Arduino sketch to cycle an RGB LED through the colour spectrum.

rgb_spectrum.c

const int redPin = 11;
const int greenPin = 10;
const int bluePin = 9;

void setup() {
  // Start off with the LED off.
  setColourRgb(0,0,0);
}

void loop() {
  unsigned int rgbColour[3];

  // Start off with red.
  rgbColour[0] = 255;
  rgbColour[1] = 0;
  rgbColour[2] = 0;  

  // Choose the colours to increment and decrement.
  for (int decColour = 0; decColour < 3; decColour += 1) {
    int incColour = decColour == 2 ? 0 : decColour + 1;

    // cross-fade the two colours.
    for(int i = 0; i < 255; i += 1) {
      rgbColour[decColour] -= 1;
      rgbColour[incColour] += 1;
      
      setColourRgb(rgbColour[0], rgbColour[1], rgbColour[2]);
      delay(5);
    }
  }
}

void setColourRgb(unsigned int red, unsigned int green, unsigned int blue) {
  analogWrite(redPin, red);
  analogWrite(greenPin, green);
  analogWrite(bluePin, blue);
 }

Wouldn't it be better to use PWM here? Leds have a nonlinear voltage response

Well written jamesotron, thanks.

Peterrus: analogWrite() is PWM.

Perfect. Just what I was looking for!

Very well written codes! Thank you so much!!

Neat code! Thank you!

Yeah, thanks!)

Hello Arduino World,
I'm trying to work out how this sketch works. I understand it up to the line 20.
int incColour = decColour == 2 ? 0 : decColour + 1;
I think it starts by declaring an integer called incColour and sets it equal to the other int decClour but then I get lost with the two = signs and then the quesion mark. Is there a long way to write this line that will be easier to understand?
Regards,
Volandi

Volandi,

? is a ternary operator. Line 20 is the same logic as:

int incColour;
if (decColour == 2) {
    incColour = 0;
} else {
    incColour = decColour +1;
}

Thanks!

You can skip a block of code by assigning values when you initialize the array...

unsigned int rgbColor[3] = {255, 0, 0};

Also, thanks for this example.

You can also write that line as:
incColour = (decColour + 1) % 2

@ManciuStefan Not really, that line only ever gives you 0 or 1 as an output. The original line with ternary operator gives you either 0, 1, or 2.

i need to control rgb light via blutooth from mobile?. any advice?

@jamesotron Your schetche works but, you need to explain stuff and add comments. Explain what this means(on line 20):

int incColour = decColour == 2 ? 0 : decColour + 1;

This works great for common cathode LEDS.
If you're using common Anode LEDS, change line 27 to:
setColourRgb(255-rgbColour[0], 255-rgbColour[1], 255-rgbColour[2]); //quick and ugly invert

Interesting code, but for me only blue changes

I just noticed a minor bug. A stickler (such as myself) may wish to call setColourRgb(rgbColour[0],rgbColour[1],rgbColour[2]); delay(5); immediately after rgbColour gets initialized to {255,0,0}, before hitting the for loops. Otherwise the LED doesn't ever hit a pure red output.

Alternatively, you could initialize rgbColour to {256,-1,-1}, but then you'd need to change the declaration of rgbColour to signed and make similar change to the arguments to setRgbColour() function. I'm pretty sure making it signed won't hurt anything; at worst analogRead() will complain about implicit casting. If that bothers you (and it should) you can safely cast it to unsigned in setRgbColour()

I noticed that the red output seemed underpowered a few months ago, but assumed I used too large a value of resistor on the red pin. That may still be true, but this fix should help a bit.

I love this gist, by the way. I've come back to it several times since I first saw it a couple years ago. Thank you, @jamesotron !

Simultaneous increasing and decreasing of 2 colours leads to the situation where the colors "meet" at midpoint between 0 and 255 so the RGB status is like 127,128,0 which gives a darker shade. If you take a look at "color picker" in google, set the color to be the brightest and play with the slider you'll notice that it always changes 1 of the RGB variables at a time leading it to reach 6 main states that gradually switch between themselves, these states are:

1. 255, 0, 0
2. 255, 255, 0
3. 0, 255, 0
4. 0, 255, 255
5. 0, 0, 255
6. 255, 0, 255
7. 255, 0, 0 (whole cycle ends, it's the same as 1st step)

Your implementation is providing the following states:

1. 255, 0, 0
2. 127, 128, 0
3. 0, 255, 0
4. 0, 128, 127
5. 0, 0, 255
6. 127, 0, 128
7. 255, 0, 0

Here's a python implementation which shows how to loop through RGB without getting the darker shade:
https://github.com/michalmonday/RGB_rainbowLoop/blob/master/RGB_rainbowLoop.py

