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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what is the resistance of your photo resistor

Photo resistors do not have set resistances. They vary wildly in resistance and performance. The values that you will get out of any given photoresistor are random. That is why you have to match the fixed resistor to the photo resistor that you have. You can choose the fixed resistor. You have no idea what the photo resistor will be.

what is the resistance of your photo resistor

what is the resistance of your photo resistor

ashish081 month ago

how can i find the value of R1

DIY Hacks and How Tos (author)  ashish081 month ago

Just use a 1kohm resistor

ashish081 month ago

if i change value of R2=2.4k,which value of R1 should i use

DIY Hacks and How Tos (author)  ashish081 month ago

R1 can be anything that is 1kohm or greater.

ashish081 month ago

when laser pointed on LDR value is 2.4Kohm and in darkness the value is 7.5Kohm what should be the correct value for R2

DIY Hacks and How Tos (author)  ashish081 month ago
Try about 2.4K for the R2
ashish081 month ago

hello.. buzzing sound is coming when the laser is pointed on LDR r even when the switch is reset..what may be the problem plz can you help me

DIY Hacks and How Tos (author)  ashish081 month ago

try a different value for R2. If it is not balanced with photoresistor it may be constantly triggering the alarm

rahul19964 months ago
thanks alot broo

can u show me your circuit diagram.. plz...

Yo bro, can you by any chance make a video showing how to assemble the circuit?
thank you

Thanks a lot bro
DIY Hacks and How Tos (author) 1 month ago

The circuit is connected according to the circuit diagram. If you need help reading a circuit diagram you can check out this instructable for a guide:

If you use the same circuit board or breadboard, you can also just put all the parts in the same location as you see in the picture and it should work.

Can you please show how you joint all the parts I.e. how you made the circuit plz reply as fast as you can
brityock2 months ago
What are the values of the resistors photo cell and 555 timer ic
DIY Hacks and How Tos (author)  brityock2 months ago
R1 can be anything kohm or higher
R2 should be approximately the same resistance as the photoresistor in the desired lighting.
The timer IC is a standard 555 timer IC
Amukim3 months ago

plz help me it keeps on ringing even i turn on d laser

DIY Hacks and How Tos (author)  Amukim3 months ago

The way the circuit is designed, the alarm will stay on until the laser is shining on the sensor and the reset switch it flipped. So make sure that the laser is on the sensor. Then flip the reset switch.

but wen m shining d laser it keeps on buzzing........

Amukim3 months ago

when m putiing nothing its still lighting up plz help me

akshay.d216 months 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. :)



DIY Hacks and How Tos (author)  akshay.d216 months ago

The best thing to do is replace the series resistor with variable resistor (potentiometer). That will let you make adjustments for different environments.

Thanks for replying ! :)
Please could you answer my last queries :(

So should i use a POT in place of R1 or R2 ?

What should be the other resistor value ? and if you want the beep to last longer what should you do ?



JonasV14 months ago

mine works the opposite way, any idea why?

DIY Hacks and How Tos (author)  JonasV14 months ago
opposite how?

the buzzer turns off when de laser doesnt hit the LDR and turns on when the laser hits the LDR

DIY Hacks and How Tos (author)  JonasV14 months ago

Check to make sure that your photoresistor is wired up correctly. It should connected to pin 6 and GND (pin 1). If the photoresistor and the fixed series resistor where switch, it could cause this problem.

AbhroM4 months ago

do this favor as fast as u can

AbhroM4 months ago

Can u show me the wireings? Please

How viable of an option would it be to replace laser/CdS with infrared emitter/detector? I assume it would be similar in all aspects, but shorter potential working distance?

That would definitely work. But the shorter range would prevent you from using mirrors to create a sensor grid.
cstierle6 months 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)  cstierle6 months 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.

Could you give an example of resistor and capacitor values that would keep the alarm on for a few seconds?

DIY Hacks and How Tos (author)  mbobletz5 months ago

Start with a 10k resistor and a 100 microfarad capacitor. Then increase the value of the resistor for longer beeps.

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