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. 
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> 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>
Author plz reply i want to complete the project plz plz help
When i connect battery n switch on the circuit the buzzer rings if i point the laser the buzzer also rings...what i do...i dont have a multimeter so i take r2 100 ohm and r1 1k ohm.....last ques to check the circuit can i put 100ohm resistor temporarily inplace of ldr...thanks in advance........
<p>The circuit is designed to sound the alarm when the beam is broken and to stay on until the circuit is reset with the switch. This means that if you turn on the circuit before the laser is in place, the alarm will sound and stay on even after the laser is on. What you need to do is set the laser so that is it shining on the sensor. Then flip the switch to reset the circuit. If everything is working, this should turn off the alarm. </p><p>If you don't have a multimeter, yes, you can use a fixed resistor for testing. You could also replace R2 with a variable resistor and just use trial ad error until you find the right value. But I highly recommend buying a cheap multimeter if you plan on doing any work in hobby electronics. They are a very useful tool and you can buy them really cheap.</p>
R2 value should be what<br>...resistance when pointing laser or resistance in normal lightning
<p>R2 Should be the value of the Photoresistor in normal lighting without the laser.</p>
R2 value should be what<br>...resistance when pointing laser or resistance in normal lightning
Does both resistors have same value r1 and r2
<p>R1 can be anything greater than 1kohm. R2 needs to be roughly the same resistance as the photoresistor in the lighting of the room where it will be set up. So position the photo resistor where it will be mounted. Then measure its resistance. The resistance value that you read is the value that you should use for R2.</p>
<p>I tried changing the value of R2 and still got static from the buzzer. Does the fact that the buzzer is only 3 volts and running off of 9 volts cause a problem?</p>
That could do it.Try adding a resistor to it.
<p>Here it is.</p>
<p>That looks like you have everything wired up right. The only thing that I can think of is that you might need to change the value of R2. It needs to be the same as the value of the photoresistor in the normal lighting of the room. </p>
Thank you very much..this worked great for my school physics project...but i have a doubt ..instead of a battery of 6 V, can i use an AC to DC converter which outputs 5 V and 2 A.if not possible...please do tell me what values should the converter output for this circuit to work without any damage to its components while connected to the domestic household power supply.. ..
<p>May I know the explanation for the circuit.</p><p>Please</p>
It is easier to explain if you first let me know what you are confused about.
<p>Thanks for the response.</p><p>Request you to throw more details in the middle part involving&quot;TRIG,THR,RESET,GND,4,8,6,1&quot; and soldering the things at the back.</p>
<p>Are you having trouble reading the schematic?</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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