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Using LEDs — Parallax Forums

Using LEDs

ArchiverArchiver Posts: 46,084
edited 2004-03-02 14:08 in General Discussion
Just a general thought to through out into the world.

You can not just put LEDs in parallel, and size one resistor to run them
all. There are small variations in the Voltage drop across different LEDs,
that will cause some to be brighter than others, and some to not turn on at
all. When in operation, LEDs act like zener diodes in the way that they
perform in the circuit. So, one cuts on at a lower Voltage and the Voltage
never gets high enough for the others to cut on.

I did a theater Markee (spelling) for the wife one Halloween, so that she
could go as the Goodyear Blimp with legs (she was seriously pregnant at the
time). To cut down on components, I did do exactly what I am saying you
shouldn't do. But in order to do this, I had to take a large pile of LEDs,
and try them all, one at a time, to find ones that had the same Voltage drop
across them. Not really worth the effort, but I didn't know that until I
was seriously into the effort, so continued trying them all until I had what
I wanted.

By the way, she won first prize at the contest.
Original Message
From: Allan Lane [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=bPrXn1KsRBaKsP0hYCY-RLGzZiJpv7JfnC0-hO6fgQqQk8nAmhOa1TbmQhAroUJPOezhFOdhq7ZzQOSsZjWn2AAe]allan.lane@h...[/url
Sent: Monday, March 01, 2004 9:52 AM
To: basicstamps@yahoogroups.com
Subject: [noparse][[/noparse]basicstamps] (unknown)


My recommendation: Start small.
Get a 10-segment LED bar from Radio Shack,
along with 10 220 ohm resistors. Interface
it to the BS2. Try stuff.

Once you've got something that works like
you want, now interface those 10 LED's to
a 74HCT595 (That's the serial to
parallel output chip.) Now do your patterns
using SHIFTOUT. Get that to work. Find out
how much current you're going to need.

Once you've gotten that to work, now you
should have something that can 'scale up'
to your full LED implementation, since you
can put LOTS of '595's in series with each
other. You may need some external memory
to store LED patterns, though.

This approach lets you perfect your ideas
BEFORE doing a lot of hardware, which you
may have to re-do as your ideas change.





--- In basicstamps@yahoogroups.com, "lac_2k4" <lasseal@o...> wrote:
> Hi!
> I'm new to this Basic Stamp thing, and dont really know what is is.
> I dont know the limits with this but i've heard that it is a great
> deal of things BasicStamp can do, so here you see:
>
> I'm building a project were i want 216 leds to light up in a row,
> one after antoher, I don't really know how to explain this =). I
> want to make it look like the light is moving along the chain of
> leds in a loop. I also want to make them all blink at a set
interval
> (random if it is possible?) and maybe some other cool effects, but
> as said, i don't know the limits.
>
> So thanks to anybody who can teach me anything! Greatly appriciate
> it, thanks.



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