Laser Tripwire


Introduction: Laser Tripwire

When I first started trying to figure out how to build a tripwire i was able to find a lot of different ways to do it but they each had their flaws. Some were too complex, requiring ICs or Arduino. Some were simple but only stayed on while the beam was broken. Finally after experimenting with a variety of methods I came up with a simple laser tripwire that stayed on when the beam was broken.

Step 1: Materials

What You need:
4 100 ohm resistors
1 switching diode
3 transistors. I used 2n2222 but anything similar will work
1 photoresistor (LDR)
1 buzzer. you can also substitute an LED or anything else you want the Tripwire to set off

You will also need a breadboard, jumpers and a laser pointer for assembly

Sorry for the poor quality picture

Step 2: Assembly

Assemble your tripwire as shown in the diagram. When you're done it should look something like the second picture. Make sure you connect the transistors in the right way or else it won't work.

Step 3: How It Works

If you don't know a lot about electronics I would skip this step. it is just an explanation of how the tripwire works. If you are interested however, just follow along in this explanation.

The tripwire is divided into 2 parts. the part that detects the light and sends out the pulse and the "switch" that turns on the buzzer and keeps it on. The reason I have "switch in quotes is because it isn't something like a rocker or toggle switch that change the direction or allow current to pass. This switch simply keeps current flowing through it when it encounters a single pulse. The only way to turn it off is to disconnect it from power.

The first picture shows the Light Detector. When light (or a laser) is shining on the photoresistor the photoresistor alows current to pass. Since we want it to send a pulse when there is no light ( or when the laser beam is broken) we run it into an inverter or not gate. This makes it so that when light is shining there is no output current, but when there is no light there is current flowing through the output.

The second picture shows the switch. this is simply an arrangement of 2 not gates. when power comes into the first gate it stops current going to the second gate. the lack of power then causes the second gate to allow power to flow through to the load ( Buzzer LED ect.) and then acts as a second pulse at the first gate. This causes the cycle to start all over again thus keeping the circuit on.

Step 4: Your Done!

Just set it up so the laser is shineing on the photoresistor, conceal it, and sit back and relax as you watch people trip it.



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    24 Discussions

    What is the out put voltage if i use a 6v battery, and a 9v battery(seperately)?

    It does not work for me....I understand and copied ur circuit connection like in the picture....but the buzzer start and did not stop when the voltage source is connected........even there is laser beam pointing in LDR.....i use PN2222 transistor instead of ur 2N2222....because i think they are the same....i really dont know what is wrong.....please help me

    Can u tell me which photo resistor to use? I will appreciate if you answer...
    Thnk u

    I just build this and am having the same problem as evilgenius32. The only thing I did differently was use 2n2222a transistors. Any idea why it won't continuously buzz after the laser has been crossed?

    2 replies

    Jperry, same issue here, using the 2n2222. Not worked as described in Instructables.

    I did some changes in the circuit with Yenka software and works well with a LED, but not with the Buzz. When I cut the laser beam, the LED turns ON and keep on. If I try with a Buzz, it works, but stop when the beam back.

    If someone have some idea, follow my circuit below:


    Jperry and others..

    I solved the question. Just put a LED in parallel with BUZZ and will work.

    see bellow my diagram.


    Hi mate, i made this week ago, but now I want to use a siren instead of pizo buzzer, so i connect the output wires to siren power inpit, but I heard a very low sound, i checked the output by multimeter, their it was showing 4 v, the problem is that it was showing enough power to sound the siren at full volume bcoz siren need only two 1.5 v aa battery, please help,

    1 reply

    Chances are it's either hooked up incorrectly, or the circuit cannot source enough current to drive the siren. I have to imagine that the siren requires more current than a typical pizo. You might try hooking the siren up via a MOSFET to try to get it more current.

    Amazing tutorial dude! but when I connect everything and substitute an LED when the laser is not broken it glows and when the laser is broken the led glows. Please help
    Thank You.

    1 reply

    Thanks a ton bro. i made it with an old cellular phone charger and it works like a dream

    really helpful project there dude.but how do i do it if i have to connect it to AC power? cud u post a circuit diagram or smthing .That'd b really helpful. Appreciate it.

    1 reply

    There are several ways you could go about connecting it to AC power. The easiest way to do it is to find an old power supply for something and use that. It will have a big block looking thing that plugs into the wall and then a connector on the end (you probably have plenty of these things plugged in around your house). You can then either cut the connector and strip the wires or better yet, get a plug that fits the connector on the supply.

    DUDE!!! Awesome 'ible Made one myself but I had some problems. I put it together and it only stayed onn when the beam was broken. I was wondering what transistors you used and if you used different ones for each switch. Thanks for the ible again!

    2 replies

    I used 2n2222 transistors for all the parts. You can use other NPN bipolar transistors instead as long as the power ratings are similar.

    Why is so important to all transistor have similar power rating?
    i used BC107A 240hfe ,BC107B 400hfe and BF182B 240hfe it does not work at all no Volts at buzzer at any time,and can i use cutted 2N3055 as solar cell instead of photo resistors?