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. 

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. 
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Great work sir!. i have this 5v laser would the resistor id be using be thesame as yours? and by the way the R1 is 1k and the R2 is 100ohm?

collinjc1 month ago
I also checked the voltage its above 4.5v please help me
collinjc1 month ago
Hi this is a great 1..i have made it but i had a few problems ..soon as i turn on the circuit the buzzer came on then i tried different values for R2 still coming on then i removed the photo resistor and R 2 and when power is connected the buzzer still sounds please help thanks alot bro can email me as well undergroundcustoms5@gmail.com
Trojan241 month ago
Great project! I'm going to build this with a micro relay in place of the buzzer to trigger a higher voltage circuit for a louder alarm and lights. Sounds good in theory eh? Haha
CraigB201 month ago
How would I connect this to where I get a phone to dial my phone when the alarm goes off? Thanks!
DIY Hacks and How Tos (author)  CraigB201 month ago
You would need a much more complex setup. You could use an Arduino with an Ethernet shield.
JcBoyDemz1 month ago

Good day,

Can i apply i2c addresing on this project using arduino?

I guess you could set something like that up but I couldn't tell you how.
thebouljello2 months ago

I just made one of these and I used three 1.5V button cells instead of AAs. Within minutes, the batteries are drained of their charge. I do not know what the problem is. I would appreciate any help.

I don't know. Check to see if you have a short somewhere.

Do you think I used too much solder on the connections?


The amount of solder doesn't really matter. Just make sure that it isn't making connections any place that it shouldn't

i used AAs instead and now it works well. Its weird how their the same voltage but the button cells lose their charge. thanks for your help

Its just a question of battery capacity. AA batteries typically have a capacity of about 2000 mAh. Button cells can be as low as 30 mAh. This also means that they can't output much power.

is mAh capacity or output?

mAh (milliAmp Hours) is the storage capacity of the battery. 2000 mAh means that a battery can output 2000 mA for 1 hour or 200 mA for 10 hours or 20 mA for 100 hours, etc.) But based on the way that the batteries are constructed, it also usually correlates to the max current that the battery can output at once. So a AA can output much higher currents than a button cell.

I always thought that volts was the storage capacity. So would a 1.5V battery with an output of 1Ah last shorter than a 3.7V battery with the same output?

To make an analogy. Think of a battery like a big tank of water. The voltage is the like the pressure of the water as it comes out of the tank. The amps is a measure of how much water comes out of the tank each second. The amp-hours (mAh) is a measure of how much total water was in the tank to start with. Does that help?

Yes it does. So after using the battery for some time, will mAh decrease with voltage?

the mAh rating of the battery is what the battery starts with. That is it's capacity. If you wanted to talk about how many mAh remain in a used battery, then you would be talking about the remaining capacity. But kind of. Again, think of it like a big tank of water. As the level goes down, the pressure drops and there isn't as much water left in the tank. So the max output of the battery goes down as it is used up.

That helps. Thank you for your help.

cstierle1 year 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?

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 don't fully understand, do I remove the part of the switch going to pin 3 and then relocate it to the buzzer or do I take the switch to pin 3 out and then connect a wire from pin 3 to the buzzer. And also which part of the buzzer do we connect it to, the positive or negative. Urgent reply please.
DIY Hacks and How Tos (author)  DavidM952 months ago

I may have misunderstood your question. Are you looking for a circuit that only beeps while the light beam is being interupted?

And could you redraw the circuit if possible

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)  mbobletz11 months ago

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

Tanmayg2 months ago
Can you give the arduino code?
DIY Hacks and How Tos (author)  Tanmayg2 months ago

There is no arduino code. I used a 555 timer IC. So no coding is required.

No. I Said about the 'larger security system' by arduino. I wanted its code
DIY Hacks and How Tos (author)  Tanmayg2 months ago

I only built the system detailed here. Sorry.

timlab2 months ago

Great thing and I plan on building it as I really do need it. However, in your instructions you don't indicate what type of resistors I need. This type of information is very important for someone (like myself) that would like to make it. In addition to this, where can a person get these items? Thanks.

timlab timlab2 months ago

Thanks for the reply. However, I know nothing about resistors, and not trying to drag a horse over and over again, but if you actually need a 100 ohm resistor and purchase 1000 ohm your telling me it will still work? In addition to this, would be permitted for you to send me a PM at timlab55_@_ aol. com? Thanks.

DIY Hacks and How Tos (author)  timlab2 months ago
The type of resistor does not matter. All the matters is the resistance value. All these parts can be purchased from any electrical component store like RadioShack, Fry's, Mouser, Digitkey, etc.
matthews292 months ago


i have just built this circuit. i am new to NE 555 timer chips.

do the need to be programmed, if so what was your program

DIY Hacks and How Tos (author)  matthews292 months ago

No programming is required

Drakelin4 months ago

I'm new to the world of electronics and circuits, would a SPST switch instead of a SPDTswitch cause this circuit to not function properly? like say not reset when turned off?

DIY Hacks and How Tos (author)  Drakelin4 months ago

Yes a SPST switch would not work. You could design a circuit that would use a SPST. But this circuit needs a SPDT.

is there a specific circuit board needed?

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