LED driver test board (giant pads question) - Page 1
Forum section: electronics / projects

author: sparkydog
author_rank: Regular Contributor
timestamp: February 20, 2024, 10:44:23 pm
content: Eventually I want to make my own LED seven-segment display. (Why? Because I can't find one in a size I like; my current design is for 1.2" — which only exists in red; I want other colors — but could easily be made just about any arbitrary size larger.) Meanwhile, each segment has a variable number of LEDs (depending on segment size), and I'm trying to figure out how to drive them, while also dealing with questions about brightness uniformity.To facilitate these experiments (and also for solder practice, because... I've never soldered before), I made this tiny board that I felt like sharing:Note thateitherthe front lower drivers will be populatedorthe driver (and ancillary components C1 and R1) on the back; never both. (I'll probably always populate C2.) The goal was to design a board that could support multiple approaches depending on what components are populated, and to allow different target currents to be tested. There are three options for driving the LEDs:Resistors. (Boring, not optimal, but simplest and cheapest.)CCRs. (Layout is dead simple, a bit pricey, limited options for current target.)"Proper" 4-channel drivers.Now, (3) is the only one that lets me fine-tune the LED current, and in general I would have expected it to be superior, but using them with theactualboard layout is a mess nearing on "disaster", and by the time the extra components they need are counted, the price is just about a wash. Therefore, for my real application, I suspect I'm going to use the CCR-per-LED approach, but I still wanted to play around with each one.Besides the usual call for comments/critiques,I do have one question:CCRs are SOD-123 components which have tiny, tiny leads and normally would have correspondingly small pads. However, because I wanted the option to use resistors instead in the same footprint, I'm using 3216 footprints. (The SOD-123 pads fit entirely within the larger 3216 pads, but are very near the outside thereof.) Is this going to be a problem for hand-soldering SOD-123 components?

author: cosmicray
author_rank: Frequent Contributor
timestamp: February 21, 2024, 12:30:59 am
content: Quote from: sparkydog on February 20, 2024, 10:44:23 pmYes, I am also aware of that desire, and have similar interests. Be aware that what we think of asseven segment displayswere in older times produced in variants up to 17 segments.QuoteYour desire is both to fine tune the LED brightness and the current, they both go hand-in-hand. You can also consider the possibility of using a PWM to generate an appearance of varying the brightness, but that also gets involved with an LED devices that might have a phosphor coating on the lens (generally white and some UV devices) and any persistence the phosphor may have. Nothing wrong with resistors, provided you know in advance what current you want. You can also look at something like a 2-terminal use of a LM317L (up to 100 ma) and use a resistor there to control the feedback to the adjust pin, and use it as a current limiter device. One LM317L can actually multiple LEDs to limit the current (series, parallel, or series-parallel). For a really large rendition of a seven-segment display, you might have multiple LEDs per segment.

