Anybody has experience with SMD RGB LEDs? - Page 1
Forum section: electronics / projects

author: laxamar
author_rank: Contributor
timestamp: August 24, 2022, 07:59:28 pm
content: I am going to show a color through a pinhole on a PCB with an SMD RGB LED.It's obviously easier to test on a through hole design, but the final result needs to be manufactured with SMD parts, and that's expensive and time consuming.I have a couple of choices - clear lens or diffused. On through hole, diffused is better at mixing colors. I don't know what will happen on an SMD, specially if viewed on a pinhole.Part number of the SMD LED I'm considering: APFA3011-AK13-Fhttps://www.kingbrightusa.com/images/catalog/SPEC/APFA3011-AK13-F.pdfBut there are diffused version available.Here's what it looks like with non SMD parts:ClearDiffusedAny experience or suggestions?===P.S. It's hard to photograph, but clear does not mix colors well when viewed from above where each LED is separate in this version

author: ataradov
author_rank: Super Contributor
timestamp: August 24, 2022, 08:40:07 pm
content: Use diffused ones. Apart from poor color mixing, clear LEDs also have weird directionality to them. Some viewing angles make it impossible to see if LED is on or off.Clear LEDs work better if used with a light pipe that sufficiently mixes the color. When viewed directly diffused ones would always be better.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: August 24, 2022, 09:04:38 pm
content: You can also mix colours in a 'light pipe'.It is simply an opaque [or clearish] plastic tube of any diameter [or even shape]; at one end the light source or sources, at the other end, the observer.A light pipe is sometimes called a 'color mixer', when used in old fashioned photographic enlargers. A good light pipe guarantees the observer sees a blend of colours without seeing hot spots of single colours. It's used all the time in product design, bending light inside a product from the LED source to somewhere on the front panel. Chances are, you're already looking at a light pipe somewhere in front of you.You could possibly 3D print a prototype.https://www.fictiv.com/articles/learn-by-example-how-to-design-light-pipes

author: laxamar
author_rank: Contributor
timestamp: August 25, 2022, 01:49:55 am
content: Completely accidentally, the RGB light is going to have a (very lightly) opaque resin bar above the pinhole. Was not thinking of the color mix, but to "spread it" across the whole width of the device. The light is smack in the middle of the device.The transparent part is going to be T shaped, with the downward bar of the T VERY short (inserted and covering the pinhole). Will that short pinhole cover act as a "light pipe"? Will it spread to the rest of the bar like I hope.If this works as a light pipe, I can continue using the part I had chosen and sent to manufacturing.I need the PCB to be manufactured while I learn enough 3D design skills to draw the box and print it on my 3D printer. It's one area I realize I completely suck at. I completely see the value of industrial designers and wish I could afford one.

author: ataradov
author_rank: Super Contributor
timestamp: August 25, 2022, 02:07:29 am
content: Technically any piece of transparent plastic is a light pipe. But the actual behaviour depends on the material properties of the plastic.I assume you would be prototyping things anyway and not order mass quantities at first. Just order both types of LEDs and see how it looks and which one you like more.

author: laxamar
author_rank: Contributor
timestamp: August 25, 2022, 05:28:53 am
content: Quote from: ataradov on August 25, 2022, 02:07:29 amNot as easy and cheap on SMD PCB cards (my device has 5 interconnecting boards). Last batch took a nice 6 months (parts missing, design issues, pandemic and shipping to China) that has created PTSD that only magic mushroom might heal.First test devices will be housed in 3D printed cases and whatever version 3 of the PCB design will yield. AT least 3 Logic bugs have been identified and fixed. I am hesitant to change parts as sourcing things was a nightmare.I have to stop myself from trying to be perfect (so hard!) I'll make it work first, then worry about visual design as it's not my forte.I used a crapload of different resistor values in my breadboard to get the colors I have now.

