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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. 
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>
<p>What Kind of printed circuit board should I use because I'm doing this project for my makers class?</p>
You can use any printed circuit board. I use a perf board usually because it is very flexible and can be used for a lot of different kinds of circuits.
<p>Hi, i'm now using a Photodiode. how many ohms should i use now for my R1 and R2 ? and Btw, im also using a 30mm peizo electric buzzer. should i use a 3 AA batteries or a 9V battery ?</p>
<p>Any battery pack over 4.5V should work. R1 and R2 can be anything that is 1kohm or greater. </p>
<p>Thank you so much sir</p>
<p>Hi, im currently using your design for my thesis research. And I've done many researches for the materials, but still don't know what to buy. These are some of my questions:<br>what kind of 555 Timer IC<br>what kind of CdS Photoresistor</p><p>and what kind of resistor ?</p><p>Btw sorry for the idiot questions but im really interested in your design. Thanks in advance Btw.</p>
<p>All 555 timer IC should function the same way. So it shouldn't matter what model you pick. </p><p>CdS photo resistors are a little tricky to work with. Photoresistors are very inconsistent. They have a wide range of performance characteristics and they are not standardized. So when you buy a photoresistor its output is essentially random. If you want much more consistent results, then you should use a phototransistor or a photodiode. </p><p>The resistor can be any type. When using the photoresistor, you need to choose the value of the resistor based on the output of the photoresistor. If you instead use a photodiode or a phototransistor, then you can probably just use a 1kohm.</p>
<p>If i instead use phototransistor or photodiode. what should i get or what's better, Phototransistor or photodiode? and please indicate the full and exact name of the item. i really don't know what to buy and use. And btw, should i use a 1/2 watt resistor? or suggest me anything please.<br><br>thanks for the fast reply. Much appreciated :D</p>
<p>Any kind of resistor can work. Phototransistors are a little easier to work with. I can't make a specific recommendation because I have not built it this system with those parts, but any photo transistor should work. </p>
i need to build something like this because my inlaws has got a big intruder problem. one modification i would like to try is gaving a dual beam setup to avoid the pets or other insects tripping a single beam.
To avoid pets just set the beams higher. Insects usually aren't a problem. But if you want, you can connect a small capacitor in parallel the photo resistor. This will create a short delay on the alarm.
Hi there<br>I am planning to use this alarm in someplace where the light will be changing constantly, so would it be a good idea to cover the LDR with black tape or something(just to keep it in dark) and then shine the laser on it. I will test its resistance accordingly
<p>Another option would be to use a PIR motion sensor.</p>
आमी सगळ्यांनी पातक केलां आनी ताचे खातीर ताका परत बंदखणीची ख्यास्त फर्मायली आनी हेर भारतीय भासांतले अणकार उर्बेभरीत दिसनात भारतीय भासांनी वेगवेगळ्या पाठांचें अणकाराचे माहितीची प्रसारणी करप जाता आनी ती आपल्या घोवाच्या देशाची नागरीक जाता आनी ती आपल्या घोवाच्या देशाची नागरीक जाता।

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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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