author: sparkydog
author_rank: Regular Contributor
timestamp: February 21, 2024, 06:35:48 pm
content: Quote from: cosmicray on February 21, 2024, 12:30:59 amOh, I already have a board design for the digits. Two, actually, which is how I know the CCR version is stupidly simpler than the 4-channel driver version."Older times"? 14-segment (not counting decimal points) are still very common, especially in larger sizes... and that's the problem. If I just wanted a larger display, I'd use an OLED panel. Once I do that, however, besides being more complicated to drive, it opens all sorts of questions like "what font should I use", and, worst, "what idle animation should I use"? The whole point of this exercise is to have a larger displaythat has exactly 27possible statesso that I'm forced to work within those confines. (Also, because a custom display gives me fine control over the aesthetics, and because it's fun. 🙂)Quote from: cosmicray on February 21, 2024, 12:30:59 amI'm hoping to avoid the complexity of PWM dimming, though in theory I could add that on top of the other approaches. The CAT4104 I have tentatively selected is actually designed to take a PWM enable signal (on this board, EN/PWM is just shorted to VIN), resistors ought to handle external PWM switching with no issues, and NSI-series CCRs are explicitly stated to handle up to 100kHz PWM switching.In principleI'd hope my final design can work with any color LED (CCRs would limit the current options to 10, 15 or 20 mA), but the one I'm planning to actually build will probably use LTW-150TKs, i.e. white. If anything, I'd think phosphor persistence would actuallyhelp, by smoothing out the flickering in much the way a capacitor is used to smooth out AC. I'd expect the worst issue might be that the perceived intensity with respect to PWM duty cycle won't be linear, but OTOH I think that's true regardless.Quote from: cosmicray on February 21, 2024, 12:30:59 am"Might"? My little 1.2" has 3-4 per segment 🙂, with the idea being to have more consistent illumination-per-area, as well as more consistent illumination (i.e. less vignetting). Alas, that means series driving with >3V LEDs is... complicated, as I have at best 12V supply. (I'm actually somewhat hoping to have everything run on 5V.) Yes, I could DC-DC boost it, but a) 5V will runoneLED nicely, and b) unless I have different supply voltages per segment, the 2-LED segment is going to have to dump more than twice the voltage as the 4-LED segments, which isn't ideal. Plus, keeping everything in parallel, while necessitating that each LED is independently driven, means I still havepartof a segment if one of the LEDs dies. (For that matter, while I certainlyplanon using diffusers, this design ought to look passable even without them. Who knows, maybe I'll even decide it looksbetterthat way!)It may turn out this is too much light, but I won't know until I actually see it in action. Naturally, I'm looking for fairly low output-per-LED, and if I end up underdriving them, so much the better. (Plus, if I can, I'm probably going to end up sticking something like a piece of smoked acrylic over the whole thing. Not energy-efficient, but the idea would be to make the segments much less noticeable when they aren't lit, which will look cool.)

author: tooki
author_rank: Super Contributor
timestamp: February 21, 2024, 09:21:31 pm
content: Quote from: sparkydog on February 20, 2024, 10:44:23 pmWhile I think making DIY 7-segment LED displays is fun, having done it myself, I will point out that large LED displays in all the usual colors (red, green, yellow, blue, white) are readily available on aliexpress.I’ve bought LED displays (or “digital tubes” as the Chinese call them, with digits referred to as “bits”) from the vendor below and was very pleased with both the product quality and the careful packaging that ensured not a single leg was bent. They have white displays up to 8” right now, but I think they’ve even had the 10” and 12” ones in white in the past.https://www.aliexpress.com/store/912680125

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 21, 2024, 09:31:12 pm
content: Large pads and small componentes will cause thombstoning or shifting (The pad drags the component).You'll have to solder everything by hand or use glue.Not a problem for testing stage or only a few DIY PCBs.

author: tooki
author_rank: Super Contributor
timestamp: February 21, 2024, 09:43:04 pm
content: Quote from: sparkydog on February 21, 2024, 06:35:48 pmWith how typical modern LEDs are, that may be way too much brightness with 4 LEDs!Whether you use 1 LED per segment or multiple ones, getting even illumination is a challenge. (With a poor diffuser, multiple LEDs simply gets you multiple hotspots. :/ )I did some experiments where I took some wood (bamboo plywood, actually) and milled slots through it to test making segments approximately 1/2” long. After cleaning off the sawdust very well, I covered the viewing side with Kapton tape, then from the other side, poured in clear epoxy adhesive into which I mixed a small amount of talcum powder. After the epoxy cured, the Kapton tape pulled off leaving a smooth-as-glass surface. The end result was really, really good. The only challenge was some air bubbles entrapped in the resin. Using a resin actually intended for casting, especially a less viscous one, would probably help, but the real solution of course is to use a vacuum to degas it before casting.

