BITE SIZE ARDUINO – RGB LED

A RGB LED is a LED that can change the colour of the light it produces depending on which of the LEDs’ Connectors have current flowing through them. The LED has 4 connectors, one connector for red, one for green, one for blue and then finally an anode or a cathode, depending if the RGB is a common anode or cathode LED.

So what is the difference between common anode and common cathode?

Well a RGB LED is actually a combination of 3 LEDs, a red LED, a green LED and a blue LED. All LEDs have 2 connectors, an anode and a cathode. So depending how these LEDs are connected together determines if they share an anode or a cathode, thus common anode RGB LED or common cathode RGB LED. The Anode\Cathode leg can be identified as it is the longest leg on the LED. Below are 2 diagrams that illustrates the difference discussed.

Common Cathode:

Common Cathode_schemCommon Anode:

Common Anode_schem

How these 2 different RGB LEDs are connected to a circuit also differs, let us first have a look at a circuit that contains a common cathode RGB LED:

common cathode arduino_bb

Here is the code used with this circuit:

int redPin = 9;
int greenPin = 10;
int bluePin = 11;
 
void setup()
{
  pinMode(redPin, OUTPUT);
  pinMode(greenPin, OUTPUT);
  pinMode(bluePin, OUTPUT);  
}
 
void loop()
{
  setLEDColour(255, 0, 0);  // red
  delay(2000);
  setLEDColour(0, 255, 0);  // green
  delay(2000);
  setLEDColour(0, 0, 255);  // blue
  delay(2000);
  setLEDColour(255, 255, 0);  // yellow
  delay(2000);  
  setLEDColour(80, 0, 80);  // purple
  delay(2000);
}
 
void setLEDColour(int red, int green, int blue)
{
  analogWrite(redPin, red);
  analogWrite(greenPin, green);
  analogWrite(bluePin, blue);  
}

Now let us have a look at a circuit that contains a common anode RGB LED:

common anode arduino_bb

Code used with this circuit:

int redPin = 11;
int greenPin = 10;
int bluePin = 9;
 
void setup()
{
  pinMode(redPin, OUTPUT);
  pinMode(greenPin, OUTPUT);
  pinMode(bluePin, OUTPUT);  
}
 
void loop()
{
  setLEDColour(255, 0, 0);  // red
  delay(2000);
  setLEDColour(0, 255, 0);  // green
  delay(2000);
  setLEDColour(0, 0, 255);  // blue
  delay(2000);
  setLEDColour(255, 255, 0);  // yellow
  delay(2000);  
  setLEDColour(80, 0, 80);  // purple
  delay(2000);
}
 
void setLEDColour(int red, int green, int blue)
{
  red = 255 - red;
  green = 255 - green;
  blue = 255 - blue;

  analogWrite(redPin, red);
  analogWrite(greenPin, green);
  analogWrite(bluePin, blue);  
}

Although the circuits and code differ between the 2 types of RGB LEDs, the end results are exactly the same.

Try changing the values passed into the setLEDColour function to see what different colours can be created.

BITE SIZE ARDUINO – RGB LED

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