The best way to determine whether an LED in on using light.

Kurt_ · Post by **Kurt_** » Sun Nov 30, 2008 3:35 pm

Well, I need to determine whether a red (or blue, which may require a different sensor) LED is on, based solely on its light output.

Here's the scenario:

A red LED flashes on and off from about a foot and a half away, behind a pane of plexiglass/glass. I need a way to detect whether or not it is on, and then send a signal to some small setup that will trigger a motor or actuator or something. The catch: Similarly-coloured lights are also flashing around it.

Here's what I'm thinking:

A few lenses to focus in on a spot the set distance away, with a light sensor (HELP! What to buy for a red LED/blue LED?) behind them. This goes to some mysterious IC, etc etc.

Thoughts?

Post by **bicostp** » Sun Nov 30, 2008 4:03 pm

What about an infrared emitter/detector pair? (You can get them at Rat Shack, and one of their detectors comes with a housing that blocks visible light.)

It may get tripped by TV remotes, though. (They use rapid blink sequences.)

DSFARGEG · Post by **DSFARGEG** » Sun Nov 30, 2008 4:07 pm

Using a laser(maybe a cheap laser pointer) and a photoresistor in a small housing with a hole only big enough for the laser beam to go in might work if you wanted the light to still be visible.

I think focusing the LED light with lenses onto a small enough spot for a sensor to pick up only one LED while there are others flashing would be quite difficult, but if you find a way, more power to you!

Kurt_ · Post by **Kurt_** » Sun Nov 30, 2008 4:13 pm

YOu fail to realize something.

The LED is not in my control. It is behind glass that I can not get in to.

DSFARGEG · Post by **DSFARGEG** » Sun Nov 30, 2008 5:19 pm

Oh. Well that changes everything...

I'll be sure to let you know if I think of anything.

Super Cameraman · Post by **Super Cameraman** » Mon Dec 01, 2008 2:26 pm

Maybe get a cheap camera with a zoom and use a computer program. That old Lego Mindstorms program could detect motion, and that was years ago. I'm sure there's some free program that could emulate a key press on a keyboard when it detects motion in a certain part of the image...

There's my $0.02.

Crispy · Post by **Crispy** » Tue Dec 02, 2008 3:52 pm

Just out of interest, what is it for?