Software PWM with RPi.GPIO in Raspberry Pi - Step-by-Step Execution

Concept Flow - Software PWM with RPi.GPIO

Start Program

↓

Setup GPIO Pin

↓

Enter Loop

↓

Set Pin HIGH

↓

Wait ON Duration

↓

Set Pin LOW

↓

Wait OFF Duration

↓

Repeat Loop

↩Back to Enter Loop

The program sets up a GPIO pin, then repeatedly turns it ON and OFF with delays to create a PWM signal.

Execution Sample

Raspberry Pi

import RPi.GPIO as GPIO
import time

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

while True:
    GPIO.output(18, GPIO.HIGH)
    time.sleep(0.001)
    GPIO.output(18, GPIO.LOW)
    time.sleep(0.009)

This code creates a software PWM on pin 18 with 10% duty cycle by turning the pin ON for 1ms and OFF for 9ms repeatedly.

Execution Table

Step	Action	Pin 18 State	Delay (seconds)	Loop Iteration
1	Set pin 18 HIGH	HIGH	0	1
2	Wait ON duration	HIGH	0.001	1
3	Set pin 18 LOW	LOW	0	1
4	Wait OFF duration	LOW	0.009	1
5	Set pin 18 HIGH	HIGH	0	2
6	Wait ON duration	HIGH	0.001	2
7	Set pin 18 LOW	LOW	0	2
8	Wait OFF duration	LOW	0.009	2
...	...	...	...	...

💡 Loop runs indefinitely until program is stopped manually.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	After Step 8	...final
Pin 18 State	LOW (default)	HIGH	LOW	HIGH	LOW	Repeats HIGH/LOW
Loop Iteration	0	1	1	2	2	Increments each cycle

Key Moments - 3 Insights

Why does the pin state change between HIGH and LOW repeatedly?

Because the code sets the pin HIGH, waits (ON time), then sets it LOW, waits (OFF time), creating a pulse. See execution_table steps 1-4.

How does the duty cycle relate to the ON and OFF delays?

Duty cycle is ON time divided by total period (ON + OFF). Here, ON is 0.001s and OFF is 0.009s, so duty cycle is 10%. See delays in execution_table.

Why is this called software PWM?

Because the timing is controlled by software delays (time.sleep), not hardware PWM modules. The loop manually toggles the pin.

Visual Quiz - 3 Questions

Look at the execution_table, what is the pin state at Step 3?

A HIGH

B LOW

C Undefined

D Still HIGH

💡 Hint

Refer to Step 3 in execution_table where pin 18 is set LOW.

At which step does the first OFF delay happen?

A Step 2

B Step 1

C Step 4

D Step 3

💡 Hint

Check execution_table for delay after pin is set LOW, which is Step 4.

If the ON delay is increased to 0.005 seconds, how does the duty cycle change?

A Duty cycle increases

B Duty cycle decreases

C Duty cycle stays the same

D Duty cycle becomes zero

💡 Hint

Duty cycle = ON time / (ON + OFF). Increasing ON time increases duty cycle.

Concept Snapshot

Software PWM with RPi.GPIO:
- Setup GPIO pin as output
- Loop: set pin HIGH, wait ON time
- Set pin LOW, wait OFF time
- Repeat for continuous PWM
- Duty cycle = ON time / (ON + OFF)

Full Transcript

This example shows how to create software PWM on a Raspberry Pi using the RPi.GPIO library. The program sets a GPIO pin as output and enters a loop. In each loop, it sets the pin HIGH, waits for the ON duration, then sets the pin LOW and waits for the OFF duration. This cycle repeats indefinitely, creating a pulse-width modulated signal. The duty cycle is controlled by adjusting the ON and OFF wait times. This method uses software delays, so timing is less precise than hardware PWM but is simple to implement.

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