PWM frequency and duty cycle relationship in Raspberry Pi - Mini Project: Build & Apply

PWM Frequency and Duty Cycle Relationship

📖 Scenario: You are working on a Raspberry Pi project that controls the brightness of an LED using PWM (Pulse Width Modulation). You want to understand how changing the PWM frequency and duty cycle affects the LED's brightness.

🎯 Goal: Build a simple Python program that sets up PWM on a GPIO pin, changes the frequency and duty cycle, and prints the current settings to observe their relationship.

📋 What You'll Learn

Create a variable for the GPIO pin number used for PWM

Create a variable for the initial PWM frequency

Create a variable for the initial duty cycle

Use a loop to change the duty cycle from 0 to 100 in steps

Print the current frequency and duty cycle values

💡 Why This Matters

🌍 Real World

PWM is used in many electronics projects to control LED brightness, motor speed, and other devices by adjusting the power delivered.

💼 Career

Understanding PWM is important for embedded systems, robotics, and hardware interfacing roles where precise control of electronic components is required.

1

Set up PWM pin and initial values

Create a variable called pwm_pin and set it to 18. Create a variable called frequency and set it to 1000. Create a variable called duty_cycle and set it to 0.

Raspberry Pi

# Your code here

Hint

Use simple variable assignments for pwm_pin, frequency, and duty_cycle.

2

Import RPi.GPIO and set up PWM

Import the RPi.GPIO module as GPIO. Set the GPIO mode to GPIO.BCM. Set up pwm_pin as an output pin. Create a PWM object called pwm on pwm_pin with the frequency variable frequency. Start PWM with the duty cycle variable duty_cycle.

Raspberry Pi

pwm_pin = 18
frequency = 1000
duty_cycle = 0
# Your code here

Hint

Remember to import the GPIO library and set the pin mode before starting PWM.

3

Change duty cycle in a loop

Use a for loop with variable dc to go from 0 to 100 in steps of 20. Inside the loop, change the PWM duty cycle to dc using pwm.ChangeDutyCycle(dc). Print the current frequency and duty cycle using print(f"Frequency: {frequency} Hz, Duty Cycle: {dc}%").

Raspberry Pi

import RPi.GPIO as GPIO

pwm_pin = 18
frequency = 1000
duty_cycle = 0

GPIO.setmode(GPIO.BCM)
GPIO.setup(pwm_pin, GPIO.OUT)
pwm = GPIO.PWM(pwm_pin, frequency)
pwm.start(duty_cycle)
# Your code here

Hint

Use a for loop with range(0, 101, 20) and update the duty cycle inside the loop.

4

Clean up and stop PWM

After the loop, stop the PWM using pwm.stop(). Clean up the GPIO pins using GPIO.cleanup(). Print "PWM test complete." to show the program finished.

Raspberry Pi

import RPi.GPIO as GPIO
import time

pwm_pin = 18
frequency = 1000
duty_cycle = 0

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

for dc in range(0, 101, 20):
    pwm.ChangeDutyCycle(dc)
    print(f"Frequency: {frequency} Hz, Duty Cycle: {dc}%")
    time.sleep(0.5)
# Your code here

Hint

Remember to stop PWM and clean up GPIO pins to avoid warnings. Then print the completion message.

Practice

1. What does the duty cycle in PWM control on a Raspberry Pi?

easy

A. The percentage of time the signal is ON during one cycle

B. The speed at which the signal pulses repeat

C. The voltage level of the signal

D. The total duration of the PWM signal

Solution

1. Step 1: Understand duty cycle meaning

   The duty cycle is the fraction of one cycle when the signal is ON (high).

2. Step 2: Differentiate from frequency

   Frequency controls how fast cycles repeat, not how long signal stays ON.

3. Final Answer:

   The percentage of time the signal is ON during one cycle -> Option A

4. Quick Check:

   Duty cycle = ON time percentage [OK]

Hint: Duty cycle = ON time percentage in each pulse [OK]

Common Mistakes:

• Confusing duty cycle with frequency
• Thinking duty cycle controls voltage
• Assuming duty cycle affects total signal duration

2. Which of the following is the correct Python syntax to set a PWM frequency to 1000 Hz on a Raspberry Pi GPIO pin using RPi.GPIO?

easy

A. pwm = GPIO.PWM(pin, 1000)

B. pwm = GPIO.setPWM(pin, 1000)

C. pwm = GPIO.PWM(pin).frequency(1000)

D. pwm = GPIO.frequency(pin, 1000)

Solution

1. Step 1: Recall RPi.GPIO PWM syntax

   The correct way to create a PWM instance is GPIO.PWM(pin, frequency).

