L5: The RGB LED

Controlling the colour and brightness of an RGB LED.
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The RGB LED

What you will learn in this lesson

In this lesson, we will cover:

  • What an RGB LED is and how it works.
  • Additive colour mixing.
  • Controlling the LED’s colour and brightness from the development board.

Prerequisites

To follow this lesson, you should have already completed Lesson 1: My first project, Lesson 2: The LED diode, Lesson 3: The LED diode (2) and Lesson 4: Variable-brightness LED. In this lesson, we will use three outputs on the board to control the three colours of a single component.

Theoretical explanation

I want to know more!

What is an RGB LED?

An RGB LED is a component that can produce different colours by combining three basic lights:

  • R stands for red.
  • G stands for green.
  • B stands for blue.

Although it looks like a single LED on the outside, it contains three small LEDs: one red, one green and one blue. Each is called a colour channel. By controlling the brightness of each channel, we can create many different colours.

The four leads of an RGB LED

A typical RGB LED has 4 leads:

  • One lead for the red channel.
  • One lead for the green channel.
  • One lead for the blue channel.
  • One common lead, shared by all three channels.

The common lead can work in two ways, depending on the type of RGB LED:

  • Common cathode: the common lead connects to GND.
  • Common anode: the common lead connects to the positive voltage.

In this lesson, we will use a common-cathode RGB LED. If the LED in your kit or classroom is a common-anode type, the wiring and the way the values behave may differ. Your teacher will tell you which type you are using.

Additive colour mixing

When we mix coloured lights, their colours add together. This is called additive colour mixing. It uses three primary colours: red, green and blue (RGB). These are the colours of the three LEDs inside our RGB LED. As we add light, the result gets lighter, and when we mix all three colours, we get white light.


Additive colour mixing (light)

This does not work in the same way as mixing paints. With paint, mixing many colours usually produces a dark colour. This kind of colour mixing is called subtractive colour mixing, and its primary colours are cyan, magenta and yellow—the colours used by printers, for example.


Subtractive colour mixing (pigments)

Some important combinations are:

RedGreenBlueApproximate colour
ONOFFOFFRed
OFFONOFFGreen
OFFOFFONBlue
ONONOFFYellow
ONOFFONMagenta
OFFONONCyan
ONONONWhite
OFFOFFOFFOff

Brightness in each channel

In the previous lesson, we learned that an LED can receive intermediate values, such as 20, 50 or 80. With an RGB LED, we can do the same for each channel.

For example:

  • Red 100, green 0, blue 0 -> bright red.
  • Red 50, green 0, blue 0 -> dimmer red.
  • Red 100, green 50, blue 0 -> orange or warm yellow.
  • Red 20, green 20, blue 100 -> blue with some white light.

You can think of each colour as a recipe. Instead of ingredients such as flour or water, we use amounts of red, green and blue.

Practical part

You will need the following materials:

  • ESP32 development board.
  • BatSTEM expansion board.
  • USB connection cable.
  • Breadboard.
  • RGB LED with 4 leads.
  • Three 100 Ohm resistors.
  • Jumper wires.

Electronic schematic and wiring diagram

We will connect a common-cathode RGB LED. Each colour channel will connect to a different GPIO pin through its own protective resistor. The RGB LED’s common lead will connect to GND.

We will use these connections:

ChannelGPIO pin
RedGPIO4
GreenGPIO16
BlueGPIO17
CommonGND


Electronic schematic


Connecting the components on the breadboard

Step by step
  1. Place the ESP32 development board on the BatSTEM expansion board.
  2. Connect a black wire between the GND connector on the expansion board and the blue rail on the breadboard.
  3. Place the RGB LED on the breadboard so that its leads are not all in the same electrically connected row.
  4. Identify the RGB LED’s common lead. On many RGB LEDs, it is the longest lead, but check the component’s diagram to be sure.
  5. Connect the common lead to the blue GND rail.
  6. Connect the red channel to GPIO4, with a 100 Ohm resistor in series.
  7. Connect the green channel to GPIO16, with a 100 Ohm resistor in series.
  8. Connect the blue channel to GPIO17, with a 100 Ohm resistor in series.
  9. Check that each channel has its own resistor.
  10. Connect the development board to the computer using the USB cable.

