Aligning Fiber optic strands to SMD LEDs

[PeterPan]

Here I go again with another hair brained self challenge folks.

I got a stack of 50 1MM thick fiber-optic strands, 2 meters long, from an ebay sale. (if you want the link, let me know... they are still available for about $10 / bundle). 1MM is pretty thick and stiff, but it cuts pretty easily and transmits  light from little SMD LEDs pretty nicely. Here I just stuck a 6" piece onto an SMD LED with a drop of glue, for the picture.




Anyway, what I want to do would look very cool, but may not be practical, so I'm hoping to hear some tips from anyone who has actually done something like this before. Either you'll spare me the nightmare, or tell me how to make it work. :-)

For small small devices where a bigger 3MM LED would be just too much, I'm hoping to use fibers to simulate little 1MM LEDs. I could make a template so I could drill 1MM holes that are well aligned with the PCB mounted underneath, where some reasonably sized (805 size) surface mount LEDs will be. Then, carefully cut short equal lengths (maybe 1/2" at most) of these fibers, smooth the ends, and place them through the holes in the case.

I think the idea would work in a factory with precision tools. But truth be told, I don't think much of my chances of aligning things that well. I'm hoping the large 805 LED sizes I'm using will give me some leeway.

I also thought of maybe gluing my pieces of fiber to the SMD LEDs with a drop of clear flexible shoe-goo or something. That way they would stay exactly aligned at the center of each LED for maximum and consistent brightness. But being the pessimist, I'm sure even if they held well, assembling the PCB to the case so all the fibers lined up with the enclosure holes would be a nightmare.

So anyone else ever try this craziness?

I know... I know... they already make LEDs with built in light pipes. But they cost a fortune by comparison, and I've not found any 1MM ones I could just cut to size.

[Tony Forestiere]

My stupid idea. Mount the LED on the bottom of the PCB with a 1mm hole/anchor/reference point. Glue your light pipe there.

[PeterPan]

 I like it! Hell... that would mean I can get the board house to do that initial alignment for me!

The issue though is that SMD LEDs (at least the ones I've found) would be pretty tough to mount upside down, because they have little blocks of some clear material in their centers to protect their sensitive innards. So what I'm saying is, upside down they wouldn't be flush with the board for soldering. Maybe somebody makes SMD LEds that are clear and shine in both directions?

[bluebunny]

If you drill a hole in the board to accept the "bump" of the SMD LED, then it should sit flush when upside-down.  You could then glue the fibre into the same hole (from the other side)?  The gluing doesn't have to be too precise.  It's just a light-pipe, not GHz telecoms!

[PeterPan]

It sounds like a good idea, but probably not for these particular LEDs.



These are 805 size LEDs, and the "bump" that lights up  is a rectangle, about 1.2mm x 1.4mm.  I can't drill a square hole, so , and those dimensions work out to be 2.1mm corner to corner. That's how big a round hole would have to be to let the bump pass through. Since the length of the whole base of the LED (including the solder tabs) is 2.0mm, that scheme wouldn't work for those LEDs.

I'll write to the manufacturer to see if they have something better suited to what you're describing, but they haven't been very responsive in the past. If you know of some LEDs that would work better, let me know. They MUST be low power (2mA) though, so that's a limitation too.

[duck_arse]

I think you're looking for leds in the "reverse mount" package. they are meant to look through the board, some w/ square holes, but there is one series at "american opto plus", L-955-RM-CX, that has gull-wings and doesn't care the shape of hole.

[PeterPan]

Thanks! That particular one required too much current for my little MCU to drive.  But there's another problem with this method. I've poked around a while now and I think I've looked t every manufacturer's offerings. At this point, don't thank anyone makes what I'd need to do this without any mods. At least not with the "reverse mount LED. approach

See, none of these reverse mount SMD LEDs are going to be flat, and even if they have round lens areas that protrude through a drilled hole, that would kind of make it impossible for me to use that hole to push in my light fiber. If someone made a reverse mount LED with NO protruding lens area at all, that might work. But nobody seems to make that.

Another idea... in my case I'm going to need a row of 8 such LEDs anyway. So maybe I could have the board house make me some thin strips of PC board with the 1MM holes drilled to line up exactly with the LEDs on my main board. Then I guess I could just glue that whole strip down over the ordinary SMD Leds, or even better, mount the strip properly with some stand-offs. Its an extra step, but I'm thinking it might be more do-able than finding some  "custom made" specialty Led.

Another possibility I guess would be to mount the regular SMD Leds I already have upside down  behind holes in the main PCB. I would just need to come up with a way to bridge the gap between the copper lands and the LED. Maybe solder bridges?

I dunno. frustrating.  Seems I always end up getting these brainless ideas to add on more features and do things that seem simple, but in the end lead me down a bunny trail in search of parts that don't exist. 

[greaser_au]

I got a stack of 50 1MM thick fiber-optic strands, 2 meters long, from an ebay sale. (if you want the link, let me know... they are still available for about $10 / bundle).

May I have the linky, please

Awhile back I bought some acrylic rod & sheet to mess with light plates.  the idea was that the status led would be replaced with a 3mm rod glued into the light plate.
Another one was IR LEDs & PTs to make 'large separation' opto-isolators for a solar inverter with 6mm rod (and a ball-end milling bit to inset them slightly)...

The light pipes used in things like network switches are pretty wild - cheap if you're into injection moulding

david

[PeterPan]

May I have the linky, please Smiley

Here  it is... looks like its still active.

http://www.ebay.com/itm/261157146489

[greaser_au]

^^   Thank You!!!!

david

[tubegeek]

If the LED holes in the surface are arranged conveniently, this is what I would do - drill your 1mm holes in the case and stuff short fiber pieces into them. Array the LEDs on their own PCB, on top of the PCB. Use flying leads to this LED-only PCB from the main board. Then glue the LED PCB under the top of the case, up against the fiber rods. Trim & smooth the fibers flush with the outside of the top and make sure the glue below is holding them in securely.

If the LEDs are spread all over the case that won't be convenient, but if they are in a nice little line close together, it could work - you'd just have to drill the case accurately enough to hit the LEDs on the PCB below.