Laser as a beam of light to a receiver across room for security

T Chap

Does anyone know a way to get the same look as in the movies where a red beam of light is across the floor, where the robbers are always trying to not break the beam?

I would like to use a very thin beam like that, similar to a "laser pen", and am trying to find a solution (not an expensive laser system) where a processor can send data on the beam (like fiber optics, only visible light), and have some type of receiver read the data. I am thinking in the order of <$100 for the beam xmitter and receiver.

Any suggestions on a method?

Mike Green

You don't need a laser. An ordinary LED and phototransistor with additional lenses to narrow the beam would work. Just like the IR emitter and detector used with the BoeBot, you'd use a modulated light beam. You'll need to make your own filter / detector and may be able to find some DIY circuits on the web, perhaps even a kit.

I've seen a commercial wireless network link using an LED and phototransistor with Fresnel lenses to focus the beams that worked over a range of about a mile, so it's doable.

T Chap

Thanks Mike, I'll read up on the Boebot scheme.

I found this site which has some interesting devices:

www.mfgcn.com/

Phil Pilgrim (PhiPi)

Getting the "look" may require some additional creativity. Without serious particulates in the air (e.g. dust, smoke, mist, etc.) even a beam of visible light will be invisible. Chances are, the "beams" you see in the movies are added after the fact by the CGI gurus.

-Phil

T Chap

Yeah you are right Phil, I was taking that in consideration, maybe at night it would be visible if it is dark.

I like the laser diode idea, cheap, and they have the look with no extra effort if they already have a lense.

More exploring:

Diode:
search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=365-1141-ND


Phototransistor:
search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=OP600A-ND

T Chap

How does modulated light work in the case of the Boebot? I noticed in another thread discussion of a carrier, but i makes no sense to me why you'd need a arrier, why not just transmitted the data as in fiber optics: low is off, high is on?

Phil Pilgrim (PhiPi)

The laser diode you've cited is just that: a diode, without any driving or regulating circuitry. Plus, it has a flat lens, so it's unfocused.

Unlike LEDs, laser diodes take more than just a current source to drive them. There also needs to be a feedback mechanism to regulate their output in order to avoid runaway situations. For a cheap, focused laser module, it's hard to beat DealExtreme.com. Here is but one example. There's no data on how quickly the beam responds to power switching, so it may or may not be suitable for modulating. But, hey, at $4.30, including shipping from Hong Kong, what have you got to lose for trying?

-Phil

BTW, I've made a couple purchases from DealExtreme and have been very satisfied with their service. HK Post takes a little time to arrive, but they use registered mail and provide a tracking number that works on usps.com once the package arrives stateside. If you have questions, their customer service department is very responsive via email. They're definitely not a fly-by-night operation. It's an abiding mystery to me how they do it and make any money!

Post Edited (Phil Pilgrim (PhiPi)) : 12/9/2008 7:45:48 AM GMT

SRLM

*I think* that a carrier is used so that you can filter out the ambient light (or signals for RF/telephone/fiber optic communication)

T Chap

Thanks Phil, I'll get a few and have some fun with it.

kwinn

You may want to try one of the IR receiver modules used in remotes. They are meant for infrared, but may be sensitive enough to a red LED or Laser diode to work and have all the required circuitry (amp, schmidt, etc.) to provide a digital output. They are cheap enough that if it doesn't work it's not the end of the world.

T Chap

www.dealextreme.com/details.dx/sku.5914

Got these in today from dealextreme.com. Neat devices, focusable dot, I will test speeds sending data this week. It would be really neat to have some permanent fog machine type device on at all times to see the beam for sci fi effect.