Introduction: Laser Tripwire Alarm

Picture of Laser Tripwire Alarm

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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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. 


MinhP35 (author)2017-11-21

I'm kinda new to this, so my questions might be a bit silly.

1/ where do I connect the "GND" to?

2/ where do I connect the positive and negative ends of the batteries to ?

Thanks, plz reply as soon a p

Bbrinnj (author)2017-05-14

Great alarm, works great. Question, can the buzzer be replaced with an LED or light bulb? Looking for a visual effect.

360hobbyist (author)2017-04-02

Hi, I accidentally left out the wire that connects pin 4 and pin 8 when I soldered the entire circuit together and it still seems to work fine. Should I solder that wire in? What does that wire do? Thanks.

Pin 8 is the positive power pin. This is needed to power the circuit. Pin 4 is the reset pin, if this pin is made LOW, it will interrupt the timing cycle and it won't start again until the next time pin 2 is triggered. If pin 4 is disconnected, it will just float to whatever voltage it feels like. Usually static electricity will keep it in a HIGH state but not always. Keeping pin 4 tied to pin 8 will guarantee that pin 4 never goes LOW and interrupts the timer. I would recommend connecting them.

EndritT1 (author)2017-03-06

Hi, I am trying to do this circuit but instead of the buzzer I want to put a small speaker so that when the laser is interrupted, a sound of my own desire shall be played. Do you know of any part, circuit that will help me store the sound and make it play through the speakers when the laser is interrupted?

You could use any sound module. I like the adafruit mp3 player.

may sam (author)2017-01-29

hello dear teacher :) (sorry if i sent this message one more time with another account .

actually i made it about 8 months ago but now i want make some differences on it because it will be more useful to me,anyway.
i want know can i make it automatic!.. without use switch!..
my mean is JUST when something or somebody is between mirror and laser the sound be ON,
after that with coming the light on cds the sound will be OFF itself without pushing any switch,
and the system will be ready for next situation!
is it possible!thanks again

Yes. You can modify the system in that way. But you might want to have the sound stay on for a few seconds to make sure that you can hear it. Otherwise if someone moves through quickly, you might not notice the sound. For this you would want to use a 555 timer in monostable mode like this:

yes, it's right

but i haven't this problem because i conected my laser pack to a remote annunciator,

and that remote annunciator with an small puls can start the sound and also continue it to several second,

anyway thank you so much for your guide and suggestion.

meysambmw (author)2017-01-29

hello dear teacher :)

actually i made it about 8 months ago but now i want make some differences on it because it will be more useful to me,anyway.
i want know can i make it automatic!.. without use switch!..
my mean is JUST when something or somebody is between mirror and laser the sound be ON,
after that with coming the light on cds the sound will be OFF itself without pushing any switch,
and the system will be ready for next situation!
is it possible!
thanks again

Digger08 (author)2016-12-25

Hi thank you for these plans. I am wanting to adapt it to be a laser timer so a switch is tripped 100m away and timer starts then dipped when a multi beam laser system like one above is tripped. It would need to be very accurate and laser 3meters apart Outsode

Would you be able help with this ? Cheer

It is possible. But you can't get super accurate timing with a 555 timer. You would have better luck with a microcontroller such as an Arduino.

As a beginner where do I start? Is there plans for something similar out there with the arduino

You can search for Arduino alarms on the Instructables search bar. You might start by taking a look at this one:

Nschack (author)2016-11-30

Zero electrical experience here. Set up as near as I can tell based off our your picture, but the buzzer isn't sounding with switch in either position. Could you possibly help? Included pic of the setup I've got right now, did I miss something?