author: ajb
author_rank: Super Contributor
timestamp: August 25, 2022, 04:53:35 pm
content: There's a difference between a lightpipe, a lens, and a diffuser, although some devices perform more than one of those functions at the same time and the terminology often gets confused.A lightpipe conveys light from one location to another. To do that effectively it must be transparent (certainly NOT opaque!) and must be designed fortotal internal reflection, so that the light does not escape the structure except at the end where it's supposed to. This is the same basic principle as an optical fiber, and has the same limitations in acceptance angle: a lightpipe will only be able to effectively accept light that enters from within a limited range of angles, and light that enters from outside of that range will not be totally internally reflected, or might not be accepted at all--so you may, in fact, want to use an LED with a narrower viewing angle to more effectively couple to the lightpipe.A lens changes the focus of a light source, so that the emitted light is distributed over a larger or smaller solid angle.A diffuser scatters incident light. It's usuallytranslucent, meaning that light will move through it but will be scattered along the way. This softens point sources and can provide a larger visible source area for the light.Getting the desired illumination effect in terms of apparent source location, luminosity (luminous flux per unit surface area) and luminous intensity (luminous flux per unit solid angle) often requires a combination of these. Off-the-shelf lightpipes often have integrated lenses or diffusion elements for these reasons.Quote from: laxamar on August 25, 2022, 01:49:55 amHow well this all works depends on a few key factors: geometry of the T-shaped piece, the optical properties of the material, and the surface conditions of the piece.- If the geometry of the part does not support total internal reflection, then it will not effectively convey light to the volume, and therefore may not produce an even level of illumination at the viewing surface. It may also 'leak' light in other places (most off-the-shelf lightpipes leak in some places, for example at mechanical mounting features, but are designed to minimize this effect). Sharp angles, like you'd presumably have at the point where the vertical part of the T joins the horizontal bar, tend to be a problem for TIR, so that part would have to be adjusted to some sort of transition shape.- If the material is not sufficiently transparent through its internal volume then you will lose intensity, either because the material itself absorbs some of the light traveling within it (especially a problem with lightpipes where the optical path length can be relatively long relative to the part geometry) or because the light is internally diffused. Internal diffusion causes the light inside the object to ends up scattering the angle of internal light rays such that more of them will meet the object's surface outside of the critical angle and will be emitted instead of reflected, again making the lightpipe leakier.- Because total internal reflection is controlled by conditions at the interface of two materials, the way the T-shaped piece is integrated into the enclosure matters. If you were to, for example, cast the piece in-place by injecting resin into a void in the 3D-printed enclosure, that will tend tofrustrateany internal reflections and cause terrible performance. That's because the resin will be in direct contact with the plastic, and since the two materials will have a similar refractive index (certainly closer to each other than to air), most of the light that strikes the internal surface of the resin will end up being transmitted into the plastic instead of reflected back into the resin. The best way to avoid this would be to cast the resin piece separately and install it mechanically into the housing. (Even adhesives on the surface of a lightpipe can cause FTIR, so while gluing is an option, it should be done sparingly and at strategic locations where it's unlikely to cause an issue.)Really, bottom line is: light pipes are tricky. If you want a sort of oblong indicator element on the surface of your box, an easier solution would be a thin diffusion layer at the surface, and a hole under that entire surface (or as much as you can manage mechanically, anyway) all the way down to the LED with just air in between. That way the LED is basically projecting onto the whole rear surface of the diffuser, so you'd just want to pick an LED with a wide enough viewing angle to cover the whole surface. You could also extend the edges of the hole down toward the LED, sort of creating a tube from the PCB surface up to the indicator surface, to ensure that light is prevented from going anywhere else. If you can make the inside of that tube white or silver you'll gain some additional diffusion and improved efficiency.

author: laxamar
author_rank: Contributor
timestamp: August 26, 2022, 03:30:37 am
content: Thank you so much for such precise and helpful info.Since I am designing and printing the translucent pinhole cover on a resin 3D printer, I have full control of the shape. I am inclined to make the top bar more like a house roof with a very slight incline that *might* diffuse and transfer the light signal throughout. This is one thing I can try a zillion different ways without too much of a cost, just wasted time waiting for the print/dry. It will probably be very easy and fast to see if it's worth the trouble by changing the angles. Or come to the conclusion to just live with a single point of light.Since this all started with a single red LED - went through a 7 segment display - and ended up on a color coded RGB, I think I should stop the feature creep (Famous last words)I was ->this close<- to go to a small SPI OLED display ...

author: ataradov
author_rank: Super Contributor
timestamp: August 26, 2022, 03:34:04 am
content: There is no way 3D printer would be able to make a real light pipe with total internal reflection. It will always be diffused, so any LED would work here. Non diffused ones may be better, since diffusion would be provided by the resin.

author: Buriedcode
author_rank: Super Contributor
timestamp: August 26, 2022, 08:12:58 pm
content: I've prototyped light guides for backlights, and HMI panels and this pdf was helpful:http://www-eng.lbl.gov/~shuman/XENON/REFERENCES&OTHER_MISC/Lightpipe%20design.pdfDeffused RGB LEDs indeed to mix rather well, but at the expense that it shoots the light in all directions - and even then the small volume of even the 5mm LED means there are still hotspots from each die, because they aren't all in exactly the same place.A longer light pipe allows the colours to mix better, then a diffuser layer on the output of the pipe should even up the hotspots. Having the entire pipe being "diffusing", as in opal or milky acrylic will scatter and absorb so much light that even a small light guide would have to be very short/small otherwise it'll just be too dim.What appearance are you after? I mean, what is the application, front panel point sources? planar backlighting?SMD LED's are somewhat better because they're generally much smaller, so the distance between the different dies is shorter. But products/systems almost always used custom polycarbonate/acrylic molds to route the light to panels, and by their nature, also mix the different colours.

