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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. 

Step 2: Parts

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

Step 3: How the Circuit Works

This alarm circuit is yet another way to use a 555 timer IC.

The light sensor that detects the laser is a CdS photoresistor (R3). This is wired in series with standard fixed resistor (R2). These two resistors form a voltage divider that is used to activate the IC. The value of R2 should be approximately the same as the resistance of the photoresistor when you are shining the laser pointer directly at the light sensitive face. Because the output characteristics of photoresistors varies considerably from one to the next, you need to measure it with a multimeter. So connect the photoresistor to the multimeter and shine the laser pointer directly at it. In my case, its resistance was about 100 ohms. So I used a 100 ohm fixed resistor for R2.

When the light beam is interrupted, the resistance of the photoresistor increases dramatically. As a result, the voltage at pin 6 also increases and goes above the reference threshold. This causes the output pin 3 to go LOW and activates the alarm.

To turn off the alarm and reset the system, a (single pole double throw) switch disconnects the speaker and sends the LOW signal from the output pin 3 to the trigger pin 2. The system is now deactivated. To reactivate it, flip the switch back to the original position. The alarm will remain off until the next time that the light beam is interrupted.

The supply voltage can be anything from 4.5V to 18V. I chose to use 4.5V (three AA batteries) because this is the same voltage that is used by the laser pointer. This gives you the option of powering the laser pointer with the same battery pack as the alarm circuit.

The resistor R1 acts as a pull-up resistor for pin 2. It helps to prevent false triggering from static electricity. This can be any value. In many cases it can be left off without causing any problems.

The alarm that I am using is a piezo buzzer. Any buzzer can work as long as it is rated to operate at the appropriate voltage.

Step 4: Assemble the Circuit

First assemble the circuit on a breadboard to test it. Set the switch to connect the buzzer. Without the laser shining on the photoresistor, the alarm should sound. Flipping the switch the other way should turn off the alarm. Now shine the laser pointer on the photoresistor and flip the switch one more time to reactivate it. As long as the laser is centered on the photoresistor, the alarm shouldn't sound. But when you move the laser away, the alarm should go off again.

If everything is working properly, solder it all together on a printed circuit board. The board that I used is a general purpose IC board. These are really convenient for circuits that are built around small ICs like the 555 timer. I also used an IC socket to attach the IC. This makes it easy to change out the IC but it is not necessary.

The batteries are mounted in individual AA battery holders. The three battery holders are soldered together in series and the end leads are soldered to the circuit board. 

When attaching the photoresistor, I mounted it with the leads sticking out about one inch from the board. This makes it easy to make small adjustments to the position of the photoresistor after it has been mounted in place.

Once the whole circuit is soldered to the board, test it again to make sure that everything is working properly. 

Step 5: Secure the Loose Parts to the Board with Hot Glue

The switch and the batteries are connected to the board with wires. I used hot glue to secure them to the circuit board. This helps to keep the whole circuit neatly together. If the wires from the battery holders are too long, you can tie them down with either tape or a rubber band. 

Step 6: Mount the Laser Pointer and the Alarm Circuit to Form a Single Beam Tripwire

The simplest way to set up your alarm is as a single beam tripwire. In this configuration the laser pointer is mounted to one side of the walk way and the alarm circuit is mounted to the other. For the tripwire to work, the laser pointer needs to be constantly on. The easiest way to accomplish this is by tightly wrapping a piece of tape around the button.

To secure the two pieces in place, you can use tape or a temporary adhesive putty such as Sticky Tack. First mount the alarm circuit in place. Then mount the laser pointer to the opposite side. Carefully adjust the position of the laser pointer so that it is pointed directly at the photoresistor. 

Once you have the light from the laser pointer centered on the photoresistor, you are ready to arm the alarm. Flip the switch to connect the buzzer and activate the alarm. Whenever someone walks through the beam, the alarm will go off. 


Step 7: Use Mirrors to Make a Multibeam Tripwire

A single tripwire beam works but with the addition of a few mirrors, you can have the laser crisscrossing all over the room making it impossible for someone to avoid detection. 

To accomplish this, you will need a lot of mirrors. There are a number of places where you can get small cheap mirrors. One place is the auto section of your favorite big box store. They often sell plastic sheet mirrors that are designed to replace car mirrors. The major advantage of these is that you can easily cut them to any size and shape that you want. Another good source for mirrors is a craft store. Many craft supplies have a mirror finish. However, the surface is not perfectly uniform. So you won't be able to get as many reflections before the beam starts to disperse. 