ZakG6 (author)2016-11-19

I was wondering how to make this so that the buzzer is always off when the laser shines on it and always on when the laser is not shining on it. (as in it does not require a reset)

That would be a lot simpler. You could just use a voltage divider with a fixed resistor and a photoresistor. The fixed resistor would be connected to the positive voltage wire and the photoresistor would be connected to ground. The center wire would then be connected to the base of a transistor and the transistor would be wired between buzzer and the power wires.

kashif361ks made it! (author)2016-11-18

I made it. Sorry simulated it in Autodesk Circuits. But i have also made it seems me ugly so i have posted the simulation images.

ron351 (author)2016-10-30

I just bought a pair of garage door laser sensors that stops the door from closing and can something more simple be made using these garage door beams so when broken it sounds a beep from a audio device? I thought of trying something basic and maybe a couple reflectors and thought it might be simple as breaking the beam and when it does sound a beep for like 5 seconds or something

If you want to use a 555 timer IC like the one used in this project you will want to wire it in monostable mode. The circuit will look something like this.

Thanks I will have to look into what the 555 timer is and what is IC? Dummy here on all this stuff. And monostable mode, all greek to me. My bro n law is pretty good with electronics and may be asking him for help on this also.

The garage door sensors have 2 wires each coming out of each of them, I have extra wire run in PVC to the garage so connecting them together won't be a issue if needed. The wire ran is 4 pair phone wire so maybe that will help if I need it

Roshan Sadath (author)2016-10-03

How do i use logic gates to make up the circuit and what will be the use of it.
Im Sorry If Im annoying you but im actually new to all this...

Try using this circuit

Thanks A Lot
Ill Get Back To You....

akonichi (author)2016-09-27

Hi. I am thinking of using an old mobile phone USB charger (5V, 1Amp) as a power source for the circuit. Will there be a problem with that?

That souls work fine.


Roshan Sadath (author)2016-09-30

Hello Sir. I want to do this alarm system using Logic Gates.

What would it be like and it would be better if u could give me some good reference projects based on the same.

Thank You

There are a lot of ways that you could do this with logic gates. You could just recreate the 555 timer based on its internal schematic. Or you could make something similar using a simple OR gate, Just have one input be the resistor voltage divider and the other input connected to the output pin through a switch. When the switch is closed it acts as a self latching gate that will stay on when the photo resistor is activated. Then open the switch to reset it.

Can u give me some reference for it

what specifically would you like to know?

John PatrickG2 (author)2016-09-11

Sir Jason Poel Smith, I am a Grade 10 student and our group conducting this laser alarm for our research project. And we need to add a feature which is sending a text message to the owner when a intruder is detected. What should I do ? I really need your help please. Do you know the code for this sending of message ? Please reply Sir, I'm begging you PLEASE help us.

You are going to want to use an Arduino micro controller instead of a 555 timer. You will need a shield that is capable of sending text messages. There are several to choose from. To get you started, check out this instructable.

Yes, it should be noted that SMS is an email system. If you are efficient in programming, you connect the arduino to your computer, have your computer collect serial out, and make your computer send an SMS.

Ashton2415 (author)2016-09-12

how can this circuit be generated on multisim?

Sorry but I am not familiar with multisim.

Evan De Santis (author)2016-08-20

Hey man my machine just buzzes no matter what

NurH32 (author)2016-08-16

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 :( )

Sorry for the slow reply. Could you post a picture of your circuit. That would help me figure out what is going wrong.

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

Try using a different value for the resistor that is in series with the LDR

NurH32 made it! (author)DIY Hacks and How Tos2016-08-18

the left one (the others is my little experiment to measure ldr resistance

Archit B (author)2016-08-17

Hey nice tutorial but it will be better if u add some better pics of schematics I mean the soldering side

yuvraj singh mavi (author)2016-08-10

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.

yuvraj singh mavi (author)2016-08-10

What should be the capacity of the resistors

Do you mean resistance values of the resistors or the wattage rating?

yuvraj singh mavi (author)2016-08-10

What should be the capacity of resistors

atwajid123 (author)2016-07-03

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.

About This Instructable




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:36 Things to Cook in a Coffee MakerHow to Make a Festivus PoleAdd Wings to an Infant's Halloween Costume
Add instructable to: