RGB LED color mixing in Raspberry Pi

Introduction

RGB LED color mixing lets you create many colors by combining red, green, and blue light. It helps make lights show different colors using just one LED.

You want to make a light that changes colors for decoration.

You want to show different statuses with colors on a device.

You want to learn how colors mix using electronics.

You want to build a simple mood light with a Raspberry Pi.

You want to control colors in a project with buttons or sensors.

Syntax

Raspberry Pi

import RPi.GPIO as GPIO
import time

# Setup pins for red, green, blue
RED_PIN = 17
GREEN_PIN = 27
BLUE_PIN = 22

GPIO.setmode(GPIO.BCM)
GPIO.setup(RED_PIN, GPIO.OUT)
GPIO.setup(GREEN_PIN, GPIO.OUT)
GPIO.setup(BLUE_PIN, GPIO.OUT)

# Create PWM objects for each color
red = GPIO.PWM(RED_PIN, 1000)
green = GPIO.PWM(GREEN_PIN, 1000)
blue = GPIO.PWM(BLUE_PIN, 1000)

# Start PWM with 0 duty cycle (off)
red.start(0)
green.start(0)
blue.start(0)

# Function to set color by duty cycle (0-100)
def set_color(r, g, b):
    red.ChangeDutyCycle(r)
    green.ChangeDutyCycle(g)
    blue.ChangeDutyCycle(b)

Use PWM (Pulse Width Modulation) to control brightness of each color.

Duty cycle 0 means LED off, 100 means fully on.

Examples

This makes the LED show bright red color.

Raspberry Pi

set_color(100, 0, 0)  # Full red, no green, no blue

This makes the LED show bright green color.

Raspberry Pi

set_color(0, 100, 0)  # Full green, no red, no blue

This makes the LED show bright blue color.

Raspberry Pi

set_color(0, 0, 100)  # Full blue, no red, no green

This mixes red and green light to create yellow color.

Raspberry Pi

set_color(50, 50, 0)  # Mix red and green to get yellow

Sample Program

This program cycles through main colors by changing the brightness of red, green, and blue parts of the LED. It prints the color name each time so you know what color is showing.

Raspberry Pi

import RPi.GPIO as GPIO
import time

RED_PIN = 17
GREEN_PIN = 27
BLUE_PIN = 22

GPIO.setmode(GPIO.BCM)
GPIO.setup(RED_PIN, GPIO.OUT)
GPIO.setup(GREEN_PIN, GPIO.OUT)
GPIO.setup(BLUE_PIN, GPIO.OUT)

red = GPIO.PWM(RED_PIN, 1000)
green = GPIO.PWM(GREEN_PIN, 1000)
blue = GPIO.PWM(BLUE_PIN, 1000)

red.start(0)
green.start(0)
blue.start(0)

def set_color(r, g, b):
    red.ChangeDutyCycle(r)
    green.ChangeDutyCycle(g)
    blue.ChangeDutyCycle(b)

try:
    while True:
        set_color(100, 0, 0)  # Red
        print("Red")
        time.sleep(1)
        set_color(0, 100, 0)  # Green
        print("Green")
        time.sleep(1)
        set_color(0, 0, 100)  # Blue
        print("Blue")
        time.sleep(1)
        set_color(50, 50, 0)  # Yellow
        print("Yellow")
        time.sleep(1)
        set_color(0, 50, 50)  # Cyan
        print("Cyan")
        time.sleep(1)
        set_color(50, 0, 50)  # Magenta
        print("Magenta")
        time.sleep(1)
        set_color(100, 100, 100)  # White
        print("White")
        time.sleep(1)
except KeyboardInterrupt:
    pass
finally:
    red.stop()
green.stop()
blue.stop()
GPIO.cleanup()

Important Notes

Make sure to connect the LED pins correctly with resistors to avoid damage.

Use BCM pin numbering mode for clarity and consistency.

Clean up GPIO pins after use to avoid warnings on next run.

Summary

RGB LED color mixing uses red, green, and blue light combined at different brightness levels.

Use PWM to control each color's brightness from 0 (off) to 100 (full).

Mixing colors lets you create many colors with just one RGB LED.

Practice

1. What does PWM control when mixing colors on an RGB LED connected to a Raspberry Pi?

easy

A. The brightness of each red, green, and blue LED component

B. The voltage level of the Raspberry Pi power supply

C. The speed of the Raspberry Pi processor

D. The temperature of the LED

Solution

1. Step 1: Understand PWM role in LEDs

   PWM (Pulse Width Modulation) changes how long the LED is on versus off, controlling brightness.

2. Step 2: Apply PWM to RGB LED components

   Each color (red, green, blue) brightness is controlled separately by PWM signals.

