Software PWM with RPi.GPIO in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PWM duty cycle print?

Consider this Python code using RPi.GPIO to create a software PWM on pin 18. What will be printed?

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 100)  # 100 Hz frequency
pwm.start(0)  # Start with 0% duty cycle

for dc in range(0, 101, 50):
    pwm.ChangeDutyCycle(dc)
    print(f"Duty Cycle set to: {dc}%")
    time.sleep(0.1)

pwm.stop()
GPIO.cleanup()

A

Duty Cycle set to: 0%
Duty Cycle set to: 50%

B

Duty Cycle set to: 0%
Duty Cycle set to: 50%
Duty Cycle set to: 100%

C

Duty Cycle set to: 0%
Duty Cycle set to: 50%
Duty Cycle set to: 100%
Duty Cycle set to: 150%

D

Duty Cycle set to: 0%
Duty Cycle set to: 25%
Duty Cycle set to: 50%
Duty Cycle set to: 75%
Duty Cycle set to: 100%

💡 Hint

Look at the range function parameters and step size.

The range(0, 101, 50) generates values 0, 50, and 100. So the loop prints three lines with those duty cycles.

❓ Predict Output

intermediate

What error does this PWM code raise?

What error will this code raise when run on a Raspberry Pi?

Raspberry Pi

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 0)  # Frequency set to 0
pwm.start(50)

A RuntimeError: PWM frequency cannot be zero

B TypeError: frequency argument must be int

C No error, PWM starts with 50% duty cycle

D ValueError: PWM frequency must be greater than 0

💡 Hint

Think about what frequency zero means for PWM.

RPi.GPIO requires a positive frequency for PWM. Setting frequency to zero raises a ValueError.

🔧 Debug

advanced

Why does this PWM code not change brightness smoothly?

This code is intended to smoothly increase LED brightness using PWM. Why does the LED jump abruptly instead of smoothly changing brightness?

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 1000)
pwm.start(0)

for dc in range(0, 101, 20):
    pwm.ChangeDutyCycle(dc)
    time.sleep(0.5)

pwm.stop()
GPIO.cleanup()

A PWM must be started with 100% duty cycle for smooth change

B The frequency 1000 Hz is too high for LED brightness control

C The step size of 20 is too large for smooth brightness change

D time.sleep(0.5) is too short to see brightness change

💡 Hint

Think about how many steps the brightness changes in.

The duty cycle changes in big jumps (0, 20, 40, 60, 80, 100), causing abrupt brightness changes instead of smooth.

📝 Syntax

advanced

Which option fixes the syntax error in this PWM code?

This code snippet has a syntax error. Which option fixes it correctly?

Raspberry Pi

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 100)
pwm.start(50
pwm.ChangeDutyCycle(75)
pwm.stop()
GPIO.cleanup()

A Add a closing parenthesis after 50: pwm.start(50)

B Replace pwm.start(50 with pwm.start[50]

C Add a colon after pwm.start(50: pwm.start(50):

D Remove pwm.start(50 entirely

💡 Hint

Look carefully at the parentheses in the pwm.start line.

The line pwm.start(50 is missing a closing parenthesis. Adding it fixes the syntax error.

🚀 Application

expert

How many PWM cycles occur in 2 seconds at 500 Hz?

You set up software PWM on a Raspberry Pi with frequency 500 Hz. How many complete PWM cycles will occur in 2 seconds?

A 1000

B 250

C 2000

D 500

💡 Hint

Multiply frequency by time in seconds.

At 500 Hz, 500 cycles happen each second. In 2 seconds, 500 * 2 = 1000 cycles occur.