Author Topic: Match LED Brightness  (Read 6953 times)

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Offline codingwithethanolTopic starter

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Match LED Brightness
« on: November 04, 2019, 12:35:03 am »
I'm helping a friend design a PCB badge for an event, I breadboarded a prototype and am now pricing out components from JLCPCB's SMT library. Something occurred to me though, which was a slight problem with the prototype. I used 2 different color LEDs, and identical resistors in parallel, but they were different brightness. How do I figure out how to keep them at identical brightness?
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Offline mariush

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Re: Match LED Brightness
« Reply #1 on: November 04, 2019, 01:06:19 am »
Well duh, human eyes have different sensitivity do different colors.

You could look into datasheets for those leds and see the numbers like mCd rating at some amount of current.

Also, different colors mean different forward voltages ... your formula is V = I x R  so  Resistor value = (Vin - Vdrop led) / I ... the resistors would be different.

You could always use a couple potentiometers instead of a resistor to limit current, and adjust the potentiometers under both leds look like the same brightness to you.

I'd suggest using a solar cell in a small box and measure the amount of current and voltage the solar cell gives when the led inside is lit... but solar cells are more sensitive to some parts of the light spectrum so it's probably not a good way to measure brightness.
 
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Offline StillTrying

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Re: Match LED Brightness
« Reply #2 on: November 04, 2019, 01:16:37 am »
"How do I figure out how to keep them at identical brightness"

By eye is the only way. I measured the max photo diode currents from some LEDs @ 5mA here, sometimes the brightness of SB green and blue were the other way around by eye, but SB green and blue were always at the top.
www.eevblog.com/forum/beginners/floating-probe!-for-$2-50/msg1858251/#msg1858251
.  That took much longer than I thought it would.
 
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Online Kasper

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Re: Match LED Brightness
« Reply #3 on: November 04, 2019, 01:16:41 am »
Edit to add: efficiency and viewing angle may be so different that these quick solutions do not help.
////////////////////////////////

Short version:
Pick resistors for each LED that cause them to have the same current.  For example if you want 5mA in each LED:
R = (Vsupply - Vf) / I
R = (Vsupply - Vf) / 0.005A

If they have similar current they should have somewhat similar brightness but you could go further...

Longer version:
Look up the amount of light output per current input and pick a current for each LED that will yield similar outputs.

Estimate Vf of each LED based on current.
This should be listed in datasheet maybe in a graph.

Use a different resistor for each LED:
R = (Vsupply - Vf) / I
« Last Edit: November 04, 2019, 05:46:50 pm by Kasper »
 

Offline codingwithethanolTopic starter

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Re: Match LED Brightness
« Reply #4 on: November 04, 2019, 01:21:51 am »
EDIT: Just checked the thread and found that some other people replied

@mariush
Yeah its just that I'm picking SMD leds I haven't used irl so the last thing I need is for my friend to get like 100 of them and one color is far brighter than the other. Just to make sure, what you're saying is I need to design it so that mCd is matched?

@StillTrying
I'm trying to spec out blue and yellow LEDs, I have some THT ones but SMD is a mystery.

@Kasper
I took mariush's advice and looked through the datasheets and found mCd ratings and "relative luminous intensity", and just wanted to make sure that I have my maths correct.
« Last Edit: November 04, 2019, 01:26:03 am by codingwithethanol »
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Offline codingwithethanolTopic starter

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Re: Match LED Brightness
« Reply #5 on: November 04, 2019, 01:32:14 am »
Not sure how to interpret the mCd ranges
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Online wraper

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Re: Match LED Brightness
« Reply #6 on: November 04, 2019, 01:39:13 am »
It's naive to expect different color LEDs to have the same brightness. Even if you take same model LEDs from different batches, brightness or color tone may be noticeably different. Also their forward voltage vary, so you will get not only different current with the same resistor but also different power even if current is the same. If you look into datasheets for similar LEDs, maximum available brightness for different colors is quite different. Not to say eyes have different sensitivity to different wavelengths.

 

Offline StillTrying

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Re: Match LED Brightness
« Reply #7 on: November 04, 2019, 01:47:38 am »
@StillTrying
I'm trying to spec out blue and yellow LEDs, I have some THT ones but SMD is a mystery.

