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For a model train layout, I am looking to route a light to a difficult part of the layout - even the 1.6mm LEDs are too big for the area. It is for display, not data communications.

One person has solved the problem on their layout using optical fibre, which seems quite sensible. Rather ingeniously, the light is transferred from an LED the fibre via thimble with a hole drilled in the top.

The fibre does not need to be connected/disconnected from the PCB - a permanent connection is fine.

I am aware of solutions for POF in ethernet, but this is far beyond what I need. I can't be the first one to have this problem since fibre optic trees are cheap and plentiful.

So, some questions:

  • If I have a surface mount LED (or an array thereof), how do I attach an optical fibre to transmit the light for this purpose? I assume there is some sort of plastic mounting I can purchase off-the-shelf.
  • What is the attachment called (i.e. does it have a standard name)?
  • How are they assembled? (e.g. manual, pick and place?)
  • What considerations are there in selecting the type of fibre to use? (it will be no longer than 1m)

(Note to moderators, "optical-fibre" would be a good new tag for this question)

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I've added "light-guide" tag, hope that's fine with you. – Vladimir Cravero Jun 8 '14 at 7:22
As for the tags, optical-fibre, fibre optics, fiber optics, light-guide, etc, all have problems. Too few questions to warrant one, too many spellings and variations to agree on a single one. – Passerby Jun 8 '14 at 8:25
You could drill a hole in the end of a 3mm or 5mm through-hole LED and glue the fiber into the LED with clear epoxy. Don't drill (quite) down to the bonding wire, which arcs above the die itself. It's also possible to solder wires to an 0402 LED, which are about 1.0 x 0.5 or 0.6 mm. – Spehro Pefhany Jun 8 '14 at 11:03
'Fibre optic trees' actually usually use a single bulb held beneath a bunch of fibre, from what I've seen. – Ollie Ford Jun 8 '14 at 18:12
up vote 12 down vote accepted

Fiber Optics, as a light source, and not a data transmission platform, is a "shoe-fits" type application. Fiber optic trees use heat shrink or electrical tape or tightly molded plastic as the method of optical coupling.
enter image description hereenter image description here

Data Transmission Optical Fiber Cable on the other hand, has closely designed standardized connectors, such as TOSLINK for consumer audio connections or SFP (small form-factor pluggable) connector used in some Cisco router Fiber Optic connections.

Using a 5mm or 3mm led with matching heat shrink tubing and enough fiber to fit is very very common in hobbyist modeling applications (which a well done scale model requires much of to look good (Model Enterprise Project)). If you need just a single thin strand, you could drill a hole in a led and hotglue the strand in (Model Train Project).
enter image description here

Fiber Optics are also used for Star Maps or Star Ceilings (The ends are mushroomed out using a lighter or other heat source, then hot glued in place).
enter image description here

Commercial applications use Light Pipes made of molded or extruded plastic, to channel a pcb mounted led (almost always smd) to an external case opening. I can't think of a single commercial/consumer/industrial product that would use fiber optic strands as internal light redirecting method, due to how unwieldily that would be in manufacturing, let alone technical support issues as connections are fragile due to the thinness.
enter image description hereenter image description here

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"SFP" is a type of E/O transceiver module. The connectors used to connect the fibers to an SFP module are called LC connectors. – The Photon Jun 8 '14 at 14:55

I have had fair success using individual plastic fibers from an inexpensive decorative lamp like this one to create headlights for N-scale model railroad vehicles: enter image description here.

For coupling, making a clean flat cut on the LED side of each fiber using a surgical knife, and then heat-shrinking or hot-gluing the fibers tightly to the LED, works well. At the other end of each fiber, as described by Passerby, create a small lens-like melted blob using a heat source.

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When you're talking about fibres you're usually talking about high speed transmission of data. What you are looking for is a light guide. They come in all shapes and can be bent easily. I would guess you're looking for something similar to this: http://www.farnell.com/datasheets/1739879.pdf

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More "light pipe assemblies" like the one above can be found here. – Damien Jun 8 '14 at 6:38
Light Pipe is the best industry term for this, though it refers to solid molded plastic and not flexible optical cable. – Passerby Jun 8 '14 at 8:00

If the LED was flat, I'd use a length of 1/16" diameter extruded acrylic rod for a light pipe, polish one end of it flat, and secure that end to the "sweet spot" on the face of the LED with superglue.

Once the glue set I'd clean off its uncured remains and, for more structural support, run a fillet of epoxy around the end of the rod where it contacts the LED.

If the LED was domed, I'd polish a flat a little larger than the diameter of the rod at its very peak and then superglue and epoxy the light pipe to it.

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