author: sparkydog
author_rank: Regular Contributor
timestamp: February 21, 2024, 10:53:58 pm
content: Quote from: tooki on February 21, 2024, 09:21:31 pmHmm, well, 0.8" is larger than I think I've seen before, but I'm still not seeing 1.2" time displays (four seven-segment digits plus a colon, preferably without decimals). Yes, I can assemble something out of single-digit displays...ifI can find a colon. And I suppose slap something over top of everything to hopefully hide the seams.I suppose the right economical constraints would also "encourage" me to go that route, but, as I said, part of the point of this exercise is that I get acustomdisplay with control the exact shapes and spacings, rather than having to settle with whatever is available "off the shelf".Quote from: tooki on February 21, 2024, 09:43:04 pmYes, part of the point of this experiment board is to see what sort of uniformity, or lack thereof, I actually have to cope with. (With one LED per segment, non-uniformity just means thesegmentsaren't uniform. If anything, multiple LEDs per segment — given sufficient diffusion — ought to help, because each segment gets the average of several LEDs.)Quote from: tooki on February 21, 2024, 09:43:04 pmRight now (not owning anything that can do CNC cutting) my plan is to 3D-print the segment outlines and, yes, fill them with resin/epoxy with something to help diffusion. Not all that different from what you did. Thanks for sharing!Quote from: tooki on February 21, 2024, 09:43:04 pmFWIW, everythingI'veheard (which, okay, is based almost entirely on Robert Tolone's videos) is that degassing can help, but thebetterapproach is toaddpressure; specifically, to let the resin harden in a pressure pot at 50-100 PSI. (I don't believe Robert ever degasses resin.Silicone, yes, but you also can't pressure-cure silicone because it's flexible.)

author: sparkydog
author_rank: Regular Contributor
timestamp: February 21, 2024, 11:06:50 pm
content: Quote from: DavidAlfa on February 21, 2024, 09:31:12 pmAh, yes, thanks for reminding me! I do wonder if the bent-under pins of the CCRs, plus that they're near the outer edge of the pads, might anyway have a somewhat mitigating influence on that tendency... but this particular board isveryDIY and I absolutely expect to hand-solder it, so hopefully it'll be okay. 🙂 The whole point of this jank, after all, is because I don't know if I want to use CCRs or resistors as current limiters, and I want to be able to build either one. In anything like "production", I'd either know, or would run separate designs with correct footprints.

author: tooki
author_rank: Super Contributor
timestamp: February 22, 2024, 07:09:41 am
content: Quote from: sparkydog on February 21, 2024, 10:53:58 pmDegassing is standard procedure for resin casting, not some experimental process. Look at the numerous channels of people doing resin casting for furniture, trinkets, etc. All the pros (and semi-pros) degas.

author: cosmicray
author_rank: Frequent Contributor
timestamp: February 22, 2024, 12:03:59 pm
content: Quote from: tooki on February 21, 2024, 09:43:04 pmIndeed. Part of this exercise, is can you do it, and the other part is is it readable. Beyond that there is thecan it be scaled up big ?I's like to do something smaller, somewhat like what OP is intending. But I want the design to belogically scalable. I want something that might scale up to one digit per 150x200mm PCB. Beyond that, the PCBs begin to become rather pricey (and I would need something more than design exploration to justify it).The LEDs I'm using are 2835 white 18.5v chips. Because of the higher voltage, I can get a finer control over the current adjustment. These are not as readily available (outside of direct industry buy/sell I suspect), but for one seller on eBay, who primarily handles damaged freight. Thanks to that seller (and his reasonable pricing) I now have enough to cary forth all my explorations and designs. If anyone wants his listing number, I will share it via DM.

author: sparkydog
author_rank: Regular Contributor
timestamp: February 22, 2024, 06:34:12 pm
content: Quote from: tooki on February 22, 2024, 07:09:41 amI never said it was "experimental" (in fact, I explicitly agreed it's better than nothing!), I saidmyexperience is that pressure-casting seems to be preferred.Casual DIYrecommends pressure-casting.So does Robert Tolone.So does the resin-manufacturer Alumilite.Now, pot life is a factor, and perhaps casting size, but just about every source I can find either prefers pressure-casting or says "it depends". Do you haveconcreteexamples of people saying that degassing is preferred over pressure-casting?

