Alt-BEAM Archive

Message #09343



To: beam@sgiblab.sgi.com
From: Senior kyled@cruzers.com
Date: Tue, 18 Jan 2000 17:18:51 -0700
Subject: [alt-beam] Re: FLEDs in Sunlight


Hey uh... is this Ben Hitchcock? :)

Kyle

Hitch Bencock wrote:
>
> Hi,
>
> ----- Forwarded message from David Simmons -----
>
> Newbie_s Here!!!
>
> All joking aside, I am a Newbie and my question is, in all likelihood, a
> real dumb one. But I_ll explain first. My son Kyle (nine years old)
> constructed this first SYMET as he calls it. Using a FLED Solarengine,
> solar panel, a pair 10000 uf Caps, etc. It is the first free form
> circuit that he_s put together. Testing all the way! The beard board
> circuit worked quite well but then for some reason when he free formed
> the circuit and assembled on a CD with a motor glued in the centre,
> LightBot One would just sit there doing nothing even in direct sunlight.
> After hours of trying to find the problem I remembered reading once that
> some LEDs are effected by direct sunlight. Once the FLED was covered
> with a little black tape LightBot One came to life. The little brat
> nearly jumped out of skin, after six hours of work he was quite excited
> to see his first design in operation. So his question, _Why are LEDs
> and FLEDs effected by direct sunlight, while some are not, and what is
> happening inside the FLED when the light hits it?
>
> Thank You
>
> Dave and Kyle
> ----- End of forwarded message from David Simmons ----
>
> To explain what's happening, I have to first clarify what's in a FLED.
> Inside every FLED are three components - a LED junction, an
> IC that makes it flash, and some form of current limiting (don't worry
> about the current limiting for now, it's not relevant).
>
> The IC is just basically a switch - it waits for a bit, then closes the
> switch to turn the LED on.
>
> All LED junctions (the bit that shines when you put current through it)
> have a photoelectric potential. That is, when you shine light on them,
> they will create a voltage. So they work a bit like transducers - you can
> either put electricity through them to make light, or put light onto them
> to create electricity.
> You can use this effect to your advantage by using LEDs as light sensors.
> I did this in my "Alf" circuit.
> Try it - grab a LED (clear lensed ones work best), take it into the
> sunlight and attach a multimeter to the legs. I can get almost a volt out
> of some LEDs.
>
> Anyway, back to the FLED SE design. THe way that the FLED triggers the SE
> is that every second, the FLED will turn on, pulling a LOT of current from
> the base of the PNP transistor. The reason it pulls current is that it
> takes current to make the FLED turn on. That's when it's dark, of
> course...
>
> In direct sunlight, the LED junction is lit up brightly, so starts
> producing electricity. When the IC inside the FLED turns the LED junction
> on, there isn't much current flow (because the junction is already lit up
> brightly and is producing electricity that opposes the 'normal' current
> flow) so the PNP doesn't turn on, so the SE doesn't fire.
>
> Think of it as the LED inside the FLED acting like a solar panel that
> opposes the flow of electricity.
>
> So no current - no base current - no SE turn-on.
>
> By the way, by adding just two extra components, you can dramatically
> improve the performance of your FLED SE especially in low light levels.
> Check out http://wollongong.apana.org.au/~ben/fred/ to see a circuit.
>
> I was experimenting last night, and it seems that replacing the 10 k
> resistor with a 33 k improves the low-light ability even more.
>
> It also seems as if this modification reduces the need to blacken the
> FLED. At least, mine doesn't lock up in direct sunlight.
>
> Anyway, hope this helps!
> Ben Hitchcock



9344 Tue, 18 Jan 2000 20:18:30 -0500 D1 beam@corp.sgi.com owner-beam@sgiblab.sgi.com [mailto:owner-beam@sgiblab.sgi.com]On
OK, I thought I would tell you all that I got Wilf's spectacular new D1
circuit to work last night. I think I have a bad bread board, not sure
though. Right now its fully charged to 2.5v and happily blinking at me on
top
my computer (I unhooked the solar cell so I can see just how long it will
last). I was wondering if anyone else has made this yet. My LED blinks kinda
strangely but it still looks cool. Its does a dim-off-dim-bright cycle
rather
then a nice on-off, but its still cool. Thanks again to all who helped

Steve

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