This is more or less a video instructable that I've created so that you can follow along at your end. All of the components talked about in the following three videos can be found at your local electronics store, such as Radioshack. I've broken this set of tutorials down into bricks. The first of three videos talks about voltage regulators (7805), voltage dividers, LDRs (Light Dependent Resistors), and comparator circuitry that can be used to detect a laser breach. I talk about the theory, the arithmetic, and I manually place the test the components on the prototyping breadboard. The parts list for the fist video is displayed at the very beginning of the video. The second and third videos give you some options as to where you can go with this project. The next two videos are add-on circuits.
Step 1: Adding Adjustable Pulse-Width to Your Alarm/LED Indicator
In this video, we introduce a 555 timer IC option (NE555). I configure a 555 timer as a non-retriggerable monostable multivibrator circuit. This means that when the 555 timer is triggered, the output will go high for a pre-set amount of time, then will go low again. This pulse width can be set by changing the timing resistor/capacitor values. The formula is in the video, and it is extremely easy to modify. A demonstration lies at the very beginning of the video. We then go through the theory, and the build procedure. From there, I change the timing capacitor values to make the output go high for more than 3 seconds to offer up another demonstration.
Step 2: Adding a Relay & Loud Siren Into the Mix
This is where you really have options. You can place a JK Flip-Flop at the output of the 555 timer and configure the flip flop as a toggling circuit, or you can do what I did, and place a relay at the output of the 555 timer. The relay has a driver circuit that is talked about in the video, and it acts to switch the loud siren on and off. Remember, you can change the duration of the pulse width of the 555 timer circuit by simply changing the timing components. This would work well as a science project, and if you've watched all three videos, the entire circuit should make sense to you. I've tried to be as practical as humanly possible, and I've kept the mathematics to a minimum.