Designing 3-digit 7-segment custom 0805 LED screen - Page 1
Forum section: electronics / projects

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: November 23, 2024, 12:49:00 pm
content: I am designing an LED screen to replace standard 4-digit module. Need only three digits without flicker caused by multiplexing and PWM modulation. I decided to use 0805 LEDs, 4 LEDs per segment, which is 84 LEDs + 2 LEDs for the dot. Each segment power is controlled by AO3400A N-channel MOSFET. For power I am planning to use SY8089A1 which will convert 3.0-4.2v Li-ion battery voltage down to 3.0v. This voltage will be used for the microcontroller and the LED screen. When all segments are ON, the screen will consume around 840 mA (with 10mA per LED). KT-0805G LEDs are used at 50% of maximum forward current:Code:derate=0.5; ' 50 % of currentIforward=20*10^-3*derateIforward = 0.01Vforward=2.8;Vsupply=3;Uresistor=Vsupply-VforwardUresistor = 0.2' U=R*IR=Uresistor/Iforward ' OhmR = 20So, there is a 20 Ohm resistor per each LED. LED forward current is within the limits if supply voltage goes up to 3.2v. I think that 0.2v it is a good safety margin. Resistors, LEDs and MOSFETs are "basic" parts and total cost for 3 digits is within $3, not sure how number of components will affect the assembly price though. I want to order this screen and test it in daylight and possibly reduce the current per LED. Schematic is pretty standard with N-channel MOSFET and 1k gate resistor. Are there any obvious flaws in my approach?

author: inse
author_rank: Super Contributor
timestamp: November 23, 2024, 01:09:27 pm
content: Without knowing the exact type of LED you are using, I expect that at 10mA the display is going to be very bright.Some time ago I replaced a seven segment display in a desktop multimeter with an actual type and I had to add some dark foil as not wanting to replace the resistors as well.Why exactly are you worried about flickering multiplex?If the scan rate is selected accordingly, it should not be visible and allows brightness control in the software.

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: November 23, 2024, 01:22:07 pm
content: Quote from: inse on November 23, 2024, 01:09:27 pmI have problems with filming the screen during the daylight. It looks even worse when recorded using slow-motion camera. I thought about adding capacitors, but it seems that nobody doing this. Therefore, I decided to make non-multiplexed version.

author: Peabody
author_rank: Super Contributor
timestamp: November 23, 2024, 03:36:44 pm
content: So you'll use 22 pins of the MCU? Which MCU are you using?I agree with @inse that a three-digit display drawing 840mA will be very bright indeed. And may possibly overheat. With a 3V supply, if 20R produces 10mA, then the Vf of the LEDs must be 2.8V. Is that right? In any case, it just seems that driving a three-digit display with nearly an amp is at least an order of magnitude too high. Perhaps I'm not thinking right about it, but that's 2.5 watts, which just seems like way too much.

author: Nominal Animal
author_rank: Super Contributor
timestamp: November 23, 2024, 03:46:08 pm
content: I'm only a hobbyist myself, but I think I'd use one or more Analog MAX6969 LED 32-segment constant current drivers myself. Mouser sells them for about6€ apiece in singles in SOIC-24. It uses a standard serial interface (SPI-like), and they can be chained, so your MCU only needs four pins to update the LED segment states (and the latch pin means they'll all update at the same time).If you need a standard 8+3 -pin interface, I'd use a small microcontroller that supports pin change interrupts on the 3 input pins, and only latch the updated states after a specific time has elapsed since the last pin change. That way all digits will change at the same time, even though the source updates them one at a time.

