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
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. 
I buy a cheap multimeter n sett it to 20 k when i shine laser on ldr it fluctuate bw 11.00--13.00 n without laser at normal lighting it fluctuate bw 0.20--0.30 . ..so what is the value or<br>R1???@nd R2????
<p>R1 use a 10 kohm. R2 try using a resistor that measures that same value with the laser off.</p>
Author plz reply i want to complete the project plz plz help
When i connect battery n switch on the circuit the buzzer rings if i point the laser the buzzer also rings...what i do...i dont have a multimeter so i take r2 100 ohm and r1 1k ohm.....last ques to check the circuit can i put 100ohm resistor temporarily inplace of ldr...thanks in advance........
<p>The circuit is designed to sound the alarm when the beam is broken and to stay on until the circuit is reset with the switch. This means that if you turn on the circuit before the laser is in place, the alarm will sound and stay on even after the laser is on. What you need to do is set the laser so that is it shining on the sensor. Then flip the switch to reset the circuit. If everything is working, this should turn off the alarm. </p><p>If you don't have a multimeter, yes, you can use a fixed resistor for testing. You could also replace R2 with a variable resistor and just use trial ad error until you find the right value. But I highly recommend buying a cheap multimeter if you plan on doing any work in hobby electronics. They are a very useful tool and you can buy them really cheap.</p>
R2 value should be what<br>...resistance when pointing laser or resistance in normal lightning
<p>R2 Should be the value of the Photoresistor in normal lighting without the laser.</p>
R2 value should be what<br>...resistance when pointing laser or resistance in normal lightning
Does both resistors have same value r1 and r2
<p>R1 can be anything greater than 1kohm. R2 needs to be roughly the same resistance as the photoresistor in the lighting of the room where it will be set up. So position the photo resistor where it will be mounted. Then measure its resistance. The resistance value that you read is the value that you should use for R2.</p>
<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>
That could do it.Try adding a resistor to it.
<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.

About This Instructable


1,792 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: Controlling Halloween Effects with DIY Infrared Remote Controls Giant Spider Web System For Automatically Limiting TV Time
Add instructable to: