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. 
Remove these adsRemove these ads by Signing Up

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. But 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

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. 
arizik7 days ago
does this alarm turn off automatically or you have to turn it off by your self once it is activated???
DIY Hacks and How Tos (author)  arizik6 days ago
The alarm will continue to go off until you turn it off.
arizik7 days ago
does this alarm turn off automatically or I have to turn it off by myself once it is activated??

i do not have a security alarm however i want to build exactly what you did then link it to something that makes a loud sound when the laser is interrupted can you please help?

Well, first you need to figure out how your noise maker is activated. This works in one of for ways.
1 The alarm input is normally connected to ground and it goes off when the connection is broken.
2 The alarm input is normally connected to V+ and it goes off when the connection is broken.
3 The alarm input is normally open and it goes off when it is connected to ground.
4 The alarm input is normally open and it goes off when it is connected to V+.
nandan310113 days ago

Hi, actually i had a problem while making this trip wire laser alarm to get the resistance R2 which you mentioned above it should be approximately equal to the resistance on cds and to calculate this we have to shine the laser on the photoresistor but when i did this the value of resistance keeps on increasing as long as laser is shining on the photoresistor so how to determine the resistance value that I shouls use for it thanks

That is very unusual. If that is the case, then you can't use that photoresistor. You need parts are unstable than the circuit will never work.
rishabh872117 days ago

does we have to connect battery to GND and Vcc. if not then where can i connect them .

mark99919 days ago

Where are the positive and negative terminals of laser pointer?

DIY Hacks and How Tos (author)  mark99919 days ago
Check where the battery terminals are connected. Then follow them to the LED. You can usually connect to any point in between.
savina dias29 days ago

can you give me the list needed to make this

I added a list to the instructable
mark9991 month ago

The siren is activated by attaching the positive and negative terminals to a 9 volt batterry or 12 volts

DIY Hacks and How Tos (author)  mark9991 month ago
The max current output of the 555 IC is 200mA. So if the siren uses 200mA or less, then you can just hook it up directly where the buzzer is located on the schematic. Otherwise you will need to use a relay or a power transistor to activate the siren.
mark9991 month ago

How can I change the buzzer with a 12 v siren what resistor should be replaced and what resistor code should be putten on the replaced resistor? Thanks for advance

DIY Hacks and How Tos (author)  mark9991 month ago
The 555 timer circuit can run on 12V just fine. If you power the laser pointer with 12V you need to add a 6.8K ( or greater) resistor to it. But to know how to hook up the siren, I would need to know how the siren is activated.
mark9991 month ago

how can i wire ir photo transistor if i removed the photo resistor and the laser and instead use ir led and ir photo transistor

DIY Hacks and How Tos (author)  mark9991 month ago
You should just be able to substitute in the photo transistor. You may need to change its resistor depending on how sensitive it is. The IR LED would just hook up to a battery. But it will have a lower voltage than the laser. So you should either use a smaller battery pack or add a series resistor to drop the voltage.
mark9991 month ago

How can I change the buzzer with the siren what resistor should be replaced and what resistor code should be putten on the replaced resistor? Thanks for advance

DIY Hacks and How Tos (author)  mark9991 month ago

That all depends on the siren that you want to use. I couldn't really say without knowing the specs

vzalta1 month ago

how much jumper wires is needed for the project?

DIY Hacks and How Tos (author)  vzalta1 month ago

Only what is visible on the main image. I would say about 12 inches of wire total.

hotcheezy2 months ago

How long will this circuit run if using a 9 volt battery with a 5 volt regulator?

DIY Hacks and How Tos (author)  hotcheezy2 months ago
The circuit uses about 7 to 10 mA. So take the capacity of your battery (550 mAh for a typical alkaline) and divide that by 10. But I should also point out that you don't need the 5V regulator. It doesn't save any power and the 555 timer can run on 9 volts. So all you need to do is add a 4.7k resistor to the laser pointer and you can just use the 9 volt battery directly.

And also how long this circuit would run for if turned on and using 3 AA?

katanahikari2 months ago
I soldered it all together and it works, except the laser doesn't stop the alarm completely. It reduces the noise of the alarm, which increases it volume significantly when the laser is broken. What should I do?
It sounds like you have a very sensitive buzzer. You can try adding a small resistor or diode between the switch and the buzzer.
eli2412 months ago


eli2413 months ago

can i use a siren instead of a buzzer? :)

DIY Hacks and How Tos (author)  eli2413 months ago
You can use anything as long as it can be activated by the 4.5 volt signal from the IC.
mnc3 months ago

I always wanted to do that. :) I was thinking for my window. Cool!

zhgutas3 months ago

awesome instructable! too bad I'm rubbish with circuit diagrams.. :(( a stripboard layout would be excellent for noobs like myself.. ;)

katanahikari3 months ago
I read the "how to read circuit diagrams" page and I think I'm getting a hang of it, but I'm having some trouble assembling it on the breadboard (this is my first time trying something like this).  

I can't quite see where exactly to put r2 and the wires for the buzzer. I made a picture on my computer of what I have so far and uploaded it to imgur (, could you take a look at it and tell me (1) if everything I have is correct and (2) which holes to put the r2 and buzzer wires in?

Also, is anything connected to pins 5 and 7?
wanzie3 months ago
I think I have all the connections figured out on the circuit board photo. I think the orange wire goes from pin 1/common to an unused connection where the other side of the photoresistor is anchored. For the life of me, I can't see where the brown wire from pin 4 ends up. It seems to me it should connect pin 4 to pin 8 but I don't see that confirmed. Let me know what's behind the IC socket.
DIY Hacks and How Tos (author)  wanzie3 months ago
Yes. Pin 4 is connected to pin 8.
katanahikari3 months ago
I'm a newbie at DIY projects like this. Could you possibly be a little more specific about how to assemble the circuit (specifically what gets connected/soldered to what, in laymen's terms)?
The connections are all shown in the circuit diagram. If you need help reading the schematic, you can check out this instructable:
It gives a good introduction.
vzalta4 months ago
can you please list all the supplies necessary to complete this project (detailed)?
DIY Hacks and How Tos (author)  vzalta4 months ago
I discussed the parts in detail in Step 2 but here it is in a simple list of the parts that I used:
555 Timer IC
A 4.5V Piezo buzzer
R1: 1Kohm resistor
R2: 100 ohm resistor
R3: CdS Photoresistor
SPDT Sliding switch
Printed Circuit board (RadioShack 276-159)
Jumper Wires
Three AA batteries
Three AA battery holders
Laser Pointer
thank you so much you helped me out bigtime

Get More Out of Instructables

Already have an Account?


PDF Downloads
As a Pro member, you will gain access to download any Instructable in the PDF format. You also have the ability to customize your PDF download.

Upgrade to Pro today!