author: laxamar
author_rank: Contributor
timestamp: August 26, 2022, 09:20:33 pm
content: Quote from: Buriedcode on August 26, 2022, 08:12:58 pmI have a series of boxes that can connect to the next just like Lego bricks. (using connectors of course). Each box determines what number it is. I wanted to light up the correct box to bring attention to it. Started with a single LED, then 7 seg counter to convey the box number (and verify they are recognized correctly) , then had the genius idea of a single RGB that light up based on the box number (BCD decimal input). there are a bunch of other info on the box, but the box number/info , was thinking a translucent bar across.So short answer, the RGB is to identify the box that should be looked at and display on control screen a matching color.Honestly would have worked with a single red LED

author: Buriedcode
author_rank: Super Contributor
timestamp: August 27, 2022, 09:47:59 pm
content: Quote from: laxamar on August 26, 2022, 09:20:33 pmGotcha, kind of like this?https://www.digikey.co.uk/en/products/detail/kingbright/KB2550SGD/1747584but rather than a PCB mount unit of single colour, a way to use your RGB to illuminate a bar?Or for just a lightguide:https://www.etsy.com/uk/listing/155006523/l-e-d-light-pipe-oval-shape-3-wide(I would would sand that emission surface with 800 grit for more even colour - that pic looks too glaring).It all depends on how many you need, whether you're willing to get your hands dirty cutting and filing acrylic, or if you just want an off the shelf thing. One very cheap and surprisingly easy/good way is to use hotmelt glue to glue the LED to the back of a panel cutout to the desired shape, then cut the cooled glue flush with the surface. Not exactly professional but it compeltely diffuses and mixes light well.Another way would be wedge shaped acrylic, with the LED glued into one edge, and another edge sandblasted/sanded for a frosted surface. The light bounces around all the different angled surfaces until it hits the sanded part, where it escapes. Like this:https://megaled.com/wp-content/uploads/2015/02/figure26.jpgQuote from: laxamar on August 26, 2022, 09:20:33 pmI too sometimes over-do things, but whilst a single colour LED is simple, its also boring and mean one doesn't start thinking about different methods, and sometimes its all worth it and you create something beautiful rather than plain

author: exmadscientist
author_rank: Frequent Contributor
timestamp: August 27, 2022, 11:16:24 pm
content: Don't forget about diffusing sheets. Those things are freakingmagic, I've seen them turn some laughably-terrible optical designs into something that almost works. We had the best luck with 3M sheets from Digi-Key (though that may have been luck of the sampler pack draw).

author: laxamar
author_rank: Contributor
timestamp: August 27, 2022, 11:19:40 pm
content: The circuit design is finished and a first test version is manufactured.. The box that will cover it is in design.The RGB is an upward facing light that will touch the box and have a pinhole to show through. Not much room to work it. So just a very thin layer of 3D printed clear resin.Here's the actual boards in action (the RGB doesn't work, cause I messed up the resistor values)I want the RGB light to be spread across the length of the underlying PCB board (which will be the width of the case). The other LEDs will be show from a side wall.God this is hard to explain.

author: Buriedcode
author_rank: Super Contributor
timestamp: August 30, 2022, 11:01:47 pm
content: Edit: I'm a moron and didn't realise that you'd already chosen and mounted the RGB LED.I'm not seeing the RGB LED or where it would go, unless its that SMD right angle one facing upwards?If you've got a single 5mm or SMD LED to try and light up the width of that board I doublt you'll get light to spread across that far.. 30cm? or is it just the width of that part of the board sticking up? 10mm?If its just that tiny RGB LED (which will have a nice wide angle) then a small diffused lens should be more than enough. The wider angle helps colour mixing in light guides and means the light guide can be smaller/shorter becuase it doesn't have to have the length to effectively mix all the colours with total reflection. If its just that LED and a small hole in the case, something as milky as hot melt glue would be more than diffuse enough to get a nice even colour/brightness and it can be small enough that it doesn't absorb too much light.Small light pipes like these:https://www.mouser.co.uk/ProductDetail/Bivar/VLP-650-R?qs=2oh8XZODtldVtbzSG%252B6vgg%3D%3D&mgh=1&vip=1&gclid=EAIaIQobChMI-LWlnNXv-QIVj-3tCh3TfQYgEAQYASABEgK00fD_BwEHave a nice roughened emission surface

author: laxamar
author_rank: Contributor
timestamp: August 31, 2022, 09:48:13 pm
content: Quote from: Buriedcode on August 30, 2022, 11:01:47 pmNo, the moron is the idiot (let's call him me) that changed the design from a single LED to RGB and put a transistor to control ON/OFF from the emitter and did not change the resistor for the base in the design he sent to manufacturing. Very pretty and useless PCB.Thankfully, this moron had 2 other major flaws, so not feeling as guilty about that as he has to fix those and re-manufacture. If you'd like to make fun of me, I'll share the details of those 2 other breadboard to PCB errors ...