[HH] Controlling lots of LEDs with a microcontroller

[Tom Metro]

Drew Van Zandt wrote:
> The flexible PCBs are insanely expensive in small quantities.

That's if you farm it out, right?

What if you etch it yourself? That should be practical if you can get by
with a single sided board with SMD components. Can you get the flexible
substrate for a reasonable price?

If not, perhaps it can be hacked together using something like a thin
sheet of polypropylene (often sold as temporary cutting boards) with
some self-adhesive copper adhered to it.

Then there is the approach of drawing the circuits using conductive
paints. Instructables.com is full of projects for making flexible circuits.

Now that I look at the picture of what David wants to make, I see the
curve in question is actually a flat 2-D circle. Not what I expected. So
there's really no need for a flexible circuit. A simple flat, rigid PCB
will do.

This is going to pose some challenges to the legibility of your text, as
you are going to have varying dot density as you go from the inner to
the outer circles.

"That's roughly a 30" circumfrence." -that's a lot of LEDs!


David Kramer wrote:
> Since the background needs to be
> white, I was thinking of using foamcore boards
> http://en.wikipedia.org/wiki/Foamcore
> 
> ...I should be able to just push the LEDs through...

Do you want the unilluminated LEDs to be visible protruding from the
mask? I had assumed the LEDs would be mounted from behind and visible
through some sort of a window (smoked or mirrored).


> ...and bend the leads out which should keep the LEDs relatively
> straight.

Rather than pushing the leads through the foamcore, another option would
be to punch holes through it that are slightly smaller than the LED
bodies (assuming you use traditional round body LEDs), such that when
they are inserted they'll be flush with the outer surface.

In theory, if you could get your math and assembly tolerances good
enough, you could still use SMD LEDs mounted to a flexible circuit. But
good luck getting 100+ LEDs to line up with 100+ hand punched holes. A
little CNC help would be good.

The simplest approach I can think of is to make the outer circle
entirely from a single-sided PCB, with no traces on the side that the
LEDs mount on (and thus you'd use through-hole LEDs), and paint the
board white.


> It's been decades since I made a PCB.  While that would certainly be the
> neatest solution (though not the lightest), I'm definitely not up to
> making a PCB with hundreds of trace lines all going to the same general
> area.

It's not trivial, but it may be much easier than you think. You wouldn't
"hand tape" the board. You'd do the layout in software. You've then got
3 options to manufacturer it: 1. farm it out for professional
production; 2. find someone with a CNC router to do it; 3. make it
yourself by using one of many techniques, such as toner transfer from a
laser print to the board.

As for weight, use as thin substrate. Or hack something together like
substitute plastiboard (similar to foamcore, but waterproof) with
self-adhesive coper applied to it.


> I feel if I go this route instead of the mechanical rotating ring
> design, though, I'll end up with a much better product and more 
> experience in embedded software.

Actually I think the best option is to ignore everything above, and
combine the two designs: rotating electronic LED display.

http://www.youtube.com/watch?v=ML6ZrWKTWPo

It eliminates a whole pile of challenges mentioned above, like getting
hundreds of LEDs wired up and evenly spaced, and the dot density
problems I mentioned. Now you only have to deal with driving 8 or 10
LEDs. And making them RGB becomes far more practical.

You just need to figure out how to get them to spin at a few hundred RPM
without being too heavy (use a stepper motor from an old floppy drive?),
being a danger to yourself and those around you, how to synchronize your
message with the rotation, and how to get power and/or signals onto the
spinning mechanism.

 -Tom