To set up a multibeam tripwire, start by mounting the laser pointer. Then at the point where the beam hits the opposite wall, mount a mirror. You can use tape or a self adhesive putty. Position the mirror at a slight angle so that it reflects the beam in a different direction. Continue this process adding more mirrors until you are satisfied with the number of beams or the light beam is starting to disperse too much. The last mirror should direct the light to the alarm circuit. 

Because this system is using one continuous laser, if any of the beams are interrupted, it will cause the alarm to go off. 

Step 8: Optional: Power the Laser Pointer with the Alarm Circuit's Battery Pack

Most laser pointers also run on 4.5V (three button cell batteries). If your alarm circuit is powered by 4.5 volts (three AA batteries), then it is possible to power the laser pointer from this battery pack as well.  All you have to do is connect the terminals of the laser pointer to the batteries of the alarm circuit. 

One terminal of the laser pointer is a spring that sticks out of the internal circuit board. The other terminal of the laser pointer is connected to the inside of the metal barrel. You can easily connect to both of these with a pair of alligator clips. The alligator clips can be connected to the positive and negative lines on the circuit board, or you can connect them directly to the terminals of the battery pack. 

By connecting the laser pointer to the larger battery pack you can extend the battery life and you only need to worry about changing one set of batteries. 

Step 9: Optional: Connect Your Laser Tripwire to a Larger Security System

The buzzer on the alarm works to alert you if you are nearby. But you can also connect the tripwire to a larger security system. As part of a whole house security system, you have more options in how the system alerts you. If also lets you confirm the alert with other sensors. 

To connect your laser tripwire to another circuit, connect the grounds of both circuits. Then connect the wire that was attached to the negative terminal of the buzzer to the signal input of the second circuit. Set your monitoring circuit to look for a LOW signal. For example, if you are using an Arduino, wire it to a digital input pin and use the digitalRead function monitor the wire. When it detects a LOW signal have it activate the alarms. 
<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>
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>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।
<p>I made it for a security system of a local shopkeeper who seems pretty happy with it.. Thanks a lot dude. However I have a slight problem... The circuit works only when the photoresistor is on complete darkness(like in a dark or very dimly lit room).</p><p>Now when buying the parts for this the shopkeeper gave me the wrong resistor for R1 (560 ohms). R2 is 10 k&Omega; which is correct according to my photoresistor. Can using a resistor below 1k&Omega; be the cause of this problem?</p><p>Once again thanks for sharing such a nice and simplified circuit...</p>
<p>If you are having trouble with too much ambient light, you can put some heat shrink tubing around the photoresistor so that it is more shielded from the normal light in the room. Or you could try changing out the resistor values. That may also help. </p>
I think it is a little complicated
<p>The output of the pin 3 goes low when the laser doesn't fall on it. How can i modify this circuit so that the output of pin 3 goes high, which then can be used to switch on a npn transistor to switch on a bigger buzzer. And can I attach a PNP transistor to to the output as it is now?</p>
<p>Yes, you can use a PNP transistor but you would need to add a resistor (1kohm could work between pin 3 and the base.</p>
<p>and how can I make the output of pin 3 go high instead of low?</p>
<p>Well, you basically have to rearrange the whole circuit. The easiest way to think of it is like this. <br>When the voltage at pin 2 drops below 1/3 of the supply voltage, then pin 3 goes HIGH. Then when the voltage at pin 6 goes above 2/3 of the supply voltage, then pin 3 goes LOW. So you want a configuration where the laser being interrupted will make the voltage at pin 2 drop. </p><p>One way to do this would be to put the photoresistor between V+ and pin two. Then put R2 between pin 2 and GND. Then put R1 between pin 6 and GND. Then connect pin 6 to the switch.</p>
how to arrange .proper method<br><br>
<p>What are the values of the resistors??</p>
R1 can be anything greater than 1kohm. R2 needs to be matched to the photoresistor. Put the photo resistor in the lighting where it will be set up, then use a multimeter to measure its resistance. Use a fixed resistor for R2 that is close to this value.
<p>i have never done anything like this before. Can i ask how easy it would be too modify the circuit so that instead of a buzzer it wirelessly communicates with another circuit with a silent led indicator.</p>

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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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