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. 
 
Remove these adsRemove these ads by Signing Up

Step 1: Safety Note: Working with Lasers

Picture of Safety Note: Working with Lasers
Image1.jpg
51N8FjlBLKL._SL1000_.jpg
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
IMG_7498.JPG
vlcsnap-2013-11-08-23h06m17s146.png
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

Step 3: How the Circuit Works

Picture of How the Circuit Works
vlcsnap-2013-11-06-11h49m24s96.png
This alarm circuit is yet another way to use a 555 timer IC.

The light sensor that detects the laser is a CdS photoresistor (R3). This is wired in series with standard fixed resistor (R2). These two resistors form a voltage divider that is used to activate the IC. The value of R2 should be approximately the same as the resistance of the photoresistor when you are shining the laser pointer directly at the light sensitive face. Because the output characteristics of photoresistors varies considerably from one to the next, you need to measure it with a multimeter. So connect the photoresistor to the multimeter and shine the laser pointer directly at it. In my case, its resistance was about 100 ohms. So I used a 100 ohm fixed resistor for R2.

When the light beam is interrupted, the resistance of the photoresistor increases dramatically. As a result, the voltage at pin 6 also increases and goes above the reference threshold. This causes the output pin 3 to go LOW and activates the alarm.

To turn off the alarm and reset the system, a (single pole double throw) switch disconnects the speaker and sends the LOW signal from the output pin 3 to the trigger pin 2. The system is now deactivated. To reactivate it, flip the switch back to the original position. The alarm will remain off until the next time that the light beam is interrupted. 

The supply voltage can be anything from 4.5V to 18V. I chose to use 4.5V (three AA  batteries) because this is the same voltage that is used by the laser pointer. This gives you the option of powering the laser pointer with the same battery pack as the alarm circuit. 

The resistor R1 acts as a pull-up resistor for pin 2. It helps to prevent false triggering from static electricity. But in many cases it can be left off without causing any problems. 

The alarm that I am using is a piezo buzzer. Any buzzer can work as long as it is rated to operate at the appropriate voltage.

Step 4: Assemble the Circuit

Picture of Assemble the Circuit
IMG_7531.JPG
IMG_7547.JPG
First assemble the circuit on a breadboard to test it. Set the switch to connect the buzzer. Without the laser shining on the photoresistor, the alarm should sound. Flipping the switch the other way should turn off the alarm. Now shine the laser pointer on the photoresistor and flip the switch one more time to reactivate it. As long as the laser is centered on the photoresistor, the alarm shouldn't sound. But when you move the laser away, the alarm should go off again.

If everything is working properly, solder it all together on a printed circuit board. The board that I used is a general purpose IC board. These are really convenient for circuits that are built around small ICs like the 555 timer. I also used an IC socket to attach the IC. This makes it easy to change out the IC but it is not necessary.

The batteries are mounted in individual AA battery holders. The three battery holders are soldered together in series and the end leads are soldered to the circuit board. 

When attaching the photoresistor, I mounted it with the leads sticking out about one inch from the board. This makes it easy to make small adjustments to the position of the photoresistor after it has been mounted in place.

Once the whole circuit is soldered to the board, test it again to make sure that everything is working properly. 

Step 5: Secure the Loose Parts to the Board with Hot Glue

Picture of Secure the Loose Parts to the Board with Hot Glue
vlcsnap-2013-11-08-23h05m16s49.png
vlcsnap-2013-11-08-23h06m03s247.png
vlcsnap-2013-11-08-23h06m17s146.png
The switch and the batteries are connected to the board with wires. I used hot glue to secure them to the circuit board. This helps to keep the whole circuit neatly together. If the wires from the battery holders are too long, you can tie them down with either tape or a rubber band. 

Step 6: Mount the Laser Pointer and the Alarm Circuit to Form a Single Beam Tripwire

Picture of Mount the Laser Pointer and the Alarm Circuit to Form a Single Beam Tripwire
vlcsnap-2013-11-08-23h19m14s233.png
vlcsnap-2013-11-08-23h18m43s174.png
The simplest way to set up your alarm is as a single beam tripwire. In this configuration the laser pointer is mounted to one side of the walk way and the alarm circuit is mounted to the other. For the tripwire to work, the laser pointer needs to be constantly on. The easiest way to accomplish this is by tightly wrapping a piece of tape around the button.

