Electronics > Beginners

Match LED Brightness

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codingwithethanol:
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?

mariush:
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.

StillTrying:
"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

Kasper:
Edit to add: efficiency and viewing angle may be so different that these quick solutions do not help.
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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

codingwithethanol:
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.

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