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Author Topic: LED intensity and enclosures  (Read 1795 times)
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Austin, TEXAS
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I have a question about LEDs and luminous intensity.  I am developing a hand-held enclosure that will have 8 LEDs.  They indicate system status type of stuff.  They are 0805 SMT LEDs and will illuminate through 8 light tubes respectively, through the front face of the enclosure.  I am down to the nitty-gritty of deciding exactly which LEDs to send to the board manufacturer.  The basic question I have is simply this: how is this usually done?  A couple of quick factoids:

I want to have different colors for different sections of the LEDs.  My current thought is to simply utilize white LEDs coupled with those colored but clear plastic inserts that I've seen somewhere on the web (don't have the link handy).  In this way the parts list for the board is simplified, I get the color difference that I want and I can change those colors simply by changing the plastic insert(s) (which is much simpler than changing the LED!).  So is this a good approach?

That leads me to the biggest question that I have about these LEDs.  And that is the intensity of the light that the LED emits (mcd).  From what I can gather, normal "indicator LEDs" are about 40 - 50 mcd.  My concern is in regard to the light needing to pass through the plastic insert.  Seems like it should be at least a little brighter than normal somehow.  So I found some white LEDs, but they are 220 mcd, which seems a bit too bright:

http://www.mouser.com/ProductDetail/Harvatek-International/HT-170TW-5510/?qs=sGAEpiMZZMseGfSY3csMkfueAjHpt4BBw6wzZSaP1uA%3d

So that's the question/issue.  Anyone have experience with this/thoughts on it?

I would much appreciate any feedback!
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If inserts seem easier to you and offer the flexibility at your price point then that will work.

Brightness can be compensated for ("attenuated") by using less current.
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If inserts seem easier to you and offer the flexibility at your price point then that will work.

Hmmmm....well what other ways are commonly used then?  In this case, there are tubes that will shield the LEDs from interfering with each other.  Those tubes will extend from the LED pcb to the backside of the front face of the enclosure.  On that front face will just be an open "hole" if you will, so it seems like you'd need to put some kind of insert there or else expose your LED/pcb to the elements.  I suppose there are other ways to shield it, but not sure.  This is my first attempt at a custom enclosure with such features.

Brightness can be compensated for ("attenuated") by using less current.

Oh certainly.  At present I am using 1K current limiting resistors for the LEDs.  Obviously that can be changed, but that would require another back-n-forth with the assembly folk, etc.  I was/am trying to get the combination as close to being "right" or "intensity combination suitable" on the first attempt.  Thus the post to see if anyone had done something like this and could share their experiences.

Thanks for the feedback!
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Hmmmm....well what other ways are commonly used then?  In this case, there are tubes that will shield the LEDs from interfering with each other.  Those tubes will extend from the LED pcb to the backside of the front face of the enclosure.  On that front face will just be an open "hole" if you will, so it seems like you'd need to put some kind of insert there or else expose your LED/pcb to the elements.
I thought you meant "light pipes".

The proper approach is to use a "light pipe" which is a moulding of clear acrylic which forms a solid, smooth, cylindrical rod - not tube - which conveys the light by total internal reflection, like fibre optics.  The end which mates to the LED is flat and goes as near as possible to touching it.  Where you have a number of these in a row, they can have a common "manifold" piece for mounting at the back of the panel.
« Last Edit: October 03, 2013, 10:24:02 pm by Paul__B » Logged

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Interesting, good information.  One question though: are these light "pipes" solid or hollow?  Have any pictures of what this looks like?

ps-Ok I did a Google search of light pipes/tubes and saw several examples.
« Last Edit: October 03, 2013, 09:51:34 pm by av8or1 » Logged

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In thinking about the two responses, perhaps I haven't explained things all that well.  I'll try again.

Whether the implementation of the system status LEDs is done by light pipes or not, in the end I don't really care how I achieve it, so long as the system state LEDs resemble the style of LEDs that you see on the front cover of a CD drive.  Everyone is familiar with those.  But this is what I had in mind when I designed the enclosure.  Now.  As I mentioned, I am open to how that is achieved technically.  But it seems to me that if the PCB is simply mounted very closely to the back side of the front face of the enclosure and a small plastic insert is placed into a hole to keep the LED covered, then we're good.  This is actually what I had in mind from the beginning and what I was trying to describe in the previous posts.  I just didn't know what the heck a light pipe was and assumed that it was necessary.  It seems like that's not so with this type of implementation, but I'm open to hearing from those with more experience.

