Instructables
Picture of Laser Tripwire Alarm

No security system is complete without lasers. So in this project I am going to show you how to build a laser tripwire alarm from a laser point, a couple of mirrors, and a few dollars of electrical parts. With this you can cover an entire house with an array of light beams. If any one of them is crossed it sets off your alarm. It can be a standalone alarm or it can be integrated into a larger DIY security system. 
 
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Step 1: Safety Note: Working with Lasers

Picture of Safety Note: Working with Lasers
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Cheap laser pointers that you find in most stores are generally restricted to 5mW or less. These are generally considered safe. However, it is still possible to damage your eyes if you are not careful. When working with lasers, it is a good idea to wear the appropriate eye protection.  Avoid looking directly at the laser diode. 

Also never point lasers at aircraft. 

Step 2: Parts

Picture of Parts
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Here are the parts that you will need for this project:

Laser Pointer
Printed Circuit Board
555 Timer IC
IC Socket (optional)
3-12 Volt Buzzer
Switch
CdS Photoresistor
2 resistors
3 AA Batteries
3 AA Battery holders
Jumper Wires
Heat Shrink Tubing
savdd20 hours ago

Hi, sorry I'm new to this but how do i know what kinds of resistors to use?

DIY Hacks and How Tos (author)  savdd19 hours ago
Take a multimeter and measure the resistance of the CdS photoresistor while it is in the room where you want to set it up. Try to have the same lighting conditions that it will usually operate under. Then whatever resistance you measure, find a fixed value resistor with close to that value. That is the resistor that you should use.
arizik9 days ago
does this alarm turn off automatically or you have to turn it off by your self once it is activated???
DIY Hacks and How Tos (author)  arizik8 days ago
The alarm will continue to go off until you turn it off.
arizik9 days ago
does this alarm turn off automatically or I have to turn it off by myself once it is activated??

i do not have a security alarm however i want to build exactly what you did then link it to something that makes a loud sound when the laser is interrupted can you please help?

Well, first you need to figure out how your noise maker is activated. This works in one of for ways.
1 The alarm input is normally connected to ground and it goes off when the connection is broken.
2 The alarm input is normally connected to V+ and it goes off when the connection is broken.
3 The alarm input is normally open and it goes off when it is connected to ground.
4 The alarm input is normally open and it goes off when it is connected to V+.
nandan310115 days ago

Hi, actually i had a problem while making this trip wire laser alarm to get the resistance R2 which you mentioned above it should be approximately equal to the resistance on cds and to calculate this we have to shine the laser on the photoresistor but when i did this the value of resistance keeps on increasing as long as laser is shining on the photoresistor so how to determine the resistance value that I shouls use for it thanks

That is very unusual. If that is the case, then you can't use that photoresistor. You need parts are unstable than the circuit will never work.
rishabh872119 days ago

does we have to connect battery to GND and Vcc. if not then where can i connect them .

mark99921 days ago

Where are the positive and negative terminals of laser pointer?

DIY Hacks and How Tos (author)  mark99921 days ago
Check where the battery terminals are connected. Then follow them to the LED. You can usually connect to any point in between.
savina dias1 month ago

can you give me the list needed to make this

I added a list to the instructable
mark9991 month ago

The siren is activated by attaching the positive and negative terminals to a 9 volt batterry or 12 volts

DIY Hacks and How Tos (author)  mark9991 month ago
The max current output of the 555 IC is 200mA. So if the siren uses 200mA or less, then you can just hook it up directly where the buzzer is located on the schematic. Otherwise you will need to use a relay or a power transistor to activate the siren.
mark9991 month ago

How can I change the buzzer with a 12 v siren what resistor should be replaced and what resistor code should be putten on the replaced resistor? Thanks for advance

DIY Hacks and How Tos (author)  mark9991 month ago
The 555 timer circuit can run on 12V just fine. If you power the laser pointer with 12V you need to add a 6.8K ( or greater) resistor to it. But to know how to hook up the siren, I would need to know how the siren is activated.
mark9991 month ago

how can i wire ir photo transistor if i removed the photo resistor and the laser and instead use ir led and ir photo transistor

