Software PWM with RPi.GPIO in Raspberry Pi

Introduction

Software PWM lets you control devices like LEDs by turning a pin on and off very fast. This makes the device look like it is dimmed or at different power levels.

You want to dim an LED connected to your Raspberry Pi.

You need to control the speed of a small motor using your Pi.

You want to create different brightness levels for a light without extra hardware.

You want to generate sound tones by turning a pin on and off quickly.

Syntax

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(pin_number, GPIO.OUT)
pwm = GPIO.PWM(pin_number, frequency)
pwm.start(duty_cycle)

# To change brightness or speed
pwm.ChangeDutyCycle(new_duty_cycle)

# To stop PWM
pwm.stop()
GPIO.cleanup()

pin_number is the GPIO pin you connect your device to.

frequency is how fast the pin switches on and off (in Hertz).

Examples

This example sets GPIO 18 to output PWM at 1000 Hz with half brightness for 2 seconds.

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 1000)  # 1000 Hz frequency
pwm.start(50)  # 50% brightness

time.sleep(2)
pwm.stop()
GPIO.cleanup()

This example gradually increases brightness on GPIO 12 from 0% to 100% in steps.

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(12, GPIO.OUT)
pwm = GPIO.PWM(12, 500)  # 500 Hz frequency
pwm.start(0)  # Start with LED off

for dc in range(0, 101, 20):
    pwm.ChangeDutyCycle(dc)  # Increase brightness
    time.sleep(1)

pwm.stop()
GPIO.cleanup()

Sample Program

This program smoothly increases and then decreases the brightness of an LED connected to GPIO 17 using software PWM.

Raspberry Pi

import RPi.GPIO as GPIO
import time

# Use BCM pin numbering
GPIO.setmode(GPIO.BCM)

# Set GPIO 17 as output
GPIO.setup(17, GPIO.OUT)

# Create PWM instance at 500 Hz
pwm = GPIO.PWM(17, 500)

# Start PWM with 0% duty cycle (off)
pwm.start(0)

try:
    # Gradually increase brightness
    for duty_cycle in range(0, 101, 10):
        pwm.ChangeDutyCycle(duty_cycle)
        print(f"Brightness: {duty_cycle}%")
        time.sleep(0.5)

    # Gradually decrease brightness
    for duty_cycle in range(100, -1, -10):
        pwm.ChangeDutyCycle(duty_cycle)
        print(f"Brightness: {duty_cycle}%")
        time.sleep(0.5)

finally:
    pwm.stop()
    GPIO.cleanup()
    print("PWM stopped and GPIO cleaned up.")

Important Notes

Always call GPIO.cleanup() to reset pins after your program ends.

Software PWM is not perfect for very high precision or very fast signals.

Use try-finally to ensure PWM stops even if your program crashes.

Summary

Software PWM lets you control brightness or speed by switching pins on and off fast.

Use RPi.GPIO.PWM to create and control PWM on Raspberry Pi pins.

Always clean up GPIO pins after use to avoid issues.

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