author: Peabody
author_rank: Super Contributor
timestamp: November 23, 2024, 05:03:50 pm
content: Further on the power issue. If you drive each LED at 1mA instead of 10mA, then you still have 4mA per segment, which should be pretty bright if it's continuous power and not multiplexed, particularly with today's high efficiency LEDs. That would let you drive each segment directly, with no mosfet.Of course the big issue is whether it would be bright enough in daylight. I don't know the answer to that, but I think it would be well worth testing before you design for 22 mosfets with their gate resistors that may not be needed.If 1mA isn't bright enough, you could do more with direct drive, with the limit being what the GPIO pins can do individually, and the aggregate maximum current the MCU's power or ground pin can handle.Will you have a brightness adjustment? It's possible that brightness sufficient for daylight would be too bright indoors.By the way, I think you are right about not using multiplexing. Even at normal 30fps video, the actual shutter speed could be very fast if you're outdoors in the sun. You would have to multiplex at a very fast clip to deal with that.

author: shabaz
author_rank: Frequent Contributor
timestamp: November 23, 2024, 06:11:59 pm
content: Just to add to the suggestions; PCA9634 is another nice option. Sure it needs 8 x current limiting resistors but they are small and cheap.I've done a similar thing in the past, for a completely flicker-free display; I stuck a PCA9634 behind each digit (but I used off-the-shelf 7-seg LED modules; I used modules large and bright enough to very easily read from 5-8 meters away, even in a showroom type of environment. I used HDSP-N150 LED modules (now obsolete). A plastic optical filter is also a good idea.

author: Benta
author_rank: Super Contributor
timestamp: November 23, 2024, 08:34:42 pm
content: Personally, I'd step up the supply voltage and put the segment LEDs in series. Fewer resistors and simpler design.For segment control, three 74HC595 in series, controlled by SPI.

author: Someone
author_rank: Super Contributor
timestamp: November 23, 2024, 09:29:06 pm
content: Quote from: Georgy.Moshkin on November 23, 2024, 12:49:00 pmVisibility of indicators/screens in varying lighting conditions is more about contrast than absolute intensity. So you need to solve the optical problems of crosstalk (stray light) and contrast ratio (reflections).

author: Someone
author_rank: Super Contributor
timestamp: November 23, 2024, 09:36:10 pm
content: Quote from: Peabody on November 23, 2024, 05:03:50 pmOP doesnt mention what the least significant digit is counting. All you need is that the transition time between values is some small fraction of their period. Even for 240 FPS being 4.1667ms, with say 4 way multiplex 100 times per frame = 10uS or 100kHz (or 96kHz if you want to be gen locked) well within the capabilities of modern micros. Size capacitance to reduce ripple as desired.

author: mariush
author_rank: Super Contributor
timestamp: November 23, 2024, 09:50:32 pm
content: Use led drivers ... you can get linear drivers with lots of channels and which don't do pwm, so the leds will be either always on or off and you won't get flicker.40 cents for TLC59283https://www.digikey.com/en/products/detail/texas-instruments/TLC59283DBQR/3458112orhttps://www.lcsc.com/product-detail/LED-Drivers_Texas-Instruments-TLC59283DBQR_C702075.htmlex 16 channels up to 45 mA per channel, set the max current on all using a single resistor.. use two of them for your 3-4 digits. You could make the resistor a potentiometer to set the current between 5mA to 40mA or whatever)also TLC59281 , TLC59284 etc etcalso you could have a small step-up regulator boost your 3-5v to around 13-14v, and have your 4 leds of each segment in series. you need 4 x 2.8v plus some headroom for the driver, usually less than 1vps. the drivers above would work fine with single common anode digits , connect all your anodes to input voltage and the n x 8 cathodes to your led driver channels.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 23, 2024, 10:04:59 pm
content: I second the TI ones, have also used them, pretty cheap and work well. Much cheaper than the MAXIM and better availability too.As to LED current, it's hard to tell without some experimenting, it also varies depending on the exact LEDs themselves. But those SMD LEDs in general are pretty bright at low currents. 1mA is typically already pretty bright for indoor use. For something sunlight-viewable, you'll definitely need to crank up the current. You could make it adjustable easily with one of those drivers.For something that must really be viewable in direct sunlight at a distance, small SMD LEDs may not be a good fit though and you may need to resort to higher-power LEDs.

