RGB LED color mixing in Raspberry Pi - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the red LED pin as an output.

Raspberry Pi

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
GPIO.setup([1], GPIO.OUT)

Drag options to blanks, or click blank then click option'

A18

B17

C22

D27

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong GPIO pin number.

Forgetting to set the pin as output.

The red LED is connected to GPIO pin 17, so we set it as output using GPIO.setup(17, GPIO.OUT).

2fill in blank

medium

Complete the code to turn on the green LED.

Raspberry Pi

GPIO.output([1], GPIO.HIGH)

Drag options to blanks, or click blank then click option'

A17

B27

C22

D18

Attempts:

3 left

💡 Hint

Common Mistakes

Using GPIO.LOW instead of GPIO.HIGH.

Using the wrong pin number.

The green LED is connected to GPIO pin 22, so turning it on requires GPIO.output(22, GPIO.HIGH).

3fill in blank

hard

Fix the error in the code to turn off the blue LED.

Raspberry Pi

GPIO.output([1], GPIO.LOW)

Drag options to blanks, or click blank then click option'

A18

B27

C22

D17

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong pin number for the blue LED.

Using GPIO.HIGH instead of GPIO.LOW.

The blue LED is connected to GPIO pin 18, so to turn it off use GPIO.output(18, GPIO.LOW).

4fill in blank

hard

Fill both blanks to create a dictionary mapping colors to their GPIO pins.

Raspberry Pi

led_pins = [1]: 17, [2]: 22, 'blue': 18}

Drag options to blanks, or click blank then click option'

A'red'

B'yellow'

C'green'

D'white'

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect color names as keys.

Forgetting quotes around string keys.

The dictionary keys for the red and green LEDs are 'red' and 'green' respectively, matching their GPIO pins.

5fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps colors to their pin states when turned on.

Raspberry Pi

led_states = [2]: GPIO.HIGH for [2], {{BLANK_4}} in led_pins.items()

Drag options to blanks, or click blank then click option'

A{

Bcolor

Cpin

D[

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets instead of curly braces.

Mixing up variable names in the loop.

The dictionary comprehension starts with '{', iterates over color and pin in led_pins.items(), and sets each color's state to GPIO.HIGH.

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