I've attached a couple of pictures of CD drives that illustrate what I am talking about.  These lights seem to just be LEDs with a plastic cover that keeps stuff from getting inside and contaminating things.  I'll want 8 of these total, in 2 rows of 4 with silkscreen labels.

Thanks


* cd drive led I.jpg (105.73 KB, 1200x705 - viewed 25 times.)

* cd drive led II.jpg (126.11 KB, 1200x798 - viewed 30 times.)
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The little plastic bezel on CD drives - or any other equipment - is a light pipe - just a rather shallow one.

They are solid, and when they are longer, total internal reflection occurs so that the light is conveyed efficiently.  This is much better than a tubular structure.
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The little plastic bezel on CD drives - or any other equipment - is a light pipe - just a rather shallow one.

They are solid, and when they are longer, total internal reflection occurs so that the light is conveyed efficiently.  This is much better than a tubular structure.


Yep. Like fiber optics.

Works like this -

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ChicagoDave-

Ok thank you for the illustration.  I've learned a lot about light pipes during the past week.  Interesting that they are used in the implementation of a CD drive light.  I've wanted to find an old one to tear into but no luck thus far.  The other half doesn't like me doing that, thinks we might use it again someday. ;-/

Anyway, about these "inserts" as I have referred to them.  Thus far, the way I see this happening is that the enclosure will have - in essence - either a rectangular or oval "hole" in the front cover and the light pipe will extend up to the bottom of that "hole" in the enclosure.  Then what?  Shouldn't there be an "insert" or a "lens" that is press-fitted into that hole to keep contaminants out?  Kinda like a lens on your turn signal light on your car?  Only press-fitted into place...

And where could I find such "inserts" or "lenses"?  I've done a fairly good search but haven't quite found what I'm referring to ... could be because they don't exist, not sure.

Thanks!
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For intensity control, you could put in trim pots so you dial back the intensity on your initial units, then measure what is being used and pick similar values for later builds.

The inserts/lenses/bezels are likely custom for each design, so you are not likely to find them off the shelf.
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Shouldn't there be an "insert" or a "lens" that is press-fitted into that hole to keep contaminants out?
That is the whole point.  The light pipe is the insert or "lens" or "bezel"; it extends to the face of the panel and it is solid.

There is no reason you cannot make them out of acrylic rod (clear or if you must, coloured) of the appropriate diameter (or square section), cut to length and the surfaces polished (using emery paper and the car paintwork "cutting" compound).

Yes, I have done it.
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That is the whole point.  The light pipe is the insert or "lens" or "bezel"; it extends to the face of the panel and it is solid.

There is no reason you cannot make them out of acrylic rod (clear or if you must, coloured) of the appropriate diameter (or square section), cut to length and the surfaces polished (using emery paper and the car paintwork "cutting" compound).

Yes, I have done it.

Ok so it sounds like we're talking about manufacturing a custom light pipe for this particular application.  I was trying to avoid that and use something off the shelf.  Also, I was told that light pipes are not colored, though it sounds like I was misinformed.  The custom enclosure guy I'm working with said that he had cylindrical light pipes in mind and that they would be affixed to the backside of the front face of the enclosure, which would leave a "hole" in that front face of the enclosure, which is something I'd prefer not to have.  Just has an unfinished look.

I'll keep digging.  Thanks for the feedback.
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There's thousands of options, and lots of colors, available for off the shelf light pipes:

http://www.digikey.com/product-search/en/optoelectronics/optics-leds-light-pipes/524541?k=light%20pipe
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Thanks Cross-

Unfortunately most of those won't fit my needs for various reasons.  I did find a couple on Mouser that might work however.  Trying to get more information on them and pass it on to the custom enclosure guy.
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I think you'd be better off designing a PCB with the LEDs mounted in the appropriate position and then attach that to the back of your faceplate.

As an aside, CD/DVD drives don't implement light pipes, or I've never torn apart one that did (and I've torn apart quite a few). The standard is to mount the LEDs, volume control, headphone jack, and eject button on a single-sided PCB which is then connected back to the main circuit board with a multiconductor cable.

Shows a pic of what I would consider typical:
http://www.instructables.com/id/Headphone-amplifier/#step1
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