Software PWM with RPi.GPIO in Raspberry Pi - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the RPi.GPIO library.

Raspberry Pi

import [1] as GPIO

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

ARPi.GPIO

Bos

Ctime

Dgpiozero

Attempts:

3 left

💡 Hint

Common Mistakes

Importing 'gpiozero' instead of 'RPi.GPIO'.

Forgetting to import the library.

The RPi.GPIO library is imported with import RPi.GPIO as GPIO to control GPIO pins.

2fill in blank

medium

Complete the code to set the GPIO mode to BCM numbering.

Raspberry Pi

GPIO.setmode([1])

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

AGPIO.IN

BGPIO.BOARD

CGPIO.OUT

DGPIO.BCM

Attempts:

3 left

💡 Hint

Common Mistakes

Using GPIO.BOARD instead of GPIO.BCM.

Confusing mode constants with pin directions.

Use GPIO.BCM to set the pin numbering to Broadcom SOC channel numbers.

3fill in blank

hard

Fix the error in setting up pin 18 as output.

Raspberry Pi

GPIO.setup(18, [1])

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

AGPIO.IN

BGPIO.PWM

CGPIO.OUT

DGPIO.OUTP

Attempts:

3 left

💡 Hint

Common Mistakes

Using GPIO.IN instead of GPIO.OUT.

Using a non-existent mode like GPIO.PWM.

Pin 18 must be set as GPIO.OUT to output signals for PWM.

4fill in blank

hard

Fill both blanks to create a PWM instance on pin 18 with 100Hz frequency.

Raspberry Pi

pwm = GPIO.[1](18, [2])

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

APWM

B50

C100

DPin

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Pin' instead of 'PWM'.

Setting frequency to 50 instead of 100.

Use GPIO.PWM to create a PWM object and set frequency to 100Hz.

5fill in blank

hard

Fill all three blanks to start PWM with 50% duty cycle and then stop it.

Raspberry Pi

pwm.start([1])
# ... some code ...
pwm.[2]()
GPIO.[3]()

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

A50

Bstop

Ccleanup

Dstart

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'start' instead of 'stop' to stop PWM.

Forgetting to call GPIO.cleanup().

Start PWM with 50% duty cycle, stop it with stop(), and clean GPIO with cleanup().

Practice

1. What does Software PWM using RPi.GPIO.PWM allow you to control on a Raspberry Pi?

easy

A. The temperature of the Raspberry Pi CPU

B. The file system permissions on the Raspberry Pi

C. The network speed of the Raspberry Pi

D. The brightness of LEDs or speed of motors by switching pins on and off quickly

Solution

1. Step 1: Understand what PWM controls

   PWM (Pulse Width Modulation) controls how long a pin stays on versus off, affecting devices like LEDs or motors.

2. Step 2: Identify the effect of switching pins fast

   Switching pins on and off quickly changes brightness or speed smoothly.

3. Final Answer:

   The brightness of LEDs or speed of motors by switching pins on and off quickly -> Option D

4. Quick Check:

   Software PWM controls brightness/speed = B [OK]

Hint: PWM controls brightness or speed by fast switching [OK]

Common Mistakes:

• Confusing PWM with CPU or network control
• Thinking PWM changes file permissions
• Assuming PWM controls temperature

2. Which of the following is the correct way to start PWM on pin 18 with a frequency of 1000 Hz using RPi.GPIO?

easy

A. pwm = GPIO.PWM(1000, 18); pwm.start(50)

B. pwm = GPIO.PWM(18); pwm.start(1000)

C. pwm = GPIO.PWM(18, 1000); pwm.start(50)

D. pwm = GPIO.PWM(18, 50); pwm.start(1000)

Solution

1. Step 1: Check PWM constructor parameters

   The PWM constructor takes pin number first, then frequency in Hz, so GPIO.PWM(18, 1000) is correct.

2. Step 2: Verify start method usage

   The start method takes duty cycle percentage, so pwm.start(50) starts PWM at 50% duty cycle.

3. Final Answer:

   pwm = GPIO.PWM(18, 1000); pwm.start(50) -> Option C

4. Quick Check:

   Pin, frequency order + start with duty cycle = A [OK]

Hint: PWM(pin, freq), then start(duty cycle) [OK]

Common Mistakes:

• Swapping frequency and pin arguments
• Passing duty cycle to PWM constructor
• Using start() with frequency instead of duty cycle

3. What will be the output on the LED connected to pin 12 after running this code snippet?

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(12, GPIO.OUT)
pwm = GPIO.PWM(12, 500)
pwm.start(75)

medium

A. LED will be at 75% brightness

B. LED will blink slowly

C. LED will be off

D. Code will raise an error

Solution

1. Step 1: Analyze PWM setup

   Pin 12 is set as output, PWM started at 500 Hz frequency with 75% duty cycle.

2. Step 2: Understand duty cycle effect on LED

   75% duty cycle means LED is on 75% of the time, so it appears bright but not fully on.

3. Final Answer:

   LED will be at 75% brightness -> Option A

4. Quick Check:

   75% duty cycle = 75% brightness [OK]

Hint: Duty cycle % = LED brightness % [OK]

Common Mistakes:

• Thinking LED is off at start
• Assuming LED blinks instead of steady brightness
• Expecting code error without setup

4. Identify the error in this code snippet for PWM control:

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(21, GPIO.OUT)
pwm = GPIO.PWM(21, 1000)
pwm.start(110)

medium

A. Duty cycle value 110 is invalid; must be between 0 and 100

B. Pin 21 is not set as output

C. PWM frequency 1000 is too low

D. Missing GPIO.cleanup() call

Solution

1. Step 1: Check duty cycle range

   Duty cycle must be between 0 and 100; 110 is invalid and causes error.

2. Step 2: Verify pin setup and frequency

   Pin 21 is correctly set as output; frequency 1000 Hz is valid.

3. Final Answer:

   Duty cycle value 110 is invalid; must be between 0 and 100 -> Option A

4. Quick Check:

   Duty cycle > 100 invalid = D [OK]

Hint: Duty cycle must be 0-100% only [OK]

Common Mistakes:

• Using duty cycle above 100
• Forgetting to set pin as output
• Assuming frequency limits cause error
• Ignoring cleanup importance

5. You want to smoothly dim an LED connected to pin 17 from off to full brightness using Software PWM. Which approach correctly achieves this?

hard

A. Set duty cycle to 100 immediately and then to 0 after 5 seconds

B. Use a loop increasing duty cycle from 0 to 100 with small delays between steps

C. Change frequency from 0 to 1000 Hz gradually while keeping duty cycle constant

D. Call GPIO.cleanup() repeatedly in a loop while PWM is running

Solution

1. Step 1: Understand smooth dimming with PWM

   Smooth dimming requires gradually changing the duty cycle from 0% (off) to 100% (full brightness).

2. Step 2: Identify correct method

   Using a loop to increase duty cycle with small delays creates a smooth brightness change.

3. Final Answer:

   Use a loop increasing duty cycle from 0 to 100 with small delays between steps -> Option B

4. Quick Check:

   Gradual duty cycle change = smooth dimming [OK]

Hint: Increase duty cycle stepwise with delay for smooth dimming [OK]

Common Mistakes:

• Changing frequency instead of duty cycle
• Setting duty cycle instantly without steps
• Calling cleanup during PWM operation