Electronics > Projects, Designs, and Technical Stuff
LED driver test board (giant pads question)
sparkydog:
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 that either the front lower drivers will be populated or the 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 the actual board 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?
cosmicray:
--- Quote from: sparkydog on February 20, 2024, 10:44:23 pm ---Eventually I want to make my own LED seven-segment display.
--- End quote ---
Yes, I am also aware of that desire, and have similar interests. Be aware that what we think of as seven segment displays were in older times produced in variants up to 17 segments.
--- Quote ---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.
--- End quote ---
Your 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.
sparkydog:
--- Quote from: cosmicray on February 21, 2024, 12:30:59 am ---
--- Quote from: sparkydog on February 20, 2024, 10:44:23 pm ---Eventually I want to make my own LED seven-segment display.
--- End quote ---
Yes, I am also aware of that desire, and have similar interests. Be aware that what we think of as seven segment displays were in older times produced in variants up to 17 segments.
--- End quote ---
Oh, 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 display that has exactly 27 possible states so 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 am ---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.
--- End quote ---
I'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 principle I'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 actually help, 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 ---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.
--- End quote ---
"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 run one LED 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 have part of a segment if one of the LEDs dies. (For that matter, while I certainly plan on using diffusers, this design ought to look passable even without them. Who knows, maybe I'll even decide it looks better that 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.)
tooki:
--- Quote from: sparkydog on February 20, 2024, 10:44:23 pm ---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.)
--- End quote ---
While 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
DavidAlfa:
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.
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