Individual Led bright match

[Matrixx]

Hello, I'm working on a project where I use 3 leds to indicate alarmc light status inside a vehicle.
So I have green, yellow and red, and my voltage should be from 3.0 to 3.2 max as I'm using a Li-ion battery rated at 3.7V,. 3.3 regulator is used.

Happen that with the 20mA as indicated, the leds brightness is not equal. Green seems to be brighter, yellow mid bright but red is kind dimmed. each one has its own resistor, of course.

I'm using an Atmega328p to control the leds. What I did is to do PWM for each one, so I can adjust for "night" time (dimmed), so the leds do not cause vission problem in the car cabin, and "day" as many bright is need to be seen.

PWM worked ok, but I still have the problem of uniform brightness: green is the strongest. Also I find green starts to be vsible with lower PWM values while the yellow and red are still off.
I decided to increment the green resistor, and for the low level, it worked, say in "night" mode. But raising the yellow and red, now the green lacks of bright, due to the incremented resistor.

Someone has dealed with this situation..?
Also I'm looking for a led part number 3mm to have enough brightness using Li-ion available voltages (not the richness of 5V).


Any comments welcomed.

[Psi]

Part of the issue is that the human eye is very sensitive to green and not very sensitive to red.
So a green LED will always seem brighter than a red when they're emitting the same amount of light.

If you only have a few set brightness levels just hardcode the values so they look correct.

If you need fine brightness control you will need to write a simple formula to correct for the brightness variations of each LED.
Work out 5 brightness levels with equal looking RGB values and plot the actual PWM values on a graph to help you visualize the problem. You can then work out some math to generate a corrected PWM value from a brightness value.

[lapm]

Also different color leds has different forward voltage. You better refer to datasheets of manufacturer.

[Matrixx]

Thanks!!

I think will have to run in the formula to adjust dinamically.
Great forum!