author: Peabody
author_rank: Super Contributor
timestamp: November 23, 2024, 11:07:40 pm
content: Quote from: Someone on November 23, 2024, 09:36:10 pmEven if the frame rate is 240 FPS, or in fact if it's only 30 FPS, the shutter speed in bright light could be 1/1000 second, or even faster. Then each digit would need to be lit up multiple times during that 1ms or 0.5ms so all digits would appear to be equal brightness in the picture. And it's not just the LS digit - all three digits are being multiplexed. I'm not saying it can't be done, but if he has the GPIO pins available, why not use them, particularly if that lets him avoid needing any additional parts.

author: Someone
author_rank: Super Contributor
timestamp: November 24, 2024, 01:59:44 am
content: Quote from: Peabody on November 23, 2024, 11:07:40 pmAdded highlighting to the bit you missed. Keep the LED's lit with some capacitance. Or, following the maths even without capacitance a 1/5000th shutter could still have multiple refreshes of the segments. So completely wrong either way.What gets harder is slew rates in the row/column drivers to avoid smear/crosstalk but thats still in similar technical scope to individual segment drivers.

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: November 24, 2024, 03:58:07 am
content: Quote from: Peabody on November 23, 2024, 03:36:44 pmYes, it's STM32F103C8T6. Currently, I have enough pins: one ADC input, 2 gpio inputs for the toggle switch and lots of unused pins.Quote from: Nominal Animal on November 23, 2024, 03:46:08 pmThat's an option. Robust but pretty expensive though.Quote from: Peabody on November 23, 2024, 05:03:50 pmI am planning to lower the voltage by soldering a different voltage setting resistor for the step-down chip.You probably right about 1mA. Single LED looks pretty bright at 2mA under the sun. But I am not sure how the whole screen would look like. Currentlly, I have a PCB with a single LED to indicate power.I thought about making my own PWM-based step-down using the same MOSFET that is planned for LEDs (AO3400A.pdf). Not sure if it's a good idea, especially using some predefined PWM without voltage monitoring.Quote from: shabaz on November 23, 2024, 06:11:59 pmI guess for three digits I will need either 3 x PCA9634 or 2 x MAX6969 chips.Quote from: Benta on November 23, 2024, 08:34:42 pmInteresting solution. Do I need to worry about uneven brightness within 4-led blocks? That's what ChatGPT's output was.I like that 74HC595 is very cheap and pretty popular compared to PCA9634 and MAX6969. Needs more external elements though.Quote from: Someone on November 23, 2024, 09:29:06 pmYou are absolutely right. I just hope that black PCB and brighter LEDs can provide acceptable performance.Quote from: Someone on November 23, 2024, 09:36:10 pmMy device was recorded on some camera or smartphone with slow-motion function, and the main problem that the digits are turned on separately and constantly flickering. All the digits on the display remain unchanged most of the time, but the user needs to bring a lot of focus to read the whole number from the video. And because of constant flickering it's difficult to spot if the screen content was changed.Quote from: mariush on November 23, 2024, 09:50:32 pmThanks, I will look into these chips and the prices.

author: Benta
author_rank: Super Contributor
timestamp: November 24, 2024, 11:32:23 am
content: "Do I need to worry about uneven brightness within 4-led blocks?"No. The LEDs are in series, so will carry the same current."I like that 74HC595 is very cheap and pretty popular compared to PCA9634 and MAX6969. Needs more external elements though."Needs external resistors. 21 in total to be exact. That's it.

author: Peabody
author_rank: Super Contributor
timestamp: November 24, 2024, 04:24:04 pm
content: Quote from: Georgy.Moshkin on November 24, 2024, 03:58:07 amI wonder if you could insert a pot between the two voltage setting resistors in the buck converter. The fixed resistors would set the acceptable output voltage range, and the pot would adjust between those two values. I suspect the processor would work fine even at a voltage that makes the LEDs almost dark. And you could go up to maybe 3.5V, which could be really bright.On the question of multiplexing, the data sheet for the STM says the current limit on a GPIO pin is 25mA. But the more significant limit is that the Vcc and Vss pins are limited to 150mA. And of course that includes whatever the STM itself needs to run on. So it should handle 1mA per LED (84mA total), but much more than that could get you into trouble.But you can still use constant drive by adding one to three cheap octal driver chips that could take some or all of the current load off the MCU. BCD-to-7segment might even work. There's still no need to multiplex, no worries about ghosting, or reflections, or flickering. I just don't see why you would want to multiplex if you have enough pins. Well, I guess board layout would be simpler if you multiplex. So there's that.Could you post a link to your LEDs?

