Laser Tripwire Alarm




About: My name is Jason Poel Smith. In my free time, I am an Inventor, Maker, Hacker, Tinker, and all around Mad Genius

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. 

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387 Discussions


3 months ago

Hello, Jason Poel Smith. The idea of the scheme is cool !!! But a question arose when considering the principle of operation of your scheme:
According to the datasheet, the NE555 uses an RS trigger built on 2OR-NOT.

1) when applying a beam of light, and then applying power (VCC) inside 555 RS-flip-flop takes the values: R = 0, S = 1, so there is no sound at the out = 1 output.

2) when turning off the light (the voltage drop on R3 increases), i.e. R = 1, S = 1 - the forbidden state for the RS-flip-flop.
How can it work in the forbidden state? Explain, please, but I'm already confused.


Question 7 months ago on Step 9

For a school project, I was wondering if there would be a way to change it so that the alarm sounds at 15hz.

1 answer

Question 10 months ago on Introduction


I've been looking for a
laser switch combination - thank you. Instead of a buzzer being activated what
would you recommend that I use in its place that would act as a trigger/release
switch for a trap/cage door? The trigger/release could possibly be something that would change the polarity on a magnet and would hold open a small
metal door and release the door when the laser beam was interrupted? Any
help is appreciated.

1 answer
DIY Hacks and How TosMiller7070

Answer 10 months ago

The simplest thing that you can do is replace the buzzer with a low power motor. Or you can use the Arduino setup in step 9 to activate a servo or a solenoid.


Question 1 year ago

Do you think you (I) could use an IR LED instead of a laser? (They won't let me use a laser at the competition)

3 answers

Answer 1 year ago

A laser is used so that light does not spread out over long distance. If IR LED is used then it will spread over a large area and there will be a decrease in accuracy and power at the receiver. You can find some cheap laser diode here:


Answer 1 year ago

I don't think use IR LED can work, because of the light that the resister to be need .not only the light but visible light.


Question 1 year ago

sir can you provide the code for arduino if i want to attach a GSM module that alerts whenever alarm goes off or a rasberry which sends photo whenever any person enters.


1 year ago

Hey master, why my circuit always make a sound at buzzer.. no matter there is a laser or not.. how fix that problem ? thx

1 reply

The problem is probably your light dependent resistor. If the lighting is bright enough, it will saturate the cell and it won't work. Try a different light sensitive resistor.


1 year ago

hey when i shine my laser on the ldr the multimeter reads 2.5 does that means i should u a 2 ohm resistor???

1 reply

That could work. A 2 ohm resistor isn't very common though. So you could either use a 2.2 ohm resistor or two 1 ohm resistors in series.


1 year ago

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

1 reply

GND or "ground" is the negative terminal. All negative terminals of the circuit are connected together

If you don't have a barrel connector, you can connect the wires from the battery directly to the Vin pin on the Arduino.


2 years ago

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


2 years ago

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.