I guess it would be better without the section list but it does the thing... On the other side it allows for other operations being executed without the need to complete the whole cycle from red to blue, also it would be easy to implement custom sequences of colours with it.

very nice to use make the prototype. and I can save time by using this function.
I will make a led lamp using contained register led(NeoPixel)that operate by data but
previous products are consist of normal type led to controlled ON or OFF voltage

I've implemented a non-blocking version of it using @michalmonday's algorithm for Arduino:
https://gist.github.com/surik00/6eb44d102d207bc94b9f9bd1c7e87c42

You can also use the HSB model for your colors (if your mc is fast enough) then cycling through all colors will start to feel very natural. C++ for this can be found here https://github.com/rvt/Arilux_AL-LC0X

i have this, and i want to implement this into my code using one of the 'case' statements i have. now, i have no clue what i'm doing so help would be appreciated. i want to add it into the lowest one (play/pause key)

---

=]ppm #include <IRremote.h>

// Define sensor pin
const int RECV_PIN = 4;

// Define LED pin constants
const int redPin = 3;
const int greenPin = 5;
const int bluePin = 6;

// Define integer to remember toggle state
int togglestate = 0;

// Define IR Receiver and Results Objects
IRrecv irrecv(RECV_PIN);
decode_results results;

void setup(){
// Enable the IR Receiver
irrecv.enableIRIn();
// Set LED pins as Outputs
pinMode(redPin, OUTPUT);
pinMode(greenPin, OUTPUT);
pinMode(bluePin, OUTPUT);
}

void loop(){
if (irrecv.decode(&results)){

    switch(results.value){
      case 0xFF30CF: //1 key
    // Toggle LED On or Off
    if(togglestate==0){
    digitalWrite(redPin, HIGH);
    togglestate=1;
    }
    else {
    digitalWrite(redPin, LOW);
    togglestate=0;
    }
    break;

    case 0xFF10EF: //4 key
    // Toggle LED On or Off
    if(togglestate==0){
    digitalWrite(bluePin, HIGH);
    togglestate=1;
    }
    else {
    digitalWrite(bluePin, LOW);
    togglestate=0;
    }
    break;

    case 0xFF18E7: //2 key
    // Toggle LED On or Off
    if(togglestate==0){
    digitalWrite(greenPin, HIGH);
    togglestate=1;
    }
    else {
    digitalWrite(greenPin, LOW);
    togglestate=0;
    }
    break;

    case 0xFFC23D: //play-pause key
    if togglestate==0){
    
    }
    
}
irrecv.resume(); 

}

}

@jdimpson

Otherwise the LED doesn't ever hit a pure red output.

??? WHY ???

 for (int decColour = 0; decColour < 3; decColour += 1) {
 // When `decColour ==2`
   int incColour = decColour == 2 ? 0 : decColour + 1;
   //  `incColour = 0` at this moment 

After finish the i loop ,
rgbColour[0]=255; and rgbColour[1]=rgbColour[2]=0; ... back to pure red again...
right ?

Another kinds of colour cycle
it might ignore darker shade problem

but it's total different spectrum with original

void loop() {
  unsigned int rgbColour[3];

  // Start off with red.
  rgbColour[0] = 255;
  rgbColour[1] = 255;    //// Different place !
  rgbColour[2] = 0;  

  // Choose the colours to increment and decrement.
  for (int decColour = 0; decColour < 3; decColour += 1) {
    int incColour = (0==decColour ) ? 2 : decColour - 1;    //// Different place !

    // cross-fade the two colours.
    for(int i = 0; i < 255; i += 1) {
      rgbColour[decColour] -= 1;
      rgbColour[incColour] += 1;
      
      setColourRgb(rgbColour[0], rgbColour[1], rgbColour[2]);
      delay(5);
    }
  }
}

for states :
255, 0, 0
255, 255, 0
0, 255, 0
0, 255, 255
0, 0, 255
255, 0, 255
255, 0, 0

void loop() {
  unsigned int rgbColour[3];

  // Start off with red.
  rgbColour[0] = 255;
  rgbColour[1] = 0; 
  rgbColour[2] = 0;  
  int decColour =0;
  int incColour =1;

void loop() {
			if (255<=rgbColour[incColour]) {
				if (0>=rgbColour[decColour]) {
					decColour++;	decColour %= 3;
					incColour++;	incColour %= 3;
				} else 	// if (0==rgbColour[decColour]) {
					rgbColour[decColour] -= 15;  // --
			} else	// if (255==rgbColour[incColour]) {
				rgbColour[incColour] += 15;  // ++
      
        setColourRgb(rgbColour[0], rgbColour[1], rgbColour[2]);
        delay(5);
    }
}