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. 
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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
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akshay.d218 hours 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.

AdithyaD9 days ago

do you need a multimeter

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.
cstierle13 days 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)  cstierle12 days 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.

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
BreganneD21 days ago

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

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? ^^

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?

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.

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.

fathead30121 days 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
fathead30121 days 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??

can i use 220 ohms for r1

R1 can basically be anything.

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!2 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)  Angads252 months ago

Nice job

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

dudes1 month ago

What is the value for r1?

DIY Hacks and How Tos (author)  dudes1 month 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.

mark9992 months ago

what is the value of r1 in this project?

DIY Hacks and How Tos (author)  mark9992 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.

What if i use another Cds photoresistor in the place of r1....??

anadeem22 months ago
I want the alarm to keep ringing once the laser beam is interrupted.
To do that, i know we can use a flip flop like 4013 at the output of pin 3 of lm555 to keep the value low/high, whatever the requirement.
But i never used a flip flop before.
So can u please help me out.
I would be really thankful to you. :)
DIY Hacks and How Tos (author)  anadeem22 months ago
The current design will keep the alarm ringing until it is manually reset at the switch.
mark9993 months ago

can i just use a battery holder that can hold 3 aa batteries?

DIY Hacks and How Tos (author)  mark9993 months ago

You can do anything that you want to get 4.5V connected to the circuit.

mark9993 months ago

what is the value of r1 in this project?

mark9993 months ago

Why did you use 3 individual aa battery holders, why not use a battery holder that can hold three aa batteries? And does that mean I should use 4.5v?

Superior Tech3 months ago

How many ohms would R1 be? Do you really need a circuit board or can you just make the connections using a bunch of wires?

R1 should be the value of your photoresistor in the lighting where it will be set up. You can always use free form soldering in place of a circuit board. It is just hard to do properly.
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