• ### RGB LED Color Balancing Experimentation

The RGB LED's are a major piece to Weather Color Strip, and mixing of the RGB elements is also important in order to accurately show the color of the temperature outside.  The colors are mixed by calculating duty cycles for Red, Green and Blue LED's based on the temperature information parsed from a weather API.  The duty cycles are determined using this graphic I have made:

The problem I have noticed is the color spectrum can be green heavy, red heavy or blue heavy depending on the color balancing of the RGB elements.  Ideally I would like to see an even spread of blue/green at lower temperature and red/green at higher temperatures.  Currently I have tried four different strategies and would like to document the results:

1. Equal Resistance
1. Each LED (Red, Green and Blue) will have the same current limiting resistor value
2. Equal Current
1. Each LED (Red, Green and Blue) will have the same current
3. Equal Intensity
1. Each LED (Red, Green and Blue) will have a scaled max current so each LED will have an equal intensity
4. 3:6:1 Color Mixing Ratio
1. Create an overall target for intensity.  30% will be red, 60% will be green and 10% will be blue.

The LED's I used are Cree SLV6A-FKB-CK1P1G1BB7R3R3.  Digikey page is below:

https://www.digikey.com/product-detail/en/cree-inc/SLV6A-FKB-CK1P1G1BB7R3R3/SLV6A-FKB-CK1P1G1BB7R3R3CT-ND/6138648

You will notice that the LED voltage drops in each formula are changing slightly.  This is change is due to changes in LED current.  This voltage drop is very important when calculating ideal LED resistor values.  Relationship between LED current and voltage drop can be found in the following graphs:

Another important section of the datasheet are the Relative Luminous Intensity curves.  These curves show the relationship between current and luminous intensity.  These are important when trying to calculation the ideal LED currents:

# Equal Resistance

For this strategy I simply used a common resistor values for Red, Green and Blue LED's.  I chose a simple 1k resistor.

Actual resistances, currents and intensities are summarized below:

 RED GREEN BLUE RESISTOR VALUE (OHM) 1000 1000 1000 LED CURRENT (mA) 10.16 9.000 9.050 INTENSITY (mcd) 355.6 960.000 181.000

# Equal Current

For this strategy I tried to make the LED current for Red, Green and Blue to be 10mA.  Below is some math on how I calculated the LED resistor values:

 RED GREEN BLUE RESISTOR VALUE (OHM) 1000 887 887 LED CURRENT (mA) 10.16 10.124 10.169 INTENSITY (mcd) 355.6 1079.895 203.382

# Equal Intensity

For this strategy I tried to scale each LED current in order to have the same intensity for each LED (Red, Green and Blue).  I was trying to achieve a target intensity of 400mcd.  Using the Relative Luminous Intensity curves, I figured out how much current each LED would need in order to achieve 400mcd. Below is some tables/math on how I calculated the LED resistors values:

 RED GREEN BLUE 20mA INTENSITY (mcd) 700 1600 400 RELATIVE INTENSITY (@ 400mcd) 0.571 0.250 1.000 REQUIRED LED CURRENT (mA) 11.429 5 20

I have calculated the necessary current values to achieve 400mcd of intensity.  Now I can perform similar calculations completed in the equal current strategy to calculate resistor values:

Actual resistances, currents and intensities are summarized below:

 RED GREEN BLUE RESISTOR VALUE (OHM) 887 2430 442 LED CURRENT (mA) 11.342 3.745 19.910 INTENSITY (mcd) 396.956 399.451 398.190

# 3:6:1 Color Mixing Ratio

This strategy involves having specific intensity ratios for red,...