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
eli2412 months ago

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

The laser needs to be shining on the photoresistor when you reset the switch. If that doesn't help, send me a picture of the circuit.

savdd5 days 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)  savdd5 days 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.

Alright thanks. I more question, if i wanted to attach a camera to it so that the camera would take a picture once the laser is interrupted, how do you suppose i do that?

DIY Hacks and How Tos (author)  savdd3 days ago

Here is a tutorial on how to remotely control a camera

http://www.instructables.com/id/Automatic-Camera-Shutter-Switch/

tried this circuit on a bread board and it doesnt work. i just flip the switch on and off to turn the buzzer on and off. HELP

savdd savddyesterday

when i point the laser at the photoresistor and then flip the switch on the buzzer just starts to sound. i have a DPDT switch and i only have 2 pins hooked up; the left and middle one

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?

hey did you get to solve your problem? if you did could you help me on how you did it. thx

Seminer3 days ago

Does anyone know what resistor to use for the second one? I get that for the first resistor you need to point the laser on the photoresistor and stuff, but whats the ohms of the other one?

DIY Hacks and How Tos (author)  Seminer3 days ago
It doesn't really matter what the second resistor is. Just use a 1kohm or whatever you have. As long as it is higher than 100 ohm, it doesn't matter. All it does is pull pin 2 HIGH when the switch is off.

alright thanks. I more thing, if i add an LED do i still need to add a resistor to it? thx

If the operating voltage of the LED is lower than the voltage of the circuit, then yes you need to add a resistor.

Resistor = (Voltage of the circuit - Voltage of the LED) / Current of the LED

arizik13 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)  arizik13 days ago
The alarm will continue to go off until you turn it off.
arizik13 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+.
nandan310119 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.
rishabh872123 days ago

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

mark99925 days ago

Where are the positive and negative terminals of laser pointer?

DIY Hacks and How Tos (author)  mark99925 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?
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