Hi guys..!!
this is my 1st easy cheap n simple instructabel 
u need following components for laser burglar alarm-
1. LDR or photocell
2.printed circuit board
3.soldering equipment
4.2N3904 transistor
5.1000uF 35V capacitors
7.laser diode
8. 9V battery and battery terminal
9. 9V buzzer
10. 5K( or more)  trim pot

n ur ready to go...........!!!!!!!!!!!! 

Step 1: Placing Components


Things You'll Need:
1 650nm 5mw 12X30mm laser 1 3.2v AC adapter 1 1000uF 35v capacitor 1 5K variable resistor 1 2N3904 transistor 1 12v siren or buzzer - 102dB 2 small project boxes 1 black plastic tube 1 photocell 1 IC board 1 9v battery and clip Tape or adhesive velcro strips Hot glue
Connect a photocell to a 5K resistor. This will pick up the laser light and enable the alarm.

Use a 2N3904 transistor to link the resistor, a 1000uF 35v capacitor, a 12v siren and a 9v battery clip. The attached schematic should demonstrate how the parts come together. Use an IC board to hold them in place and make sure the board is small enough to fit into a project box. Do not connect the battery yet.
Cut a small hole into the side of the project box big enough to fit the photo cell. Glue a short (1-inch) length of black plastic tube on top of the hole. This will protect the photocell from any ambient light.
Mount the photo cell carefully in the length of tube, running the wires back in to the project box. Mount the IC board in the project box using hot glue, making sure there is enough room for all the components.
Cut 3 small holes in a second project box. The first should be large enough to hold a 5mw 12X30mm laser, the second should be large enough to hold a toggle switch, and the third should be just large enough to fit the wires from an AC adaptor.
Slide the positive wire from a 3.2v AC adaptor into the hole and connect it to the positive outlet from the laser. Positive wires are often marked by red insulation.
Connect the negative wires from the laser and the AC adaptor into a standard toggle switch. Secure them all inside the project box with glue.
Use tape, glue, or strips of velcro to mount the two project boxes on either side of the door you wish to protect.
Connect a 9v battery to the first project box and replace it in the exact position is was in.
Turn the laser on with the toggle switch and make sure it lines up perfectly with the photo cell. The alarm is now activated. If anything breaks the beam, the siren will go off.

Step 2: Testing Your System

1st test ur circuit on breadboard..!!
adjust sensitivitiy of 5k trim pot so that system works at day along with night
finally test ur circuit by braking laser beam on LDR. if the buzzer ring ur system is working....
now solder all the components on printed circuit board according to the given circuit diagram and protect ur home with ur own cheap burglar alarm.....!!! 



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    Is this a continuous circuit(one u block the rays it doesn't stop beeping)

    Our normal.

    I want to buy the cuircuit can anybody help me to buy it from online sites plzz tell me now

    what's the total cost of making this model. I want to make one for my science project.Can anybody suggest more projects for class seventh????

    You can make a motion detector

    i dont get how to connect the variable resistor to the photocell/LDR

    can somebody please help

    i have made the whole circuit, and there are no connection problems, but as soon as i plug in the battery the siren starts, even when the laser is pointed on the ldr.
    can any1 please help?

    ebay have the 2n3094 or i can mail ya one.