author: tooki
author_rank: Super Contributor
timestamp: February 23, 2024, 04:05:38 pm
content: Quote from: sparkydog on February 22, 2024, 06:34:12 pmMost sources I found (these are just a random sampling) don’t even mention pressure:https://www.epotek.com/docs/en/Related/Tech%20Tip%204%20Removing%20Bubbles%20from%20Epoxy.pdfhttps://www.masterbond.com/techtips/avoid-air-bubbles-mixed-epoxy-systemshttps://www.artmolds.com/pressure-pot-vacuum-chamberhttps://lvs.lacotech.com/system/resources/attachments/000/000/093/original/SMT-04-1013_revA1_%28App_06-14_Vacuum_Degassing_Epoxy_and_Silicone%29.pdf?1479854733https://www.alumilite.com/content/PDFs/alumilite-coporation-bartop-coatings-instructions.pdfYes, the same company. They seem to recommend one or the other depending on the product. And like many, they appear to recommend doing both when crystal clarity is desired:https://alumilite.freeforums.net/thread/407/clear-casting-resinAnecdotally, none of the casting channels I’ve ever watched have ever even mentioned pressure casting, perhaps because they often do large objects that would require a gargantuan pressure changer capable of holding items of furniture or vehicles. They all vacuum degas their resins, though. I’ve definitely heard repeatedly (on “how it’s made” shows and the like) of vacuum degassing being used in industrial use of resins, while I’ve never heard of pressure being used this way.It seems to me that the main downsides to vacuum degassing are the higher cost of the equipment, the risk of introducing new bubbles while pouring, and difficulty in using resins with very short pot life, since they’d harden while degassing. The advantage is that it can be used with all the types of resins, including soft ones like silicone rubber, and with items of any size, since you can cast in layers if need be. Pressure’s advantage seems to be lower cost, and no risk of introducing new bubbles. It can’t be used with soft resins because the bubbles will expand once pressure is released, and molds themselves have to be compatible with the pressure used.It seems like most of the info online concerns hobby resin casting, which of course may be a distorted perspective anyway.As an aside, look at this thing: they sell a KitchenAid stand mixer modified to mix under vacuum! 😂https://www.sprucescience.com/resources/vacuum-mixer-epoxy-casting/

author: sparkydog
author_rank: Regular Contributor
timestamp: February 23, 2024, 07:10:03 pm
content: Quote from: tooki on February 23, 2024, 04:05:38 pmThat's actually a good point. Again, I never said degassing isbad, just that the sourcesI'veencountered use pressure casting instead. My impression remains that pressure casting is preferred, but I never claimed degassing doesn't have it's uses. In particular, you're talking about a) castings that don't fit in any reasonable pressure chamber, and b) are definitelynotusing the sort of quick-set resins that Robert Tolone uses. Under those conditions, degassingabsolutely issuperior to doing nothing....but we were talking about seven-segment displays. 🙂 The sort of display with 1.2" or 2" digits probablywillfit in a very reasonably-sized pressure pot, especially if you're lucky enough to have the sort of horizontal pots that Robert uses. I may also have gotten a bit fixated on those and forgotten that people also use resin formuchlarger things.Quote from: tooki on February 23, 2024, 04:05:38 pmYup, sounds about right! If I had to summarize, I think it's true to say that pressure is bestwhen it's usable at all, and degassing is better than neither.That said, especially for larger stuff, careful pouring technique (and judicious use of air and/or a torch) also goes a long way.

author: tooki
author_rank: Super Contributor
timestamp: February 24, 2024, 06:53:13 pm
content: Indeed!I managed to get surprisingly good results with literal 5 minute epoxy adhesive, several years past its expiration, with baby powder stirred in, and then shoved into the holes. Way too thick an epoxy to begin with, and I certainly didn’t degas the baby powder! :pI think any method of bubble reduction would be an improvement, as would choosing a slower-curing resin that’s less viscous.Or if the mold/frame is of a heatproof material: in advance, mix up hot melt glue with talc or whatever opacifier used. Let that cool and cut it into pieces that can be melted into the mold. Fill it, then shove the whole shebang in the oven and let it melt, giving enough time for bubbles to rise to the surface (where the LEDs will be, so irrelevant to diffusion). Then let cool.Ann Reardon (the YouTube food scientist and baker) just did a video where she recreated Vegemite, and she used her home washing machine’s spin cycle as a centrifuge. With a slow-curing resin (or air-curing one like silicone sealant) one could mix in the opacifier, put it in a syringe, and then centrifuge it to degas it. (Apparently that’s how adhesives are degassed industrially before application.)