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. 
<p>I tried changing the value of R2 and still got static from the buzzer. Does the fact that the buzzer is only 3 volts and running off of 9 volts cause a problem?</p>
<p>Here it is.</p>
<p>That looks like you have everything wired up right. The only thing that I can think of is that you might need to change the value of R2. It needs to be the same as the value of the photoresistor in the normal lighting of the room. </p>
Thank you very much..this worked great for my school physics project...but i have a doubt ..instead of a battery of 6 V, can i use an AC to DC converter which outputs 5 V and 2 A.if not possible...please do tell me what values should the converter output for this circuit to work without any damage to its components while connected to the domestic household power supply.. ..
<p>May I know the explanation for the circuit.</p><p>Please</p>
It is easier to explain if you first let me know what you are confused about.
<p>Thanks for the response.</p><p>Request you to throw more details in the middle part involving&quot;TRIG,THR,RESET,GND,4,8,6,1&quot; and soldering the things at the back.</p>
<p>Are you having trouble reading the schematic?</p>
<p>check out this tutorial</p><p>http://www.instructables.com/id/HOW-TO-READ-CIRCUIT-DIAGRAMS/</p>
<p>I can't seem to get my buzzer to sound, I read through previous comments and no one seems to have the same problem. I've tried everything I can think of, and I've checked to make sure the circuit is set up right several times. I have a 3 volt buzzer because that's the only thing I had, I also tried getting the voltage to go down to 3 volts for the buzzer but that didn't work either. The buzzer just makes quiet static sounds and usually I have to move the circuit or the battery around to get it to do that. can you tell me what's wrong?</p>
<p>Take a picture of your circuit and post it here so that I can see what you are working with.</p>
This was a great success as my school physics project.. thank you very much.... : D
<p>Learned how to read the majority of the schematics, just confused on what THR, TRIG and RESET meant </p>
<p>those are just labels for the pins of the 555 timer IC. You can ignore those and just pay attention to the pin numbers</p>
<p>So Yesterday i posted a question, and upon further research, i was able to better learn how to use the multimeter. So after many accidential discoveries and hours of tinkering, my question boils down to this: what is going on with that schematic? I have the 555. Pin 1 to neg trmnl. 8 to pos. 6 to 8 with 100 ohm rstr, neg term to ldr to pin 6. 4 to one side of switch, 3 to pole, 2 to 10k rstr to other side of switch. Now how do i wire my led?</p>
<p>The laser LED is wired in series with its resistor and connected to the positive and negative of the power supply. </p>
<p>Ok so I'm new to a lot of this technology and figured I'd might as well try and build this; So I'm wondering if you have any pointers, or can you give me a link that explains this better for new people.</p><p>Tnx- Dalton</p>
My best advice is to just try it. You have to get started somewhere.
<p>Ill make tHis short. Beginner with prior experience. Quit my retail job to learn this stuff. You, DIY are one of my few influences, thank you. Past few weeks since i quit my job been garbage surfing and have taken apart so many electronics i have about 15 pounds in screws. Yep, screws, grommets, circuit boards... From a variety of electronics. From cmptr mthbrds to toaster oven circuit boards to printer scanner cboards, house phones, audio systems, tvs toys you name it... Am willing to contribute in any way possible. ... Anyway... having trouble following schematic, heres my progress</p>
<p>So i made the circuit and have a problem ( i used an led instead ) , my led becomes brighter when the laser is interrupted and vice-versa when interruption is gone , i solved the problem with a capacitor but the led doesnt stay on after the interruption , whats going on ?</p>
Take a picture of your circuit and post it here. Then I will because to see what you have.
<p>How cool would that be if an alarm system would actually have a trip wire feature. Oh, that would be a top seller to buy and one that's going to get the attention of others. Well, it reminds me of something that I once saw at a certain office building where a crew of installers were installing a system.http://www.telepluscorp.com/alarm-systems.php</p>
<p>When I try measuring the resistance of the photoresistor, it flashes different numbers ranging from 100-900 and anywhere inbetween. How do I know what grade of resistor to get for R2?</p>
