Instructables
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
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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
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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
Switch
CdS Photoresistor
2 resistors
3 AA Batteries
3 AA Battery holders
Jumper Wires
Heat Shrink Tubing
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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!1 month 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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can you give me your circuit digram and value of resistors you used

DIY Hacks and How Tos (author)  Angads251 month 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....??

anadeem21 month 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)  anadeem21 month ago
The current design will keep the alarm ringing until it is manually reset at the switch.
mark9992 months ago

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

DIY Hacks and How Tos (author)  mark9992 months ago

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

mark9992 months ago

what is the value of r1 in this project?

mark9992 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 Tech2 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.
Metalsnow2 months ago

What type of arduino is use in this ?

DIY Hacks and How Tos (author)  Metalsnow2 months ago
An Arduino Uno is pictured but any type can work.
Quin52 months ago

I'm looking to purchase a lazertag system to start a lazertag business,
would you be able to produce or lead me to the guns and equipment I need
that would be safe and certifiable, that would be a cheaper option than
the $600/gun price range that I've seen advertised by the lazertag
manufacturers? email if so at ulrichinvesting@gmail.com

DIY Hacks and How Tos (author)  Quin52 months ago
Sorry. I can't really help you with that.
lelouch2193 months ago

this circuit is work.... i already made it...!! thanks...

eli2415 months ago

CIRCUIT DOES NOT WORK :( TRIED IT ON A BREADBOARD.. IM SURE EVERYTHING IS CONNECTED WHERE IT SHOULD BE. HELP!

DIY Hacks and How Tos (author)  eli2413 months ago

The laser needs to be shining on the photoresistor when you reset the switch. If that doesn't help, send me a picture of the circuit.

savdd3 months ago

Hi, sorry I'm new to this but how do i know what kinds of resistors to use?

DIY Hacks and How Tos (author)  savdd3 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.

Alright thanks. I more question, if i wanted to attach a camera to it so that the camera would take a picture once the laser is interrupted, how do you suppose i do that?

DIY Hacks and How Tos (author)  savdd3 months ago

Here is a tutorial on how to remotely control a camera

http://www.instructables.com/id/Automatic-Camera-Shutter-Switch/

tried this circuit on a bread board and it doesnt work. i just flip the switch on and off to turn the buzzer on and off. HELP

savdd savdd3 months ago

when i point the laser at the photoresistor and then flip the switch on the buzzer just starts to sound. i have a DPDT switch and i only have 2 pins hooked up; the left and middle one

katanahikari6 months ago
I read the "how to read circuit diagrams" page and I think I'm getting a hang of it, but I'm having some trouble assembling it on the breadboard (this is my first time trying something like this).  

I can't quite see where exactly to put r2 and the wires for the buzzer. I made a picture on my computer of what I have so far and uploaded it to imgur (http://imgur.com/wpZypYC), could you take a look at it and tell me (1) if everything I have is correct and (2) which holes to put the r2 and buzzer wires in?

Also, is anything connected to pins 5 and 7?

hey did you get to solve your problem? if you did could you help me on how you did it. thx

Seminer3 months ago

Does anyone know what resistor to use for the second one? I get that for the first resistor you need to point the laser on the photoresistor and stuff, but whats the ohms of the other one?

DIY Hacks and How Tos (author)  Seminer3 months ago
It doesn't really matter what the second resistor is. Just use a 1kohm or whatever you have. As long as it is higher than 100 ohm, it doesn't matter. All it does is pull pin 2 HIGH when the switch is off.

alright thanks. I more thing, if i add an LED do i still need to add a resistor to it? thx

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