2. Step 2: Check options for correct method and parameters

   pwm = GPIO.PWM(pin, 1000) matches the correct syntax; others use invalid methods or chaining.

3. Final Answer:

   pwm = GPIO.PWM(pin, 1000) -> Option A

4. Quick Check:

   GPIO.PWM(pin, freq) creates PWM [OK]

Hint: Use GPIO.PWM(pin, frequency) to set PWM frequency [OK]

Common Mistakes:

• Using non-existent methods like setPWM or frequency()
• Trying to set frequency after creating PWM without parameters
• Confusing method chaining syntax

3. Consider this Python code snippet controlling an LED brightness with PWM on Raspberry Pi:

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
pin = 18
GPIO.setup(pin, GPIO.OUT)
pwm = GPIO.PWM(pin, 500)
pwm.start(0)
pwm.ChangeDutyCycle(75)

What is the effect of setting the duty cycle to 75%?

medium

A. LED will be off

B. LED will be at full brightness

C. LED will blink 75 times per second

D. LED will be at 75% brightness

Solution

1. Step 1: Understand duty cycle effect on LED brightness

   Duty cycle of 75% means LED is ON 75% of the time, so brightness is about 75%.

2. Step 2: Confirm frequency and duty cycle usage

   Frequency 500 Hz controls pulse speed; duty cycle controls brightness level.

3. Final Answer:

   LED will be at 75% brightness -> Option D

4. Quick Check:

   Duty cycle 75% = 75% brightness [OK]

Hint: Duty cycle % roughly equals LED brightness % [OK]

Common Mistakes:

• Confusing frequency with blinking count
• Thinking duty cycle 75 means LED off
• Assuming full brightness only at 100% duty cycle

4. This code tries to change PWM frequency on Raspberry Pi but does not work as expected:

pwm = GPIO.PWM(18, 1000)
pwm.start(50)
pwm.changeFrequency(2000)

What is the error and how to fix it?

medium

A. ChangeFrequency is not a valid method; use pwm.ChangeFrequency(2000)

B. ChangeFrequency method name is incorrect; use pwm.ChangeFrequency(2000)

C. Frequency cannot be changed after PWM.start(); recreate PWM object

D. Method name is case sensitive; use pwm.ChangeFrequency(2000)

Solution

1. Step 1: Identify method name case sensitivity

   RPi.GPIO does not provide a method to change frequency after PWM object creation.

2. Step 2: Correct approach to change frequency

   To change frequency, stop PWM and create a new PWM instance with the new frequency.

3. Final Answer:

   Frequency cannot be changed after PWM.start(); recreate PWM object -> Option C

4. Quick Check:

   RPi.GPIO PWM frequency fixed at creation [OK]

Hint: To change frequency, recreate PWM object [OK]

Common Mistakes:

• Using wrong method name spelling
• Ignoring case sensitivity in Python
• Trying to change frequency by creating new PWM instance

5. You want to control a motor speed using PWM on Raspberry Pi. The motor requires a minimum frequency of 1000 Hz for smooth operation. If you set frequency to 1000 Hz and duty cycle to 20%, what happens if you increase frequency to 2000 Hz but keep duty cycle at 20%?

hard

A. Motor speed doubles because frequency doubled

B. Motor speed stays roughly the same because duty cycle is unchanged

C. Motor stops because frequency is too high

D. Motor speed halves because frequency doubled

Solution

1. Step 1: Understand frequency vs duty cycle effect on motor speed

   Motor speed depends mainly on duty cycle (power ON time), not frequency.

2. Step 2: Analyze frequency change with constant duty cycle

   Increasing frequency while keeping duty cycle constant keeps average power same, so speed stays similar.

3. Final Answer:

   Motor speed stays roughly the same because duty cycle is unchanged -> Option B

4. Quick Check:

   Speed ~ duty cycle, frequency affects smoothness [OK]

Hint: Motor speed depends on duty cycle, not frequency [OK]

Common Mistakes:

• Assuming frequency change affects speed directly
• Thinking doubling frequency doubles speed
• Ignoring duty cycle's role in power delivery