How to Use an RGB LED with Arduino — Elegoo Kit Lesson 4

GGG ·
  • #arduino
  • #led
  • #rgb
  • #pwm
  • #elegoo
  • #beginner
  • #tutorial

A standard LED has two legs: anode and cathode. An RGB LED has four: one cathode shared by all three colors, and three separate anodes — one each for red, green, and blue. Each color channel behaves exactly like an individual LED, which means each needs its own current-limiting resistor and its own Arduino pin. The payoff is that by mixing those three channels at different intensities, you can produce a wide palette of colors from a single component.

This is lesson 4 in the Elegoo Arduino kit series. The circuit is simple — three PWM outputs, three 220 Ω resistors, and one RGB LED — but it introduces analogWrite() and pulse-width modulation, which show up constantly in more complex projects.

The RGB LED pinout

The four legs are easy to identify. The longest leg is the common cathode — it goes to GND. The other three, arranged around the cathode, are the color anodes. In the most common through-hole RGB LEDs:

  • Leg to the left of the cathode: Red
  • Leg second-left: Green (with cathode between them)
  • Leg to the right of the cathode: Blue

When in doubt, check the datasheet for your specific LED — pin ordering can vary between manufacturers. A quick test: put 3.3V through a 220 Ω resistor to each leg in turn and the color it lights tells you which is which.

The wiring

Each anode leg gets its own 220 Ω resistor in series, then connects to a PWM-capable Arduino pin. The cathode goes directly to GND.

RGB LED legResistorArduino pin
Red220 ΩD6
Green220 ΩD5
Blue220 ΩD3
Cathode (longest)—GND

Why D3, D5, D6 — and why skip D4? PWM on the Arduino Uno is only available on certain pins: D3, D5, D6, D9, D10, and D11. These are marked with a tilde (~) on the board silkscreen. D4 doesn’t have a tilde — it’s a plain digital pin that can only output HIGH or LOW, not a PWM signal. Using a non-PWM pin for a color channel means that color is either fully on or fully off, with no brightness control in between.

How PWM controls brightness and color

analogWrite(pin, value) doesn’t output a true analog voltage. Instead it rapidly switches the pin between HIGH and LOW at about 490–980 Hz, with the duty cycle — the fraction of each cycle spent HIGH — proportional to the value: 0 is always off, 255 is always on, 128 is 50% duty cycle. The LED can’t respond fast enough to see the flicker, so it appears as a dimmer version of full brightness.

To mix colors, you set each channel’s duty cycle independently. Red at 255, green and blue at 0 gives pure red. Red at 0, green at 255, blue at 0 gives pure green. Red at 255, green at 255, blue at 0 gives yellow. All three at the same value gives white (or warm gray at lower intensities). The full 24-bit RGB color space works this way.

The Elegoo sketch

The kit’s lesson 4 sketch fades through colors by incrementing and decrementing the red, green, and blue values in a loop:

const int redPin   = 6;
const int greenPin = 5;
const int bluePin  = 3;

int redVal   = 255;
int greenVal = 0;
int blueVal  = 0;

int fadeDelay = 5;  // ms between each step

void setup() {
  pinMode(redPin,   OUTPUT);
  pinMode(greenPin, OUTPUT);
  pinMode(bluePin,  OUTPUT);
}

void loop() {
  // Red → Yellow (increase green)
  for (int i = 0; i < 255; i++) {
    greenVal++;
    analogWrite(redPin,   redVal);
    analogWrite(greenPin, greenVal);
    analogWrite(bluePin,  blueVal);
    delay(fadeDelay);
  }

  // Yellow → Green (decrease red)
  for (int i = 0; i < 255; i++) {
    redVal--;
    analogWrite(redPin,   redVal);
    analogWrite(greenPin, greenVal);
    analogWrite(bluePin,  blueVal);
    delay(fadeDelay);
  }

  // Green → Cyan (increase blue)
  for (int i = 0; i < 255; i++) {
    blueVal++;
    analogWrite(redPin,   redVal);
    analogWrite(greenPin, greenVal);
    analogWrite(bluePin,  blueVal);
    delay(fadeDelay);
  }

  // Continue cycling...
}

The colors cycle through red → yellow → green → cyan → blue → magenta → red. Each transition takes 255 steps × 5 ms = about 1.3 seconds. Reducing fadeDelay speeds up the cycle; increasing it slows it down.

What to try next

Once the basic cycle is working, the interesting variations are easy to explore. Set all three channels to the same value (analogWrite(redPin, x); analogWrite(greenPin, x); analogWrite(bluePin, x)) and increase x from 0 to 255 — the LED fades from off to white. Try fixed color recipes: (255, 165, 0) is orange, (128, 0, 128) is purple, (0, 255, 127) is spring green. The Elegoo documentation PDF includes a color chart with a full set of values worth bookmarking.

PWM shows up again in motor speed control (analogWrite to a motor driver’s enable pin), display dimming, buzzer frequency modulation, and more. Learning to think about analogWrite as “set duty cycle to N/255” rather than “output voltage X” is the mental model that makes the rest of those applications click.

References and further reading

Watch on YouTube