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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. 
Hi there<br>I am planning to use this alarm in someplace where the light will be changing constantly, so would it be a good idea to cover the LDR with black tape or something(just to keep it in dark) and then shine the laser on it. I will test its resistance accordingly
<p>Another option would be to use a PIR motion sensor.</p>
<p>I made it for a security system of a local shopkeeper who seems pretty happy with it.. Thanks a lot dude. However I have a slight problem... The circuit works only when the photoresistor is on complete darkness(like in a dark or very dimly lit room).</p><p>Now when buying the parts for this the shopkeeper gave me the wrong resistor for R1 (560 ohms). R2 is 10 k&Omega; which is correct according to my photoresistor. Can using a resistor below 1k&Omega; be the cause of this problem?</p><p>Once again thanks for sharing such a nice and simplified circuit...</p>
<p>If you are having trouble with too much ambient light, you can put some heat shrink tubing around the photoresistor so that it is more shielded from the normal light in the room. Or you could try changing out the resistor values. That may also help. </p>
<p>The output of the pin 3 goes low when the laser doesn't fall on it. How can i modify this circuit so that the output of pin 3 goes high, which then can be used to switch on a npn transistor to switch on a bigger buzzer. And can I attach a PNP transistor to to the output as it is now?</p>
<p>Yes, you can use a PNP transistor but you would need to add a resistor (1kohm could work between pin 3 and the base.</p>
<p>and how can I make the output of pin 3 go high instead of low?</p>
<p>Well, you basically have to rearrange the whole circuit. The easiest way to think of it is like this. <br>When the voltage at pin 2 drops below 1/3 of the supply voltage, then pin 3 goes HIGH. Then when the voltage at pin 6 goes above 2/3 of the supply voltage, then pin 3 goes LOW. So you want a configuration where the laser being interrupted will make the voltage at pin 2 drop. </p><p>One way to do this would be to put the photoresistor between V+ and pin two. Then put R2 between pin 2 and GND. Then put R1 between pin 6 and GND. Then connect pin 6 to the switch.</p>
how to arrange .proper method<br><br>
<p>What are the values of the resistors??</p>
R1 can be anything greater than 1kohm. R2 needs to be matched to the photoresistor. Put the photo resistor in the lighting where it will be set up, then use a multimeter to measure its resistance. Use a fixed resistor for R2 that is close to this value.
<p>i have never done anything like this before. Can i ask how easy it would be too modify the circuit so that instead of a buzzer it wirelessly communicates with another circuit with a silent led indicator.</p>
It is possible but it would be fairly difficult if you aren't familiar with electronics.
can u send me a detailed breadboarding pic
<p>step 4 has a picture of all the parts for this project arranged on a breadboard.</p>
OK,does the ic keeps sending the signals to buzzer if it is not reset even after the battery is plugged out....
<p>Sorry. I don't think that I understand your question. What do you mean by battery plugged out?</p>
Oh, I see. That was just the answer I was looking for. Ill try using a 1.5k resistor since that seems to be the mean resistance value. Thanks!
Hi! I have a little question about the LDR and R1. At full brightness, the LDR measured a resistance of 1k, which is what I used for R1. The problem is I didn't notice that the laser was a little problematic. At startup, it would make the LDR measure 1k, but as time went on, the resistance went up to 2k and sometimes goes back down to 1k. So my question right now is if using a 1k ohm resistor for R1 is still ok. Im thinking that the voltage at the THR pin goes up, but it doesn't go up so much that it goes to twice the threshold voltage. But then again, I'm fairly new to 555 Timer ICs so i really dont know.
<p>With a 555, the internal sensor is triggered when the voltage at pin 6 goes above 2/3 the supply voltage. This means that when the resistance of the LDR is twice as big as R2, then the alarm is triggered. So you want to pick the value of R2 so that the circuit is activated when the laser is interrupted. </p>
Oh and by R1, I meant R2, sorry
now is it right. thanks
<p>It is mostly correct. You also need to connect pin 1 to ground (negative of the power supply).</p>
ok thanks
please explain with image
please see the image and explain.
<p>The center pin of the switch goes to pin 3. The other pin of the switch connects to the negative terminal of the buzzer.</p>
Can you please explain how to join which wire of 555 to which and set resistors .
<p>The timer has pins 4 and 8 connected to positive voltage. Resistor 1 is connected between pin 2 and positive voltage. Resistor 2 is connected between pin 6 and positive voltage. The photo resistor is connected between pin 6 and ground.</p>
<p>would nine volts be too much voltage? Because all I have is a nine volt battery and I already made the circuit.</p>
<p>The 555 timer can work on anything between 4.5 volts and 16 volts. But the laser pointer needs 4.5 volts. So if you want to power the laser pointer with the battery, then you need to add a resistor. </p>
<p>thank you very much?</p>
How much cost for all the components ?
<p>It depends on where you buy them. At a store like Radio Shack, it will cost about $10 - $15 dollars. If you buy them online, you can get them a little cheaper.</p>
New experience
<p>Thank you<br>for the nice system. It&rsquo;s more beautiful because there is no Arduino.</p><p>I like to<br>modify it for the target(receiver) of my laser-gun-game: the buzzer(or LED) should<br>sound only when the laserbeam is detected on the LDR (not when it is interrupted).<br>So my idea is to put the buzzer from the UP to the LOW (change Source/Sink of<br>555 timer) of the 555 timer. What do you<br>think, should I change anything else?</p><p>And how<br>could the duration of the sound be cutted after 1 second? (or be set at a<br>determined value). Thank you<br>for all ideas.</p>
<p>You probably want to use a 555 timer in monostable mode</p><p><a href="https://en.wikipedia.org/wiki/555_timer_IC#Monostable">https://en.wikipedia.org/wiki/555_timer_IC#Monosta...</a></p><p>When triggered, the alarm will sound for a set period of time, then cut off. </p>
<p>Thank you for the fast answer!</p>
<p>Very nice. now to find a way to wirelessly send the alarm activation to a remote board so i can mount this at work and let me know when my boss is coming :D</p>
<p>This project is awesome ! Can it run on a 9v battery? </p>
<p>Yes. You can use a 9V battery but add a 220 ohm resistor in series with the laser pointer so that you don't burn it out. </p>
<p>Thanks!</p>
<p> Now I am ready with all the components the only doubt I have is :</p><p> how you solder the components at back of the pcb</p><p> and</p><p>strictly I should multimeter or no</p>
<p>You might try putting it all together on a breadboard first.</p>
I made this laser tripiwre alarm by myself. First I tried it without R1, but when i turned the alarm on the buzzer starts making noise. <br>After that i tried a 8 Kohm resistor for R1 but i still have the same problem. What could I do??
<p>It needs R1. But the value of R1 doesn't matter. What are you using for R2?</p>
<p>This project is awesome. Can it run on a 9v battery?</p>
Yes... as long as al comonents (buzzer, LED or others) can run on 9v
I buy a cheap multimeter n sett it to 20 k when i shine laser on ldr it fluctuate bw 11.00--13.00 n without laser at normal lighting it fluctuate bw 0.20--0.30 . ..so what is the value or<br>R1???@nd R2????
<p>R1 use a 10 kohm. R2 try using a resistor that measures that same value with the laser off.</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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