To secure the two pieces in place, you can use tape or a temporary adhesive putty such as Sticky Tack. First mount the alarm circuit in place. Then mount the laser pointer to the opposite side. Carefully adjust the position of the laser pointer so that it is pointed directly at the photoresistor. 

Once you have the light from the laser pointer centered on the photoresistor, you are ready to arm the alarm. Flip the switch to connect the buzzer and activate the alarm. Whenever someone walks through the beam, the alarm will go off. 


Step 7: Use Mirrors to Make a Multibeam Tripwire

A single tripwire beam works but with the addition of a few mirrors, you can have the laser crisscrossing all over the room making it impossible for someone to avoid detection. 

To accomplish this, you will need a lot of mirrors. There are a number of places where you can get small cheap mirrors. One place is the auto section of your favorite big box store. They often sell plastic sheet mirrors that are designed to replace car mirrors. The major advantage of these is that you can easily cut them to any size and shape that you want. Another good source for mirrors is a craft store. Many craft supplies have a mirror finish. However, the surface is not perfectly uniform. So you won't be able to get as many reflections before the beam starts to disperse. 

To set up a multibeam tripwire, start by mounting the laser pointer. Then at the point where the beam hits the opposite wall, mount a mirror. You can use tape or a self adhesive putty. Position the mirror at a slight angle so that it reflects the beam in a different direction. Continue this process adding more mirrors until you are satisfied with the number of beams or the light beam is starting to disperse too much. The last mirror should direct the light to the alarm circuit. 

Because this system is using one continuous laser, if any of the beams are interrupted, it will cause the alarm to go off. 

Step 8: Optional: Power the Laser Pointer with the Alarm Circuit's Battery Pack

Picture of Optional: Power the Laser Pointer with the Alarm Circuit's Battery Pack
IMG_7587.JPG
vlcsnap-2013-11-08-23h29m34s36.png
vlcsnap-2013-11-08-23h29m47s160.png
vlcsnap-2013-11-08-23h30m19s218.png
Most laser pointers also run on 4.5V (three button cell batteries). If your alarm circuit is powered by 4.5 volts (three AA batteries), then it is possible to power the laser pointer from this battery pack as well.  All you have to do is connect the terminals of the laser pointer to the batteries of the alarm circuit. 

One terminal of the laser pointer is a spring that sticks out of the internal circuit board. The other terminal of the laser pointer is connected to the inside of the metal barrel. You can easily connect to both of these with a pair of alligator clips. The alligator clips can be connected to the positive and negative lines on the circuit board, or you can connect them directly to the terminals of the battery pack. 

By connecting the laser pointer to the larger battery pack you can extend the battery life and you only need to worry about changing one set of batteries. 

Step 9: Optional: Connect Your Laser Tripwire to a Larger Security System

Picture of Optional: Connect Your Laser Tripwire to a Larger Security System
vlcsnap-2013-11-08-23h32m27s226.png
The buzzer on the alarm works to alert you if you are nearby. But you can also connect the tripwire to a larger security system. As part of a whole house security system, you have more options in how the system alerts you. If also lets you confirm the alert with other sensors. 

To connect your laser tripwire to another circuit, connect the grounds of both circuits. Then connect the wire that was attached to the negative terminal of the buzzer to the signal input of the second circuit. Set your monitoring circuit to look for a LOW signal. For example, if you are using an Arduino, wire it to a digital input pin and use the digitalRead function monitor the wire. When it detects a LOW signal have it activate the alarms. 
1-40 of 110Next »
Anasshahul00117 hours ago

can i use 220 ohms for r1

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....:)

IMG_8626.JPGIMG_8628.JPGIMG_8627.JPG

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

DIY Hacks and How Tos (author)  Seminer3 months ago

If the operating voltage of the LED is lower than the voltage of the circuit, then yes you need to add a resistor.

Resistor = (Voltage of the circuit - Voltage of the LED) / Current of the LED

1-40 of 110Next »