It wouldn't surprise me if the yellow needs 50%+ more current than the blue to look similar brightness.
.  That took much longer than I thought it would.
 

Offline mariush

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Re: Match LED Brightness
« Reply #8 on: November 04, 2019, 01:47:54 am »
Not sure how to interpret the mCd ranges

The led will produce a certain amount of light intensity at a particular current amount.
If you restrict the current by adjusting the resistor, you'll have less brightness.

In your pictures, it's not really 100% clear what they mean. I may be wrong, but I read it as the led being capable of around 45-55mCd and that's probably 1 on the Y axis on that chart.
Increasing the current to around 20mA would result in 1.2 x 45-55 mCd and lowering to 10mA would result in 0.8x45-55 mCd.
Of course, may be totally wrong.

I'd suggest buying better leds, from better distributors like Digikey, Mouser, Farnell/Newark etc
At least you get proper datasheets...
 

Online wraper

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Re: Match LED Brightness
« Reply #9 on: November 04, 2019, 01:56:45 am »
I'd suggest buying better leds, from better distributors like Digikey, Mouser, Farnell/Newark etc
At least you get proper datasheets...
It's not like buying there will provide you with brighter LEDs. You can easily buy expensive LED from reputable manufacturer made with old technology and piss poor brightness. As far as it seems those LEDs have good enough datasheets according to screenshots.
 

Offline mariush

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Re: Match LED Brightness
« Reply #10 on: November 04, 2019, 02:05:59 am »

It's not like buying there will provide you with brighter LEDs. You can easily buy expensive LED from reputable manufacturer made with old technology and piss poor brightness. As far as it seems those LEDs have good enough datasheets according to screenshots.

Wasn't referring to brightness, but about how clear and easy to parse/read/understand datasheets are.
For example, see these which are 0.08$ at 100 : https://www.digikey.com/product-detail/en/lite-on-inc/LTST-C190KFKT/160-1434-1-ND/386812
Here's datasheet: http://optoelectronics.liteon.com/upload/download/DS-22-99-0186/LTST-C190KFKT.PDF

Has everything he needs to know... mCd rating , mCd rating depending on bin version (page 6) , charts on page 7 of the pdf
 

Offline codingwithethanolTopic starter

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Re: Match LED Brightness
« Reply #11 on: November 04, 2019, 02:38:39 am »
@StillTrying
Thats a big yikes! The whole thing is going to run off a CR2032 so i'm trying to keep current to a minimum. Alot of these LED's i'm seeing are trying to pull 20mA.

@mariush
Yeah i'm restricted to parts from JLCPCB's part library, so we can get the badges fully assembled. Trying to see if I can find some not so scuff LEDs in the list.

@wraper
Not so much brightness that i'm concerned with, just uniformity.
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Online wraper

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Re: Match LED Brightness
« Reply #12 on: November 04, 2019, 02:59:00 am »
@StillTrying
Thats a big yikes! The whole thing is going to run off a CR2032 so i'm trying to keep current to a minimum. Alot of these LED's i'm seeing are trying to pull 20mA.
They are not trying to pull anything. Small LEDs usually have 20 mA maximum current rating. Actual current depends only on you. If you are concerned about battery life, select brightest (most efficient) LEDs thus needing lowest current for required brightness.
Quote
@wraper
Not so much brightness that i'm concerned with, just uniformity.
Then you don't need to change LEDs to begin with. Just select current limiting resistors to get required brightness.
 

Offline StillTrying

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Re: Match LED Brightness
« Reply #13 on: November 04, 2019, 03:06:07 am »
@StillTrying
Thats a big yikes! The whole thing is going to run off a CR2032 so i'm trying to keep current to a minimum. Alot of these LED's i'm seeing are trying to pull 20mA.

I don't know about SMD, but close up I can see a Super bright 5mm blue lit at less than 1uA, they're very bright at 1 or 2mA.
.  That took much longer than I thought it would.
 

Offline sleemanj

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Re: Match LED Brightness
« Reply #14 on: November 04, 2019, 05:03:23 am »
The best way to match an led for brightness, with a resistance decade box and your eyeball.

Decide on a resistor for one led that produces the brightness you want, then hook your decade box up for the second led and adjust until you find something that makes them match.