DIY Hacks and How Tos (author)  mark9991 month ago
You should just be able to substitute in the photo transistor. You may need to change its resistor depending on how sensitive it is. The IR LED would just hook up to a battery. But it will have a lower voltage than the laser. So you should either use a smaller battery pack or add a series resistor to drop the voltage.
mark9991 month ago

How can I change the buzzer with the siren what resistor should be replaced and what resistor code should be putten on the replaced resistor? Thanks for advance

DIY Hacks and How Tos (author)  mark9991 month ago

That all depends on the siren that you want to use. I couldn't really say without knowing the specs

vzalta1 month ago

how much jumper wires is needed for the project?

DIY Hacks and How Tos (author)  vzalta1 month ago

Only what is visible on the main image. I would say about 12 inches of wire total.

hotcheezy2 months ago

How long will this circuit run if using a 9 volt battery with a 5 volt regulator?

DIY Hacks and How Tos (author)  hotcheezy2 months ago
The circuit uses about 7 to 10 mA. So take the capacity of your battery (550 mAh for a typical alkaline) and divide that by 10. But I should also point out that you don't need the 5V regulator. It doesn't save any power and the 555 timer can run on 9 volts. So all you need to do is add a 4.7k resistor to the laser pointer and you can just use the 9 volt battery directly.

And also how long this circuit would run for if turned on and using 3 AA?

katanahikari2 months ago
I soldered it all together and it works, except the laser doesn't stop the alarm completely. It reduces the noise of the alarm, which increases it volume significantly when the laser is broken. What should I do?
It sounds like you have a very sensitive buzzer. You can try adding a small resistor or diode between the switch and the buzzer.
eli2412 months ago

CIRCUIT DOES NOT WORK :( TRIED IT ON A BREADBOARD.. IM SURE EVERYTHING IS CONNECTED WHERE IT SHOULD BE. HELP!

eli2413 months ago

can i use a siren instead of a buzzer? :)

DIY Hacks and How Tos (author)  eli2413 months ago
You can use anything as long as it can be activated by the 4.5 volt signal from the IC.
mnc3 months ago

I always wanted to do that. :) I was thinking for my window. Cool!

zhgutas3 months ago

awesome instructable! too bad I'm rubbish with circuit diagrams.. :(( a stripboard layout would be excellent for noobs like myself.. ;)

katanahikari3 months ago
I read the "how to read circuit diagrams" page and I think I'm getting a hang of it, but I'm having some trouble assembling it on the breadboard (this is my first time trying something like this).  

I can't quite see where exactly to put r2 and the wires for the buzzer. I made a picture on my computer of what I have so far and uploaded it to imgur (http://imgur.com/wpZypYC), could you take a look at it and tell me (1) if everything I have is correct and (2) which holes to put the r2 and buzzer wires in?

Also, is anything connected to pins 5 and 7?
wanzie3 months ago
I think I have all the connections figured out on the circuit board photo. I think the orange wire goes from pin 1/common to an unused connection where the other side of the photoresistor is anchored. For the life of me, I can't see where the brown wire from pin 4 ends up. It seems to me it should connect pin 4 to pin 8 but I don't see that confirmed. Let me know what's behind the IC socket.
DIY Hacks and How Tos (author)  wanzie3 months ago
Yes. Pin 4 is connected to pin 8.
katanahikari3 months ago
I'm a newbie at DIY projects like this. Could you possibly be a little more specific about how to assemble the circuit (specifically what gets connected/soldered to what, in laymen's terms)?
The connections are all shown in the circuit diagram. If you need help reading the schematic, you can check out this instructable: http://www.instructables.com/id/HOW-TO-READ-CIRCUIT-DIAGRAMS/
It gives a good introduction.
vzalta4 months ago
can you please list all the supplies necessary to complete this project (detailed)?
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