, a resistor, a light dependent resistor (LDR), and an active 5v buzzer. This video shows you how to build it, and how it works. The video offers a theory lesson, and a lab build. I used a 10k ohm resistor, a 2N2222 NPN transistor, a random LDR (Most will work well with this circuit), and a 5v active buzzer.
I also show you how to employ a 5v regulator at the end of the video. Just in case you want to use a 9v battery to power your circuit. I use a 78L05 5v regulator in the video, but you can just as easily use a 7805.
On another unrelated note, If any of you are Arduino fans, then I'd love for you to have a gander at my most recent Arduino Shield at kickstarter: ICU-Duino Arduino Kickstarter Campaign
I hope that you enjoy the video. Thanks again, everyone! Happy building!