RGB LED color mixing in Raspberry Pi - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does RGB stand for in RGB LED color mixing?

RGB stands for Red, Green, and Blue. These are the three colors combined in different amounts to create many colors.

beginner

How do you create the color yellow using an RGB LED?

To make yellow, turn on the Red and Green LEDs together with full brightness, and keep Blue off.

intermediate

Why do we use PWM (Pulse Width Modulation) when controlling RGB LEDs on Raspberry Pi?

PWM lets us control the brightness of each color by turning the LED on and off very fast, making it look dimmer or brighter.

beginner

What happens if you set all three colors (Red, Green, Blue) to full brightness on an RGB LED?

The LED will show white light because mixing all three colors at full brightness creates white.

beginner

In Raspberry Pi programming, which library is commonly used to control GPIO pins for RGB LEDs?

The RPi.GPIO library is commonly used to control GPIO pins on Raspberry Pi for RGB LED color mixing.

Which two colors do you mix to get cyan on an RGB LED?

A Red, Green, and Blue

B Red and Blue

C Red and Green

D Green and Blue

Cyan is made by mixing Green and Blue light.

What does PWM control when used with RGB LEDs on Raspberry Pi?

A The brightness of each color

B The color temperature

C The power supply voltage

D The LED size

PWM controls how bright each color LED appears by adjusting the on/off timing.

If you want to make purple using an RGB LED, which colors should you mix?

A Red and Green

B Red and Blue

C Green and Blue

D All three colors

Purple is made by mixing Red and Blue light.

Which Raspberry Pi pin type is used to control an RGB LED?

A Ground pins only

B Power pins only

C GPIO pins

D Audio pins

GPIO pins are used to send signals to control the RGB LED.

What color results from turning off all three RGB LED colors?

A Black (LED off)

B White

C Yellow

D Purple

Turning off all colors means no light, so the LED appears black or off.

Explain how you can mix colors using an RGB LED on a Raspberry Pi.

Think about how turning on different LEDs at different brightness levels changes the color.

Describe the role of GPIO pins and PWM in controlling an RGB LED on Raspberry Pi.

Consider how the Raspberry Pi talks to the LED and changes its light.

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