This instructable will teach you how to make a laser trip mine.
If someone steps in the lasers path I have gotten it to set off a L.E.D., but you can make it set off other things.

You are going to need:

1. Laser.

2. Photocell, perferably a big one.

3. PNP switching transistor

4. 2 nine volts

5. potentiometer

6. L.E.D

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Step 1: The Laser

Attach the laser to the switch and the 9 volt. Make sure everything is compact so that later if you want you can put it in a box. Also to make sure the laser doesnt get burned use a resistor.

Step 2: The Circuit

First, you connect the negative side of the 9 volt to the emitter of the PNP transistor. Next, put the negative side of the L.E.D to the collector side of the PNP transistor. Then, attach one side of photocell to the base of the PNP transistor and the other side to positive side of the L.E.D. Finally, attach the positive side of the 9 volt to the left side of the potentiometer and the middle part of the potentiometer to the positive side of the L.E.D and the photocell. Make sure the potentiometer is set on low so you dont burn out the L.E.D.

The PNP transistor:
If you dont know which sides are what on the PNP transistor. Put the flat side North and the far left one emitter, middle is the base, and the far right one is collector.

To make it work:
Put the potentiometer on low so the L.E.D is totally off. Shine the laser directly on the photocell. When the laser is on the photocell turn the potentiometer slowly till it turns on, once its on turn it until it has just gone off, so it should be only a little to the opposite direction. Then to test it works put your hand through the laser and if the L.E.D turns on(should be brightly) then you've finished the project. If it doesnt make sure when the laser is on it the L.E.D is only barely off. Try to experiment with buzzers, relays, and other stuff.

If anyone knows Circuit and breadboard design software so I can make it easier to visualize the curcuit comment me. And if you have any questions or problems comment me.
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Dearden8053 years ago
So basically when the laser loses contact with the photoresistor it supplies power to the LED? That means you could wire in anything that ran on a nine-volt. Please correct me if I am wrong.
lukeD3 years ago
I have a program for you, Yenka called. it is free if you take the home license. good luck.

amruth3 years ago
Can someone give me the PNP transistor number like for NPN we have 2N2222 or 2N3904.
for 2n3904 its 2n3906
would a dvd player's laser work?
Sorry, no. You need a laser BURNER to make it at least cut through paper. I know there has been some confusion about that....

Laser readers like DVD ones read tiny bumps in the CD/DVD. The burner makes those bumps.
ARJOON GameNox3 years ago
can i use a laser pointer
hard drives contain magnetic reading heads not lasers, right?
Yeah, I think so.
techno guy3 years ago
Can you make some schematics for this?
Sparklesam6 years ago
I tried hooking this up to a buzzer, but It isn't working at all. Please if anybody has any advice I'd like to know
Most buzzers will not just work because they have electricity flowing through them, in fact if you were to open something like say... The Operation game you would find that the buzzer will usually even have a transistor of its own.
Digy Cubie24 years ago
 you should try this instructable if you want a buzzer I am building one and it works except it won't stay on when tripped, so I am in the process of finding a different type of transistor.
rrrmanion Digy4 years ago
Google set reset transistor then either get the buzzer to run of the maximum (~5v) or use a relay type thing, to up the voltage
Digy rrrmanion4 years ago
I actually figured out u need a relay to get it to stay on after activation.
erveli4 years ago
How many ohms is the potentiometer?
is it pnp or npn?? cuz for most switichng applications ppl use npn
Do you need the potentiometer? I can't seem to find one so if it wasn't there I wouldn't mind.
 Yes, it affects the sensitivity of the photocell, basically it is a variable resistor. You can find one at tri-state electronics on northwest highway in mt propect illinois if you around cook county (Chicago-land).
minerug5 years ago
It would be better to get a B216 kit from Jaycar and hook it up to a relay, which in turn is hooked up to a siren or something 
Colonel885 years ago
Can you post a simple circuit diagram, like maxxxem's but just the bare bones approach?
maxxxem5 years ago
I modded your circuit a little to make LED light or 15 or 30 sec
Laser Tripwire Circuit Diagram.bmp
Cool project. What rating is the resistor to protect the laser?
For my laser, i didnt have to have one. My laser worked on 4.5 volt first so i tried 9 volts and it worked but just as long as you can see the laser on a wall or the photocell the project will work. If the laser is powered with high amps it will burn the photocell and everything it comes in contact with so watch out.
that is a waste of energy.
so how do u raise the amps??!?!?!
halo456456 (author)  pyro137 years ago
either a transformer or get power with high amps. there is an instructables that tells you how to do that
ok can u please explain, for a dummy who knows nothing about electronics, how to turn a 5$ lazer pointer from walmart into something that can burn paper with minimal cost.
jman11 pyro135 years ago
go to and pick up a blue ray laser diode
halo456456 (author)  pyro137 years ago
ok, hmmm, have you search the 60$ Laser engraver/ cutter, if you have or dont want to you can take apart an adapter(like a one you plug it a wall, dont shock or kill yourself) that has around 3-6 volts and and 100-450 milliamps(mA= 0.001 so 100 mA equal 1 Amp). The adapter should say how much voltage and amperage it has. Heres a link that can explains it (if the link doesnt work here
hope that helps
halo456456, you have same misconceptions about basic electricity/electronics. I'm not saying that to offend you, but to try to get you to do some studying/research (example - 100mA does not equal 1amp, it equals .1amp) You can't take a power source and a fixed load and "increase the amps". You can increase the power source capacity, but it will deliver no more current than the load requires.
whats it called?
halo456456 (author)  pyro137 years ago
60$ Laser engraver/cutter" by cgosh
jman115 years ago
do you think this would work for a multi stage coil gun?
santy225 years ago
put it into a HL laser tripmine papercraft and it would be VERY COOL
imakethings5 years ago
when the laser beam is tripped will the buzzer ring continuously?
PKTraceur5 years ago
try they have a PCB maker which you can order from them, and a schematic maker, which I use alll the time. It's free, and awesome. -RoAr
ian256 years ago
This is how I visualized the schematic. Hope it helps. Great Instructable.
Schematic.psd(612x792) 456 KB
sdaw6 years ago
could you just make a schematic using paint? it would be much easier to understand.
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