i will show you how to make a laser powered burglar alarm

Step 1: Materials Required

1] LDR
2] buzzer
3] coin cell - cr2032
4] one laser 2mv or more (most important =D)
5] pref board or bead board or you can do it without using both
6] foam (for the stands)
7] 2n2222 transistor
7] resistor 100 ohm and 4.7k ohm
8] Two Neodymium (NdFeB) magnet
9] metal strip
10] tape and double sided tape
11] a long nail or toothpick

Step 2: The Circuit Digram

solder the components according to the circuit digram
the transistors back is facing you.
base-middle pin
collector-left pin
emitter-right pin

Step 3: How the Circuit Works

The 4.7k ohm resistor acts as a sensitivity controller for the LDR
you can change it between 1k to 100k to change the sensitivity
for this circuit i recommend 4.7 or lower

Step 4: Solder the Components

Step 5: Make the Stand for the Laser

1] cut a small piece of foam
2] then cut a small slit kind of a thing as shown in the pics
3] pierce a toothpick to give stability

Step 6: Make the Stand for the Circuit

1] cut a small piece of double sided tape
2] stick it on the side of the door or where ever you kept your precious things
3] stick a metal strip on it
4] stick a magnet on the circuit board with the help of double sided tape

Step 7: Now Set Up the Alarm

both the metal strip and the foam piece should be parallel to each other.
now keep the laser on the slit in the foam piece and on it such that the beam falls on the photo transistor
when you will interrupt the beam a sound goes off

Step 8: Make It With a Phototransistor

you can make this circuit with a phototransistor
heres how-
you will need
1] buzzer
2] photo transistor [dont have one see next step i will show you how to make one]
3] 2n3904 transistor
4] coin cell

Step 9: Make the Photo Transistor

you will need
1] carbon tetrachloride
2] AC126 transistor

if you cant get a photo transistor you can make it with a AC126 transistor
first take a AC126 transistor and remove its metallic cap.then wash the inside with carbon tetrachloride till the white paste like substance is completely removed.Now cover the transistor with a transparent glass cap to protect it from moisture and damage.in this way your photo transistor is ready.