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Online wraper

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Re: Match LED Brightness
« Reply #15 on: November 04, 2019, 05:18:48 am »
The best way to match an led for brightness, with a resistance decade box and your eyeball.

Decide on a resistor for one led that produces the brightness you want, then hook your decade box up for the second led and adjust until you find something that makes them match.
Not everyone has resistance decade box. Using potentiometer instead of fixed resistor to adjust the brightness and then measuring it's resistance would do the job just as well.
 

Offline BravoV

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Re: Match LED Brightness
« Reply #16 on: November 04, 2019, 05:26:23 am »
I don't know about SMD, but close up I can see a Super bright 5mm blue lit at less than 1uA, they're very bright at 1 or 2mA.

In my limited experiences, looking straight directly at the LED light path is very poor way to perceive it's brightness level at 1 or 2 mA (@20 mA rated LED) as they're still blinding, try shine it to a wall or bright object, or use two identical LEDs with different current to see the "optimal" brightness vs current.

Offline Whales

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Re: Match LED Brightness
« Reply #17 on: November 04, 2019, 05:26:58 am »
Side question: Do you need the LEDs to appear similar at close range or very long range (eg crowds)?

Keep in mind that different LEDs have different output angles.  At close range things can match, but if you care about 50-100m away then make sure to tune them at that distance.  Different output cones == different rates of decay with distance, ignoring the problems of off angles.

Offline tooki

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Re: Match LED Brightness
« Reply #18 on: November 04, 2019, 09:31:08 am »
Blue LEDs are all made with “modern” ultra high efficiency compositions. Red, yellow, and green (especially yellow) are often older, less efficient chemistries. (Modern emerald green LEDs can be insanely efficient. As power indicators, I usually run them at about 0.5mA.) The result is that at a given current, the blue LED can be massively brighter.

You can get a sort of broad idea as to the chemistry by the forward voltage: for a given color, a higher voltage tends to be more efficient. So for example a traditional green LED might be 2.2V, while a modern high efficiency one is 3.2V.

In any case, I totally agree with the recommendation to do some testing and basically adjust it by eye using potentiometers until you’ve found what values work. Between LED chemistry, human eye characteristics, and optical characteristics of different LEDs, there’s just no real way to rely on calculations alone.
 

Offline Psi

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Re: Match LED Brightness
« Reply #19 on: November 04, 2019, 10:20:40 am »
Short version:
Pick resistors for each LED that cause them to have the same current.  For example if you want 5mA in each LED:
R = (Vsupply - Vf) / I
R = (Vsupply - Vf) / 0.005A

If they have similar current they should have somewhat similar brightness but you could go further...

Doesn't really work.
At the same current, or even the same wattage there can be a huge difference in brightness.
The efficiency of LEDs varies a lot by color and by the chemistry used.
Even for one color, like yellow, there are a few different chemistry that can be used and they can have wildly different efficiency.

Best thing to do is look at the mcd rating and only compare leds that have the same viewing angle.
That will match them up pretty well.
Greek letter 'Psi' (not Pounds per Square Inch)
 
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Offline exe

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Re: Match LED Brightness
« Reply #20 on: November 04, 2019, 10:29:19 am »
I had the same problem and I used my eyes and individual resistors for three different leds (red, green, and orange).

BTW leds vary wildly when it comes to efficiency. I bought more than 10 different kinds to understand what I need. At the end I used leds with highest efficiency because why wasting power.
 

Offline tooki

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Re: Match LED Brightness
« Reply #21 on: November 04, 2019, 11:23:12 am »
Short version:
Pick resistors for each LED that cause them to have the same current.  For example if you want 5mA in each LED:
R = (Vsupply - Vf) / I
R = (Vsupply - Vf) / 0.005A

If they have similar current they should have somewhat similar brightness but you could go further...

Doesn't really work.
At the same current, or even the same wattage there can be a huge difference in brightness.
The efficiency of LEDs varies a lot by color and by the chemistry used.
Even for one color, like yellow, there are a few different chemistry that can be used and they can have wildly different efficiency.

