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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. 

Step 1: Safety Note: Working with Lasers

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. 
Hey man my machine just buzzes no matter what
<p>hello mates, really glad i found your design. In my case i need to make the object and the simulation (i'm using protheus). The problem is, when i press the button for resetting the alarm while the LDR didn't get the laser (Laser not used properly), the states became error and the output of ic 555 is changed between 0 and 1 rapidly? if that's normal, why it is happening? i need to make sure i can answer it on my presentation (my experimental became fruitless :( )</p>
<p>Sorry for the slow reply. Could you post a picture of your circuit. That would help me figure out what is going wrong.</p>
<p>look like it happens because the input of TR is changed between 0 and 1 rapidly, i dont know the reason tho, it happens when i changed it the input of TR from the switch to logic state (kind of tools to determine the state on proteus). and the output become a toggle if i change the state of TR from 0 to 1 to 0 continuously when the ldr isnt getting a light</p>
<p>Try using a different value for the resistor that is in series with the LDR</p>
<p>the left one (the others is my little experiment to measure ldr resistance</p>
Hey nice tutorial but it will be better if u add some better pics of schematics I mean the soldering side
Will 9v buzzer work
Maybe. Most buzzers have a range of voltages that they can operate at. Most will work at a lower voltage than they are labeled for. Or you can replace the power supply with a 9V battery. Just be sure that you don't try to power the laser pointer with 9V. That could damage it.
What should be the capacity of the resistors
Do you mean resistance values of the resistors or the wattage rating?
What should be the capacity of resistors
<p>PLease tell me where to connect +ve and -ve connection to this laser. Please edit the pic and mark the points where the connections are to be made.</p>
<p>Its always fun with Lasers . Thanks Mr.Jason Poel Smith for such a nice n interesting project. <br>I just used the Arduino Board as a power source . :p<br>I J</p>
How can I connect an lm 358 voltage regulator to the circuit because I am using a 9v battery and how can I connect use a variable resistor with three pins for R2
<p>As long as you are not connecting to a microcontroller, there is no need for a voltage regulator. The 555 timer circuit can operate just fine on 9 volts. To connect a variable resistor, connect the center lead to one point and either of the two side leads to the other. The remaining side lead will be unattached to anything. </p>
I followed the circuit diagram the alarm sounded when on. But when I pointed the laser pointer on the ldr it didn't work....how do I know if the ldr is bad
<p>The thing to remember with this design is that the laser needs to be on the light sensor first. Then you reset it with the switch. Also the fixed resistor needs to be balanced with photo resistor. You can test the photo resistor with a multimeter to see how its resistance is changing with different amounts of light.</p>
I followed the circuit diagram the alarm sounded when on. But when I pointed the laser pointer on the lor it didn't work....how do I know if the lor is bad
Hiii I am a novice in this field so m just gonna ask u very idiotic ques.<br>Does this need any programming or even assembling the parts as instructed will do??
No. No programming is necessary.
Thnx ...<br>I assembled as instructed and it worked perfectly.<br>
<p>AWESOME!!!I </p><p>I am less than a beginner at electronics, and I had it up and running first try! Thank you so much.Is there a way to switch it to when the laser hits it comes on instead of the other way around?</p><p>Thank you again!</p>
<p>Try just switching R2 and R3</p>
<p>That is perfect! Thank you so much. You are the best insrtuctor on here!</p>
There it is, Vcc is fine(6v) but it keeps buzzing even when i close(no light) the photoresistor (which i manage to find, and i'm not sure what its resistance my friend said its 10k) r2 is 10kohm and r1 is 100ohm i follow the exact diagram u posted, please help me its due next week.
You can use a variable resistor to make it easier to adjust the value for prototyping.
<p>thank you for the info, my alarm finally works, but there's a new problem, when you interrupt the beam alarm goes on but it turns off by itself as you stop interrupting the laser beam. it supposed to keep buzzing until i manually reset it with the switch, but it's not. according to my friend he said that the timer has no reset option, but your post tells the otherwise, how can i fix this? thank you once again for answering my questions</p>
<p>That is weird. The switch that is connected to pin 3 is supposed to be the reset switch. I am sorry but I can't quite make out the details from your pictures. Could you post another picture please.</p>
<p>here is the picture</p>
I try making it, I still failed, <br>These are my components, <br>2 100ohm resistor <br>Phototransistor 2 terminal<br>555 timer, <br>Switch<br>Tried on breadboard still not working <br>
Post a picture of the setup
I have pictures of it might be hard if post it here can i send it to your email instead? Im building this for my class project and i need your guidance, im still new it this things
<p>will 3 AAA batteries work instead of 3 AA Batteries?</p>
Yes. AA batteries and AAA batteries have the same voltage. But AAA batteries will not last as long.
<p>Thank you, I tried it and it worked! Great instructable!</p>
<p>please which mode is the 555 timer in?</p>
This project doesn't use any of the standard 555 timer modes. It uses a different configuration that I came up with.
<p>thank u for every think!</p><p>i made it.</p><p>it works,actually your a good man,because i saw that u answered to every pure questions even to people,</p><p>actually i love your morality ,thanks again.</p>
Cool. I am glad that it worked for you.
<p>thanks a lot,thats very kind of you,</p><p>these are its photos , and i added some things other to it, because i connected it to a remote alarm, and 2 LED for when its armed and unarmed ,and a pressed switch.its so fun and useful.</p><p>(Maysam,Iran)</p>
I have a question, I can't get a photoresistor, instead i bought a phototransistor, what should i have to accompany it and is the step on making it different? Thanks and sorry for bad english
<p>If it is a two terminal phototransistor, then you should be able to just substitute it in for the photoresistor in the same place.</p>
How much cost for all the components ?
<p>can i ask?what is the value of resistor both?thank u...</p>
<p>It depends on where you buy them. At a store like Radio Shack, it will cost about $10 - $15 dollars. If you buy them online, you can get them a little cheaper.</p>
<p>This is fun and easy to make. Great !</p>
<p>Nice and easy to make. Thank you for nice instructions </p>
<p>I have completed the circuit and it works my question is, where can i wire an LED to alert me that the alarm is set? i have tried a few things such as from pin 2 to pin 3 but that produces the opposite result. when armed the light is off but when unarmed the light is on.</p>
<p>Try putting an LED between pin 3 and GND or pin 3 and pin 1</p>

About This Instructable

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Bio: My name is Jason Poel Smith I am a Community Manager here at Instructables. In my free time, I am an Inventor, Maker, Hacker, Tinker ... More »
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