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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Amukim17 days ago

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

DIY Hacks and How Tos (author)  Amukim17 days 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........

Amukim17 days ago

when m putiing nothing its still lighting up plz help me

akshay.d213 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.d213 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 ?



JonasV11 month ago

mine works the opposite way, any idea why?

DIY Hacks and How Tos (author)  JonasV11 month 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)  JonasV11 month 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.

AbhroM1 month ago

do this favor as fast as u can

AbhroM1 month ago

Can u show me the wireings? Please

rahul19961 month ago
thanks alot broo

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.
cstierle3 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)  cstierle3 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)  mbobletz2 months ago

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

AdithyaD3 months ago

do you need a multimeter

DIY Hacks and How Tos (author)  AdithyaD3 months ago
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.
Anasshahul0013 months ago

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
BreganneD3 months ago

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

DIY Hacks and How Tos (author)  BreganneD3 months ago
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? ^^

DIY Hacks and How Tos (author)  BreganneD3 months ago

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?

DIY Hacks and How Tos (author)  BreganneD3 months ago

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.

Anasshahul0013 months ago

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.

fathead3013 months 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
fathead3013 months 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??
Anasshahul0013 months ago

can i use 220 ohms for r1

R1 can basically be anything.

Anasshahul0014 months ago

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!5 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....:)

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