Code

We will control the RGB LED using the blocks designed for this component. First, we will define it with the “RGB LED with name” block, then change its colour with the “set RGB LED” blocks.

I want to know more!

Although the RGB LED is a single component, the setup block needs to know which pins its three channels—R, G and B—are connected to. We can then control it as a single named device.

If you are using a common-anode RGB LED, the values may behave in reverse: a low value may make a channel brighter, while a high value may turn it off. In this lesson, we will follow the common-cathode example to keep the main activity straightforward.

Lighting up the basic colours

Build and run a sketch that shows this sequence:

  1. Red for 1 second.
  2. Green for 1 second.
  3. Blue for 1 second.
  4. Off for 1 second.
Step by step
  1. Click Devices and drag an “RGB LED with name” block into the setup block.
  2. Enter MyRGBLED as the name.
  3. Select GPIO4 for the R channel.
  4. Select GPIO16 for the G channel.
  5. Select GPIO17 for the B channel.
  6. Drag a “set RGB LED” block into the main loop.
  7. Select MyRGBLED and choose red in the colour picker.
  8. Add a wait of 1 second.
  9. Add another “set RGB LED” block with green and another wait of 1 second.
  10. Add another “set RGB LED” block with blue and another wait of 1 second.
  11. Finally, add a “set RGB LED” block with black and a wait of 1 second. Black means that all three channels are off.
  12. Run the sketch and watch the RGB LED change colour.

Mixing colours

Now modify the sketch to show mixed colours too. You can use the colour picker or the block that lets you enter the R, G and B values separately.

Colour we want to seeRedGreenBlue
Yellow1001000
Magenta1000100
Cyan0100100
White100100100
Step by step
  1. Keep the “RGB LED with name” block in setup.
  2. In the main loop, add a sequence for yellow. If you use the block with R/G/B values, enter R 100, G 100, B 0.
  3. Add a wait of 1 second.
  4. Add a sequence for magenta: R 100, G 0, B 100.
  5. Add a wait of 1 second.
  6. Add a sequence for cyan: R 0, G 100, B 100.
  7. Add a wait of 1 second.
  8. Add a sequence for white: R 100, G 100, B 100.
  9. Add a wait of 1 second.
  10. Finish by setting R 0, G 0, B 0 for 1 second.

If everything is working correctly, you will see a continuously repeating sequence of colours.

Creating softer colours

Finally, try intermediate values. For example:

RedGreenBlueExpected result
100400Orange
200100Purplish blue
1002060Pink
2010020Soft green

The colours do not all need to turn out perfectly. The aim is to see that changing each channel’s brightness changes the result.

Additional exercises or challenges

  1. Create a sequence showing these colours: red, yellow, green, cyan, blue, magenta and white.

  1. Invent a colour by adjusting the red, green and blue values. Give it a name.

  2. Make red gradually turn into green.

  1. If you have two RGB LEDs, connect the second to three other GPIO pins, create another “RGB LED with name” block and design a sequence where one shows warm colours and the other shows cool colours.

Summary of what we have learned

  • We have learned that an RGB LED combines three colour channels: red, green and blue.
  • We have connected a four-lead RGB LED using one resistor for each channel.
  • We have seen that light mixes through additive colour mixing.
  • We have created basic and mixed colours by turning on several channels at once.
  • We have used intermediate values to adjust the brightness of each colour.

Next steps

  • Learn to communicate between the board and the computer using the serial port in Lesson 6.

Solutions to additional exercises

Show

A seven-colour sequence

One possible sequence is:

ColourRedGreenBlue
Red10000
Yellow1001000
Green01000
Cyan0100100
Blue00100
Magenta1000100
White100100100

We can add a wait of 1 second after each colour.

Turning red into green

To move gradually from red to green, we can use this table:

StepRedGreenBlue
110000
280200
360400
440600
520800
601000

Add a short wait between steps so you can see the change.

Two RGB LEDs

If you have two RGB LEDs, you can repeat the same circuit using three other GPIO pins. For example:

  • First RGB LED: GPIO4, GPIO16 and GPIO17.
  • Second RGB LED: three other available GPIO pins.

To create contrast, the first LED can show red, yellow and orange, while the second shows blue, cyan and green.