author: tggzzz
author_rank: Super Contributor
timestamp: November 24, 2024, 04:36:40 pm
content: If you use a little imagination then you can get very pleasing displays with only 20 LEDs per digit.First implemented in 1971

author: tooki
author_rank: Super Contributor
timestamp: November 24, 2024, 04:48:00 pm
content: Quote from: shabaz on November 23, 2024, 06:11:59 pmI used the PCA9634* to make a giant 7 segment display using 3V LED filaments. Great driver, thanks to ~97KHz PWM. One driver per digit, like yours.*actually I used the now-discontinued PCA9624, which is identical to the 9634, but with integrated 40V, 100mA output FETs. Not pin-compatible, but is software-compatible.

author: themadhippy
author_rank: Super Contributor
timestamp: November 24, 2024, 05:00:05 pm
content: QuoteI likes that,got any more info on it? although not as impressive as the one we build with 4ft flory tubes

author: tggzzz
author_rank: Super Contributor
timestamp: November 24, 2024, 05:56:41 pm
content: Quote from: themadhippy on November 24, 2024, 05:00:05 pmHP special, see the HP5300A service manual. Not much info there!

author: themadhippy
author_rank: Super Contributor
timestamp: November 24, 2024, 06:57:05 pm
content: Quotecheers for that,page 4-4 has given me the clue for further tinkering.

author: Nominal Animal
author_rank: Super Contributor
timestamp: November 24, 2024, 08:20:34 pm
content: Quote from: Georgy.Moshkin on November 24, 2024, 03:58:07 amTheTLC59283andTLC59284as suggested by mariush and SiliconWizard look more affordable but just as suitable for this to me, as doesSTP16CP05(especiallySTP16CP05TTR, 1.8€ apiece in singles at Mouser) which only needs a 0.3V drop. These are all constant current sources that do no PWM at all; there would be no flicker/banding even at extreme shutter speeds.I assume KT-0805G refers to LCSC C2297 (datasheet), 523nm (Green) LEDs, with forward voltage about 1.9V at 10mA. TLC59283 and TLC59284 need about 0.6V voltage at output (i.e., supply voltage must be at least LED Vf + 0.5V or so for current regulation; otherwise the current will be lower but depend on the supply voltage), so in this particular case they seem to be best matched price-performance wise.If you used 64 LEDs inO O O O O O O O O O O OO O O O O OO O O O O OO O O O O O O O O O O OO O O O O OO O O O O O O OO O O O O O O O O O O O O Opattern, you'd need four TLC59283/TLC59284 ICs.One interesting option would be to use a 5kΩ + 22kΩ resistors in series for the reference current, but with a N-channel MOSFET in parallel with the 22kΩ ones, with an instrumentation amplifier over the 5kΩ (as if it was a current sense resistor) to the microcontrolled ADC, and DAC channel per MOSFET gate, so that the microcontroller could control the current exactly. The 5kΩ limits the maximum current to 10.6mA (when the 22kΩ is shorted out), and when the MOSFET is high-Z, the 5k+22kΩ sets the minimum LED current to about 2mA. I believe BSS138 and NX138AKR would be suitable MOSFETs here. The maximum voltage drop over the 5kΩ resistor would be 1.208V (maximum current), minimum about 0.224V. (Thus, with a MCU with lots of ADC channels, one could use two ADC channels per TLC5928x.)Another option would be to use 5kΩ each plus a four-gang 20kΩ linear potentiometer like Bourns PTD904 series, as an analog LED intensity control (2mA to 11mA LED current). As the reference voltage is 1.2V or so, and with 5kΩ fixed resistor limits the current to 0.24mA or less, the potentiometer only needs to dissipate at most 0.3mW per gang.