Best thing to do is look at the mcd rating and only compare leds that have the same viewing angle.
That will match them up pretty well.
Exactly. When I was writing my prior reply, I dug out some old green LEDs from around 1990 and a modern green LED. At 20mA, the old green LED is far dimmer than the modern LED at 0.5mA. The modern LED at 20mA is blindingly bright.
 

Online Kasper

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Re: Match LED Brightness
« Reply #22 on: November 04, 2019, 04:28:39 pm »
Short version:
Pick resistors for each LED that cause them to have the same current.  For example if you want 5mA in each LED:
R = (Vsupply - Vf) / I
R = (Vsupply - Vf) / 0.005A

If they have similar current they should have somewhat similar brightness but you could go further...

Longer version:
Look up the amount of light output per current input and pick a current for each LED that will yield similar outputs.

Estimate Vf of each LED based on current.
This should be listed in datasheet maybe in a graph.

Use a different resistor for each LED:
R = (Vsupply - Vf) / I


Short version:
Pick resistors for each LED that cause them to have the same current.  For example if you want 5mA in each LED:
R = (Vsupply - Vf) / I
R = (Vsupply - Vf) / 0.005A

If they have similar current they should have somewhat similar brightness but you could go further...

Doesn't really work.
At the same current, or even the same wattage there can be a huge difference in brightness.
The efficiency of LEDs varies a lot by color and by the chemistry used.
Even for one color, like yellow, there are a few different chemistry that can be used and they can have wildly different efficiency.

Best thing to do is look at the mcd rating and only compare leds that have the same viewing angle.
That will match them up pretty well.
Exactly. When I was writing my prior reply, I dug out some old green LEDs from around 1990 and a modern green LED. At 20mA, the old green LED is far dimmer than the modern LED at 0.5mA. The modern LED at 20mA is blindingly bright.

Quoting people doesn't really work if you don't read their whole post or the original post.

My post was clearly broken into a quick solution for someone who seems to want a quick solution and then a better solution with an explanation that the quick solution is not great.

You quote the 1st half of my post, say it doesn't really work (as if I didn't just say that) and then you go on to repeat the 2nd half of my quote which you for some reason left out.

Your whole post could have been better written in 1 sentence: 'viewing angle should also be matched'.

OP also said parts are coming from JLCPCB's SMT library so I doubt 1 is 30 years old and the other is modern.




 

Offline Siwastaja

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Re: Match LED Brightness
« Reply #23 on: November 04, 2019, 04:46:39 pm »
My post was clearly broken into a quick solution for someone who seems to want a quick solution and then a better solution with an explanation that the quick solution is not great.

Doesn't matter. Sorry to say, but your "quick" "solution" is just plainly wrong, and won't work as a "first order estimate" to be improved upon. It's waste of time, and misleading.

LED efficiencies easily vary by two orders of magnitude. Assuming current has anything to do with brightness is just wrong. It's not even about the age of the LED - very inefficient and dim LEDs are still being made, especially green.

The quick solution is to look at mCd numbers in datasheets, if available. The better solution is finetuning by eye.

Choosing the most efficient (bright) LEDs is a good start, also to save energy. Then fine-tune by adjusting resistors - this is done by eye.

So following your solution, you should go in the opposite direction, from step 2 to step 1.
« Last Edit: November 04, 2019, 05:02:24 pm by Siwastaja »
 
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Online Kasper

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Re: Match LED Brightness
« Reply #24 on: November 04, 2019, 05:21:14 pm »
My post was clearly broken into a quick solution for someone who seems to want a quick solution and then a better solution with an explanation that the quick solution is not great.

Doesn't matter. Sorry to say, but your "quick" "solution" is just plainly wrong, and won't work as a "first order estimate" to be improved upon. It's waste of time, and misleading.

LED efficiencies easily vary by two orders of magnitude. Assuming current has anything to do with brightness is just wrong. It's not even about the age of the LED - very inefficient and dim LEDs are still being made, especially green.

The quick solution is to look at mCd numbers in datasheets, if available. The better solution is finetuning by eye.

Choosing the most efficient (bright) LEDs is a good start, also to save energy. Then fine-tune by adjusting resistors - this is done by eye.

So following your solution, you should go in the opposite direction, from step 2 to step 1.

Quick solution would most likely be better than using the same resistor for each LED.
 


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