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

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. 
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JonasV13 days ago

mine works the opposite way, any idea why?

DIY Hacks and How Tos (author)  JonasV13 days ago
opposite how?

the buzzer turns off when de laser doesnt hit the LDR and turns on when the laser hits the LDR

Check to make sure that your photoresistor is wired up correctly. It should connected to pin 6 and GND (pin 1). If the photoresistor and the fixed series resistor where switch, it could cause this problem.

AbhroM6 days ago

do this favor as fast as u can

AbhroM6 days ago

Can u show me the wireings? Please

rahul19967 days ago
thanks alot broo

How viable of an option would it be to replace laser/CdS with infrared emitter/detector? I assume it would be similar in all aspects, but shorter potential working distance?

That would definitely work. But the shorter range would prevent you from using mirrors to create a sensor grid.
cstierle2 months ago

I am sorry if this has already been answered but I didn't see it. Is there a way to have the alarm reset automatically when the laser is re-pointed at the photoresister?

DIY Hacks and How Tos (author)  cstierle2 months ago

There are several ways that you could do this. Here is one example. Remove the switch at pin 3 and connect pin three directly to the buzzer. Then add a capacitor between pin 2 and ground. Lastly, add a resistor between pin 3 and pin 2. The values of the resistor and the capacitor will determine the delay before automatically resetting.

Could you give an example of resistor and capacitor values that would keep the alarm on for a few seconds?

DIY Hacks and How Tos (author)  mbobletz1 month ago

Start with a 10k resistor and a 100 microfarad capacitor. Then increase the value of the resistor for longer beeps.

akshay.d211 month ago

Hi Jason,

Please can you help me trouble shoot my problem ? i saw this amazing project on as well,

My Photoresistor is showing different resistance in different rooms ( Obviously ! ) But how do you make it compatible for different intensities of light as well as different atmospheric conditions. i.e Cloudy or sunny weather.

Thanks for sharing this amazing project. :)



The best thing to do is replace the series resistor with variable resistor (potentiometer). That will let you make adjustments for different environments.

AdithyaD2 months ago

do you need a multimeter

DIY Hacks and How Tos (author)  AdithyaD2 months ago
If you don't use a multimeter, then you need to do a lot of blind trial and error to find a matching resistor for your circuit.
Anasshahul0012 months ago

i dont hav a multi meter so ..
how do i measure the resistance of photo resistor ?

Buy a multimeter. You can get them for $6 to $10 at harbor freight
BreganneD2 months ago

the cicuit is not working. isn't it okay to use 12-v horn(motorcycle)? and Battery in series three 1.5 volts.

DIY Hacks and How Tos (author)  BreganneD2 months ago
The 4.5V from the circuit may not be enough to activate the 12V horn. You can try powering the circuit with 12V and see if that works. Just make sure that the horn doesn't require more than 200 mA (the max output of the 555 chip). Otherwise you will need to add a power transistor to activate the horn.

Thank you for the reply. Okay2 I will use powersupply to my circuit. I'm just curious with the difference of Horn and Buzzer. Last question, How to connect battery to the laser when battery is already connected to your circuit? ^^

DIY Hacks and How Tos (author)  BreganneD2 months ago

Step 8 shows how you can connect to the laser pointer with a set of alligator clips. But if you want to power it with a 12V battery, then you are going to need to add a resistor so that you won't overload the laser pointer. Use a multimeter to see how much current it uses. Then divide 7.5 by the current in amps. This is the resistor that you will need. So if it uses 20 mA, then you should use about 375 ohms.

Thank you again for the reply. I already try 12-v apply to my circuit. The IC overheated and my breadboard melt a little bit, But its okay. What other material should I use when there is no buzzer? 3-12v bulb?

DIY Hacks and How Tos (author)  BreganneD2 months ago

There must have been a short somewhere. Because that shouldn't have happened with this circuit. You should be able to use any load as long as it doesn't exceed 200 mA.

Anasshahul0012 months ago

i used 3.7 (mobile battery )
the alarm is beeping :D
this alarm is also beeping :( , when i " Switch off the alarm and shine laser pointer on the photoresistor and then turn on "

The 555 timer chip is designed to use 4.5 volts or greater. That may be causing the problem.

fathead3012 months ago
ive built the circuit with the alarm on it but I want the alarm to be removed and in place have a arduino board or propeller board to send sms or email to my phone and I do not have much experience with programming
fathead3012 months ago
what is the programming code to get it to where it will send a text message if it is within the WiFi of the house?? or an email??
Anasshahul0012 months ago

can i use 220 ohms for r1

R1 can basically be anything.

Anasshahul0012 months ago

Where i suppose to connect the other end of R1 ??? plz repy fast

R1 can be connected between pins 2 and 4

I was looking for something like this. Its more simpler so i'll try this first.
Thanks :)

Angads25 made it!3 months ago

Finally Completed my first instructable..... Just made few changes to the original circuit.

1). Used an photoresistor in place of R1 as its value was based on resistance of R3 in present light. It would do same.

2). Used a variable resistor for R2 (10k ohm to be specific) because resistance of R3 kept on changing for different distances from laser.

3). Used a LED instead of as i didnt had one....:)


can you give me your circuit digram and value of resistors you used

DIY Hacks and How Tos (author)  Angads253 months ago

Nice job

DaREALMastaD3 months ago

Awesome, this was my first attempted instructables and it turned out great. Awesome job and thanks.

dudes3 months ago

What is the value for r1?

DIY Hacks and How Tos (author)  dudes3 months ago

Take a multimeter and measure the resistance of the CdS photoresistor while it is in the room where you want to set it up. Try to have the same lighting conditions that it will usually operate under. Then whatever resistance you measure, find a fixed value resistor with close to that value. That is the resistor that you should use.

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