author: mariush
author_rank: Super Contributor
timestamp: November 24, 2024, 09:43:35 pm
content: I should also mention ... if you want segment currents lower than 5mA (usually the low limit of these cheap driver chips), you could use lo-side switches and resistors to limit the current to lower valuesFor example, ULN2003V12 (the mosfet version of the classic darlington uln2003A) is very cheap, something like 20-30 cents, and gives you 7 switches. Resistor in series with each segment, connect segment to a channel, then you can turn on or off channels to turn on individual segments.You could use classic shift registers to control multiple chips with just 2-3 wires.

author: cosmicray
author_rank: Frequent Contributor
timestamp: November 24, 2024, 09:58:38 pm
content: I have a vague notion to do something similar to this concept. I will be using some of my ample stock of 18v white LEDs. Normally I would consider a LM317L operating as a constant current source, but that would imply a regulator for each segment, which feels a bit like overkill. Lately I have been tinkering with an LM317L operating as a voltage regulator, and carefully tweaking the R2 value to get the desired current flow (down the knee curve of the LEDs). Now I'm wondering how resistant the LM317L would be to turning on/off various segments, if the current fluctuated, but it was regulating voltage. If the voltage held steady then one regulator might be able to power all the selected segments, and turn the segments on/off with a mosfet (e.g. AO3400 or AO6800). YAPTE (Yet another project to explore).

author: Benta
author_rank: Super Contributor
timestamp: November 24, 2024, 10:55:18 pm
content: @cosmicrayPlease start a new thread on your project instead of hijacking this one.

author: Benta
author_rank: Super Contributor
timestamp: November 24, 2024, 11:02:12 pm
content: Quote from: tggzzz on November 24, 2024, 04:36:40 pmLooks really nice!Basically a classic 5x7 display reduced to 4x7 with some LEDs missing/unneeded.How did you drive/decode it back then? My guess would be a diode matrix (messy, but still doable today) or a bipolar PROM (unobtainium now).

author: themadhippy
author_rank: Super Contributor
timestamp: November 24, 2024, 11:54:57 pm
content: Quotenot a diode matrix in sight ,just 2 unobtainium ic's.From the service manualQuote

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: November 25, 2024, 03:00:59 am
content: Quote from: Nominal Animal on November 24, 2024, 08:20:34 pmI have this LED used for the battery charging indication.Using a 1.2 kOhm series resistor (U=2.53 V drop measured on the resistor)Current I = U/R = 2.53 / 1200 = ~2.1 mAVforward = 5.0-2.53 = 2.47 VUsing a 120 Ohm series resistor:I = U/R = 1.58 / 120 = ~13.2 mAVforward = 5.0-1.58 = 3.42 VI don't understand the datasheet, maybe Iforward vs Vforward graph is wrong?Or I calculated something wrong here.

author: Nominal Animal
author_rank: Super Contributor
timestamp: November 25, 2024, 03:19:37 am
content: Quote from: Georgy.Moshkin on November 25, 2024, 03:00:59 amWhat's that 5V you're using in your calculations? Are yousureyour supply is 5V there? Did you verify it by measuring while loading it with the LED?

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: November 25, 2024, 04:25:06 am
content: Yes, I have 5 V coming directly from the USB port. With a 1.2 kOhm resistor voltage drop across the resistor and the LED is roughly 2.5 V, and the combined series connection voltage drop is 5.0 V. Battery charge IC controls LED by opening internal mosfet to ground.Attached image shows 2.47v voltage drop on LED terminals. LED part is KT-0705G from JLCPCB

author: Nominal Animal
author_rank: Super Contributor
timestamp: November 25, 2024, 07:43:33 am
content: Some voltage is dropped in U6; the datasheet indicates max. 0.4V at 5mA (section 8.5).