<p>Finally Completed my first instructable..... Just made few changes to the original circuit.</p><p>1). Used an photoresistor in place of R1 as its value was based on resistance of R3 in present light. It would do same.</p><p>2). Used a variable resistor for R2 (10k ohm to be specific) because resistance of R3 kept on changing for different distances from laser.</p><p>3). Used a LED instead of as i didnt had one....:)</p>
<p>THANK YOU! I was having trouble trying to determine the strength of the resistors I would need. The potentiometer idea was a great one. I attached the POT in the R2 space and tweaked it I to find the sweet spot. Once things were working consistantly, I then measured the pot resistance using my multimeter and knew the resistance I needed.</p>
<p>can you give me your circuit digram and value of resistors you used</p>
<p>Nice job</p>
<p>Hi - I want to make this circuit. What laser did you use?</p>
Its just a cheap key chain laser pointer that you can buy at any dollar store.
<p>thanks but does it have any particular brand name and do you know if I could get if off amazon or something? </p>
<p>Hi Sir... I would like to make this topic as my physics project... pls tell me what more do i need than the the stuff i already have now : 4 AA battery holder, Photo Resistor, 4 AA batteries, resistor of 100 ohms and 6.8 K ohms, PCB and Soldering materials, 3-24 V buzzer and a switch.. I want to know if i can use a 2n7000 transistor instead of an 555 timer IC.Also..please tell me the sequence of connection of these components to the PCB.</p>
You can use a transistor instead of a 555 timer but it will behave differently. I used the 555 timer because the alarm will stay on after someone passes through. If you just replace it with a transistor, the alarm will only be one while the beam is interrupted. In terms of the assembly process, I would say, just follow the schematic and it should work.
<p>Great work sir!. i have this 5v laser would the resistor id be using be thesame as yours? and by the way the R1 is 1k and the R2 is 100ohm?</p>
I also checked the voltage its above 4.5v please help me
Hi this is a great 1..i have made it but i had a few problems ..soon as i turn on the circuit the buzzer came on then i tried different values for R2 still coming on then i removed the photo resistor and R 2 and when power is connected the buzzer still sounds please help thanks alot bro can email me as well undergroundcustoms5@gmail.com
Great project! I'm going to build this with a micro relay in place of the buzzer to trigger a higher voltage circuit for a louder alarm and lights. Sounds good in theory eh? Haha
How would I connect this to where I get a phone to dial my phone when the alarm goes off? Thanks!
You would need a much more complex setup. You could use an Arduino with an Ethernet shield.
<p>Good day,</p><p>Can i apply i2c addresing on this project using arduino?</p>
I guess you could set something like that up but I couldn't tell you how.
<p>I just made one of these and I used three 1.5V button cells instead of AAs. Within minutes, the batteries are drained of their charge. I do not know what the problem is. I would appreciate any help.</p>
<p>I don't know. Check to see if you have a short somewhere. </p>
<p>Do you think I used too much solder on the connections?</p>
<p>The amount of solder doesn't really matter. Just make sure that it isn't making connections any place that it shouldn't</p>
<p>i used AAs instead and now it works well. Its weird how their the same voltage but the button cells lose their charge. thanks for your help</p>
<p>Its just a question of battery capacity. AA batteries typically have a capacity of about 2000 mAh. Button cells can be as low as 30 mAh. This also means that they can't output much power.</p>
<p>is mAh capacity or output?</p>
<p>mAh (milliAmp Hours) is the storage capacity of the battery. 2000 mAh means that a battery can output 2000 mA for 1 hour or 200 mA for 10 hours or 20 mA for 100 hours, etc.) But based on the way that the batteries are constructed, it also usually correlates to the max current that the battery can output at once. So a AA can output much higher currents than a button cell.</p>
<p>I always thought that volts was the storage capacity. So would a 1.5V battery with an output of 1Ah last shorter than a 3.7V battery with the same output?</p>

About This Instructable


1,787 favorites


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 »
More by DIY Hacks and How Tos: Giant Spider Web System For Automatically Limiting TV Time Remotely Control Your Halloween Props
Add instructable to: