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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
Switch
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. 
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?
<p>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)</p>
<p>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. </p>
<p>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.</p>
<p>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</p>
<p>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.</p>
<p>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.</p><p>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</p>
How do i use logic gates to make up the circuit and what will be the use of it.<br>Im Sorry If Im annoying you but im actually new to all this...<br>
<p>Try using this circuit</p>
Thanks A Lot <br>Ill Get Back To You....
<p>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?</p>
That souls work fine.
<p>Thanks!</p>
<p>Hello Sir. I want to do this alarm system using Logic Gates.</p><p>What would it be like and it would be better if u could give me some good reference projects based on the same. </p><p>Thank You</p>
<p>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.</p>
Can u give me some reference for it
<p> what specifically would you like to know? </p>
<p>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.</p>
<p>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.</p><p>http://www.instructables.com/id/Intruder-Alarm-With-Text-Message-Notification/</p>
<p>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.</p>
<p>how can this circuit be generated on multisim?</p>
Sorry but I am not familiar with multisim.
Hey man my machine just buzzes no matter what
<p>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 :( )</p>
<p>Sorry for the slow reply. Could you post a picture of your circuit. That would help me figure out what is going wrong.</p>
<p>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</p>
<p>Try using a different value for the resistor that is in series with the LDR</p>
<p>the left one (the others is my little experiment to measure ldr resistance</p>
Hey nice tutorial but it will be better if u add some better pics of schematics I mean the soldering side
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.
What should be the capacity of the resistors
Do you mean resistance values of the resistors or the wattage rating?
What should be the capacity of resistors
<p>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.</p>
<p>Its always fun with Lasers . Thanks Mr.Jason Poel Smith for such a nice n interesting project. <br>I just used the Arduino Board as a power source . :p<br>I J</p>
How can I connect an lm 358 voltage regulator to the circuit because I am using a 9v battery and how can I connect use a variable resistor with three pins for R2
<p>As long as you are not connecting to a microcontroller, there is no need for a voltage regulator. The 555 timer circuit can operate just fine on 9 volts. To connect a variable resistor, connect the center lead to one point and either of the two side leads to the other. The remaining side lead will be unattached to anything. </p>
I followed the circuit diagram the alarm sounded when on. But when I pointed the laser pointer on the ldr it didn't work....how do I know if the ldr is bad
<p>The thing to remember with this design is that the laser needs to be on the light sensor first. Then you reset it with the switch. Also the fixed resistor needs to be balanced with photo resistor. You can test the photo resistor with a multimeter to see how its resistance is changing with different amounts of light.</p>
I followed the circuit diagram the alarm sounded when on. But when I pointed the laser pointer on the lor it didn't work....how do I know if the lor is bad
Hiii I am a novice in this field so m just gonna ask u very idiotic ques.<br>Does this need any programming or even assembling the parts as instructed will do??
No. No programming is necessary.
Thnx ...<br>I assembled as instructed and it worked perfectly.<br>
<p>AWESOME!!!I </p><p>I am less than a beginner at electronics, and I had it up and running first try! Thank you so much.Is there a way to switch it to when the laser hits it comes on instead of the other way around?</p><p>Thank you again!</p>
<p>Try just switching R2 and R3</p>
<p>That is perfect! Thank you so much. You are the best insrtuctor on here!</p>
There it is, Vcc is fine(6v) but it keeps buzzing even when i close(no light) the photoresistor (which i manage to find, and i'm not sure what its resistance my friend said its 10k) r2 is 10kohm and r1 is 100ohm i follow the exact diagram u posted, please help me its due next week.
You can use a variable resistor to make it easier to adjust the value for prototyping.
<p>thank you for the info, my alarm finally works, but there's a new problem, when you interrupt the beam alarm goes on but it turns off by itself as you stop interrupting the laser beam. it supposed to keep buzzing until i manually reset it with the switch, but it's not. according to my friend he said that the timer has no reset option, but your post tells the otherwise, how can i fix this? thank you once again for answering my questions</p>

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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 »
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