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: November 25, 2024, 08:31:03 am
content: I think that LED datasheet (KT-0805G pdf) If vs Vf graph may be incorrect. On the previous page they state Vf=2.8v with If=20mA (table 4 about brightness measurement). But on the graph it's 2.1v for 20mA. 0.9v difference. Voltage across LED + 1.2kOhm in series is still 5v (photo attached).

author: tggzzz
author_rank: Super Contributor
timestamp: November 25, 2024, 09:23:23 am
content: Quote from: themadhippy on November 24, 2024, 11:54:57 pmI have an HP5300, and the display is refreshingly easy and pleasant to read; better than the standard 7-segment displays.The important concept is the visual use of the LEDs.It would be silly to use the half century old technology to create a new display. The OP has an MCU, so life is much simpler than in 1971, the year the 4004 was released.

author: Nominal Animal
author_rank: Super Contributor
timestamp: November 25, 2024, 10:01:34 am
content: Quote from: Georgy.Moshkin on November 25, 2024, 08:31:03 amYup, I agree: you've convinced me at least.Thus, for this particular case, the STP16CP05 with their lower voltage drop for current regulation, especially STP16CP05TTR (12.10€ for ten at Mouser), might be more appropriate here. (ST product page,datasheet PDF.)Note that if you use a step-up switchmode supply for the LEDs, you can put several LEDs in series.For Vf = 2.7V, STP16CP05 can regulate the current down to a 3.0V supply. (Actually, the datasheet says that for 10mA, running off 3.3V supply, the average dropout is only 79mV ≃ 0.08V. Anyway, at lower supply voltages, the current will only be lower and depend on the supply voltage.) So, for the LED you measured, you should be able to drive them at something like 18mA-20mA from a 3.0V supply. Only the supply to the IC (Vdd) needs to be regulated (to 3.0V - 5.5V); the LEDs can be powered from unregulated supply (say, directly from the battery). For the current set resistor, you'd need 7370Ω (3mA, minimum) to 2056Ω (10mA) to 1006Ω (20mA).I'll need to make a note of get a couple of these ICs for testing in my next Mouser order.Here's the hexadecimal glyph set I'd use with the 20-LED digits:. ▅ ▅ . . . . ▅ . ▅ ▅ . . ▅ ▅ . ▅ ▅ ▅ ▅ ▅ ▅ . ▅ ▅ ▅ ▅ ▅ ▅ ▅ . ▅ ▅ . . ▅ ▅ .▅ ▅ . ▅ ▅ ▅ ▅ ▅ ▅ ▅ ▅ . ▅ . . ▅ ▅ ▅ ▅ ▅▅ ▅ . ▅ . ▅ . ▅ ▅ ▅ ▅ . ▅ . . ▅ ▅ ▅ ▅ ▅▅ . . ▅ . . . ▅ . ▅ ▅ . . . ▅ . ▅ ▅ ▅ ▅ ▅ ▅ ▅ . ▅ ▅ ▅ . . . ▅ ▅ . ▅ ▅ . . ▅ ▅ ▅▅ ▅ . ▅ ▅ . . ▅ . ▅ . ▅ ▅ ▅ . ▅ ▅ ▅ . ▅▅ ▅ . ▅ ▅ . ▅ ▅ . ▅ . ▅ ▅ ▅ . ▅ ▅ ▅ . ▅. ▅ ▅ . . . . ▅ ▅ ▅ ▅ ▅ . ▅ ▅ . . . . ▅ ▅ ▅ ▅ . . ▅ ▅ . . . . ▅ . ▅ ▅ . ▅ ▅ ▅ .. ▅ ▅ . ▅ ▅ ▅ . . ▅ ▅ . ▅ ▅ ▅ . ▅ ▅ ▅ ▅ ▅ ▅ ▅ ▅▅ ▅ ▅ ▅ ▅ ▅ ▅ ▅ ▅ . ▅ .▅ ▅ ▅ ▅ ▅ . ▅ ▅ ▅ . ▅ .▅ ▅ ▅ ▅ ▅ ▅ ▅ . ▅ . . . ▅ . . ▅ ▅ ▅ ▅ . ▅ ▅ ▅ .▅ ▅ ▅ ▅ ▅ . ▅ ▅ ▅ . ▅ .▅ ▅ ▅ ▅ ▅ ▅ ▅ ▅ ▅ . ▅ .▅ . . ▅ ▅ ▅ ▅ . . ▅ ▅ . ▅ ▅ ▅ . ▅ ▅ ▅ ▅ ▅ . . .