3. Final Answer:

   The brightness of each red, green, and blue LED component -> Option A

4. Quick Check:

   PWM controls brightness = D [OK]

Hint: PWM changes LED brightness, not voltage or speed [OK]

Common Mistakes:

• Confusing PWM with voltage control
• Thinking PWM controls processor speed
• Assuming PWM changes LED temperature

2. Which of the following is the correct way to start PWM on a Raspberry Pi GPIO pin using Python's RPi.GPIO library?

easy

A. pwm = GPIO.PWMStart(pin, frequency, duty_cycle)

B. pwm = GPIO.PWM(pin, frequency); pwm.start(duty_cycle)

C. pwm = GPIO.startPWM(pin, duty_cycle); pwm.frequency(frequency)

D. pwm = GPIO.PWM(pin); pwm.begin(frequency, duty_cycle)

Solution

1. Step 1: Recall RPi.GPIO PWM syntax

   The correct syntax is to create a PWM object with GPIO.PWM(pin, frequency).

2. Step 2: Start PWM with duty cycle

   Then call pwm.start(duty_cycle) to begin PWM signal.

3. Final Answer:

   pwm = GPIO.PWM(pin, frequency); pwm.start(duty_cycle) -> Option B

4. Quick Check:

   Correct PWM start syntax = C [OK]

Hint: Use GPIO.PWM(pin, freq) then start(duty) [OK]

Common Mistakes:

• Using non-existent methods like PWMStart or startPWM
• Calling start before creating PWM object
• Mixing method names or parameters

3. Given this Python code controlling an RGB LED on Raspberry Pi:

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
red = 17
green = 27
blue = 22
GPIO.setup(red, GPIO.OUT)
GPIO.setup(green, GPIO.OUT)
GPIO.setup(blue, GPIO.OUT)
pwm_red = GPIO.PWM(red, 1000)
pwm_green = GPIO.PWM(green, 1000)
pwm_blue = GPIO.PWM(blue, 1000)
pwm_red.start(100)
pwm_green.start(0)
pwm_blue.start(0)

What color will the RGB LED show?

medium

A. Bright green

B. White

C. Bright blue

D. Bright red

Solution

1. Step 1: Analyze PWM duty cycles for each color

   Red PWM is started at 100% duty cycle (full brightness), green and blue at 0% (off).

2. Step 2: Determine resulting LED color

   Only red LED is on fully, so the LED shows bright red color.

3. Final Answer:

   Bright red -> Option D

4. Quick Check:

   Red=100%, Green=0%, Blue=0% = Bright red [OK]

Hint: 100% duty cycle means full brightness for that color [OK]

Common Mistakes:

• Assuming 0 means half brightness
• Mixing pin numbers with colors
• Thinking all LEDs light up by default

4. This code is intended to mix red and green colors on an RGB LED using Raspberry Pi PWM, but it does not work as expected:

pwm_red.start(50)
pwm_green.start(50)
pwm_blue.start(0)
GPIO.cleanup()

What is the main issue causing the LED to not show the mixed color?

medium

A. Calling GPIO.cleanup() immediately stops PWM signals

B. Duty cycle values must be 0 or 100 only

C. PWM frequency is not set before starting

D. Blue LED must be started with 50% duty cycle

Solution

1. Step 1: Understand GPIO.cleanup() effect

   GPIO.cleanup() resets all GPIO pins, stopping PWM signals immediately.

2. Step 2: Identify timing issue in code

   Calling cleanup right after starting PWM stops the LEDs before visible mixing.

3. Final Answer:

   Calling GPIO.cleanup() immediately stops PWM signals -> Option A

4. Quick Check:

   cleanup() stops PWM = A [OK]

Hint: Don't call GPIO.cleanup() before PWM runs [OK]

Common Mistakes:

• Thinking duty cycle must be 0 or 100 only
• Ignoring cleanup() effect on GPIO pins
• Assuming frequency defaults to zero

5. You want to create a smooth color fade from red to blue on an RGB LED using Raspberry Pi PWM. Which approach correctly mixes colors to achieve this effect?

hard

A. Turn off green and blue, then blink red rapidly

B. Keep red at 100 and blue at 100 simultaneously for full brightness

C. Gradually decrease red duty cycle from 100 to 0 while increasing blue from 0 to 100

D. Set all colors to 50% duty cycle and keep constant

Solution

1. Step 1: Understand color fading by PWM mixing

   Fading from red to blue means red brightness decreases while blue brightness increases smoothly.

2. Step 2: Apply PWM duty cycle changes

   Decrease red duty cycle from 100% to 0%, increase blue from 0% to 100%, green stays off.

3. Final Answer:

   Gradually decrease red duty cycle from 100 to 0 while increasing blue from 0 to 100 -> Option C

4. Quick Check:

   Fade red down, blue up = A [OK]

Hint: Fade one color down, other up for smooth transition [OK]

Common Mistakes:

• Turning both colors fully on causes white, not fade
• Blinking one color doesn't create smooth fade
• Keeping all colors constant prevents fading