Laser Trip Wire?

Thread Starter


Joined Dec 25, 2009

I'm trying to create a laser trip wire that will light an led in my room when someone goes up the stairs. I know that I need a laser and a photo transistor but how would I set this up. Please I don't know much about circuitry so explain thoroughly.


Joined Apr 15, 2009
Here is a simple schematic of the circuit, when someone walks through the beam it will turn the LED on but when they get walk through the laser and are not in the way anymore the LED will turn off, if you want the LED to turn on and stay on you need to make the relay 'Latch' this means basically when it flicks across it has been wired to power itself through the relay contacs, like a light powering itself through a solar panel

You will have to add the 330Ω resistor to your parts list.

You need to make sure the laser is pointed directly at the Light dependant resistor and that there is no other light that will get in the way

You need to adjust the Pot to get the sensitivity right for your area, a quick hint, when i made these circuits i stuck mirrors on the walls with bluetac and directed the beam right around different rooms and back to the main box, be carful the people you are monotoring dont hit the off switch and catch you in your room doing whatever you dont want them to see ;), to unlatch the relay you have to turn the circuit off or make a seperate switch to the relay, or you could connect a timer circuit so the relay stays latched for a specific amount of time

Of course there are simple-er circuits that do the same thing just search dark activated relay, some only require 5 components, Transistor NPN usually 2N3904 or 2N2222, a resistor values vary, a pot usually 10K (10,000 ohms), an LED and a light sensitive component, photo transistor, LDR and you would add the relay where the LED goes and if it wont click across you just bypass the resistor to the LED with some wire

Good luck



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