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: December 15, 2024, 11:57:12 am
content: I've made it. 4 LEDs in series as suggested bymariushandNominal Animal.Step-up to 11~12v, 2 resistors + 1 transistor per LED. Overall seems like a great approach but need to use some kind of diffusor to make it more readable under direct sunlight. When all segments are on, the PCB becomes warm (designed for 5mA per LED, but haven't checked the actual current yet). Writing a script that automatically places LEDs according to entered text may be a good idea.https://youtu.be/1Jrrw7OF31UAttached photo: Bottom-left is the screen photographed in the dark. The rest of photos are taken under direct sunlight.

author: tooki
author_rank: Super Contributor
timestamp: December 15, 2024, 09:19:49 pm
content: Quote from: Georgy.Moshkin on November 25, 2024, 08:31:03 amThe datasheet for the green LEDs you’re using (KT-0805G), which is linked in an earlier post, is almost certainly mistaken, as you guys figured out. They almost certainly copied the graphs from an old-school lime green LED when making this datasheet for a modern emerald green LED.The datasheet you linked to just now, and for which you wrote the link text “KT-0805G”, is actually for the KT-0805W, the white version.

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: December 16, 2024, 02:35:20 am
content: Quote from: tooki on December 15, 2024, 09:19:49 pmThanks for pointing this out!Here is what I measured on the prototype:Supply voltage ≈ 11.56v3 LEDs in series + R=3.3 kOhm series resistorResistor voltage drop U=4.4vI=U/R=4.4/3300≈0.0013≈1.3 (single LED voltage drop ≈ 2.4v)2.4*3+4.4=11.6v4 LEDs in series + R=560 Ohm series resistorResistor voltage drop U=1.55vI=U/R=1.55/560≈0.0027≈2.8 (single LED voltage drop ≈ 2.51v)2.51*4=11.59vThere is some small error in the measurements.I guess it gives two points on the forward current vs forward voltage graph:1.3mA for 2.4v2.8mA for 2.5vCurrent rises rapidly with voltage, and when there are a lot of LEDs need to be careful. I recall that at I=20mA (absolute maximum rating) LED light was kind of yellow and not green anymore. I think that I can go close to 10mA if required but need to check how the PCB temperature rises.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 17, 2024, 03:43:42 am
content: Quote from: Georgy.Moshkin on December 16, 2024, 02:35:20 amSuch a low drop across the R's is making this sensitive to brightness changes due to LED VF variations and temperature.You have just 387mV of drop per LED, not much for PVT.You may be better to current feed the LED strings. A simple BJT+source resistor sense can give a slight negative tempco.

author: Nominal Animal
author_rank: Super Contributor
timestamp: December 17, 2024, 11:08:06 am
content: 387mV would be ample for STP16CP05TTR, although the minimum current per the datasheet is 3mA. Note that I haven't tested these myself yet, though.

author: I wanted a rude username
author_rank: Frequent Contributor
timestamp: December 22, 2024, 04:42:30 pm
content: You may also want to take a lesson from the old masters who managed to make relatively dim LED displays legible in the 1970s. They did this by using a fairly dark plastic filter of the colour of the LEDs (e.g. red LEDs, red filter). It trades off some brightness for much higher contrast, so overall the display becomes easier to read.Although a suitably coloured filter is best, even just placing the lens of a pair of sunglasses in front will give you an idea of the effect ... the point is not to make the LEDs diffuse, but to block reflected incident light not coming from the LEDs. The improvement is more visible when ambient light is brighter.