Software PWM with RPi.GPIO in Raspberry Pi - Mini Project: Build & Apply

Software PWM with RPi.GPIO

📖 Scenario: You have a Raspberry Pi and want to control the brightness of an LED using software PWM (Pulse Width Modulation). This means turning the LED on and off very quickly to simulate different brightness levels.We will use the RPi.GPIO library to control the GPIO pin connected to the LED.

🎯 Goal: Create a Python program that uses software PWM with RPi.GPIO to control the brightness of an LED connected to GPIO pin 18.

📋 What You'll Learn

Use the RPi.GPIO library to control GPIO pins

Set up GPIO pin 18 as an output

Create a software PWM instance on GPIO pin 18 with a frequency of 100 Hz

Start PWM with 0% duty cycle

Change the duty cycle to 50%

Stop PWM and clean up GPIO at the end

💡 Why This Matters

🌍 Real World

Software PWM is used to control devices like LEDs, motors, and buzzers by simulating analog output using digital pins.

💼 Career

Understanding PWM and GPIO control is essential for embedded systems, robotics, and IoT device programming.

1

Set up GPIO and pin

Import the RPi.GPIO library as GPIO. Set the GPIO mode to GPIO.BCM. Set up GPIO pin 18 as an output pin.

Raspberry Pi

import RPi.GPIO as GPIO

# Set GPIO mode to BCM
# Set up GPIO pin 18 as output
# Your code here

Hint

Use GPIO.setmode(GPIO.BCM) to select pin numbering by BCM. Use GPIO.setup(18, GPIO.OUT) to set pin 18 as output.

2

Create PWM instance

Create a PWM instance called pwm on GPIO pin 18 with a frequency of 100 Hz.

Raspberry Pi

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)

# Create PWM instance pwm on pin 18 with 100 Hz frequency
# Your code here

Hint

Use GPIO.PWM(pin, frequency) to create a PWM object.

3

Start PWM with 0% duty cycle and change to 50%

Start the PWM on pwm with a duty cycle of 0 percent. Then change the duty cycle to 50 percent.

Raspberry Pi

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)

pwm = GPIO.PWM(18, 100)

# Start PWM with 0% duty cycle
# Change duty cycle to 50%
# Your code here

Hint

Use pwm.start(0) to start PWM with 0% brightness. Use pwm.ChangeDutyCycle(50) to set brightness to 50%.

4

Stop PWM and clean up GPIO

Stop the PWM on pwm and clean up the GPIO pins using GPIO.cleanup(). Then print "PWM stopped and GPIO cleaned up".

Raspberry Pi

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)

pwm = GPIO.PWM(18, 100)

pwm.start(0)
pwm.ChangeDutyCycle(50)

# Stop PWM
# Clean up GPIO
# Print confirmation message
# Your code here

Hint

Use pwm.stop() to stop PWM and GPIO.cleanup() to reset GPIO pins.

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