PWM frequency and duty cycle relationship in Raspberry Pi

Introduction

PWM helps control devices by turning power on and off quickly. Frequency and duty cycle decide how fast and how long the power is on.

To control the brightness of an LED by changing how long it stays on.

To adjust the speed of a motor by changing how fast the power pulses.

To create sounds by changing the frequency of the pulses.

To control heating elements by adjusting power delivery timing.

Syntax

Raspberry Pi

import RPi.GPIO as GPIO
import time

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

pwm = GPIO.PWM(pin, frequency)  # frequency in Hz
pwm.start(duty_cycle)          # duty_cycle in percent (0-100)

# To change duty cycle later:
pwm.ChangeDutyCycle(new_duty_cycle)

# To change frequency later:
pwm.ChangeFrequency(new_frequency)

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

Frequency is how many times the signal repeats per second (Hz).

Duty cycle is the percentage of time the signal is ON in each cycle.

Examples

This sets a PWM on pin 18 with 1000 pulses per second, ON half the time.

Raspberry Pi

pwm = GPIO.PWM(18, 1000)  # 1000 Hz frequency
pwm.start(50)               # 50% duty cycle

Now the signal stays ON 75% of each cycle, making the device brighter or faster.

Raspberry Pi

pwm.ChangeDutyCycle(75)  # Change duty cycle to 75%

This slows the pulses to 500 times per second, which can affect sound or motor smoothness.

Raspberry Pi

pwm.ChangeFrequency(500)  # Change frequency to 500 Hz

Sample Program

This program starts PWM on pin 18 at 1000 Hz with 10% duty cycle. It then increases the duty cycle in steps, making the device get brighter or faster. After that, it changes the frequency to 500 Hz and waits before cleaning up.

Raspberry Pi

import RPi.GPIO as GPIO
import time

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

pwm = GPIO.PWM(pin, 1000)  # 1000 Hz frequency
pwm.start(10)             # Start with 10% duty cycle

try:
    for dc in range(10, 101, 30):  # Increase duty cycle from 10% to 100%
        print(f"Setting duty cycle to {dc}%")
        pwm.ChangeDutyCycle(dc)
        time.sleep(1)
    pwm.ChangeFrequency(500)  # Change frequency to 500 Hz
    print("Frequency changed to 500 Hz")
    time.sleep(2)
finally:
    pwm.stop()
    GPIO.cleanup()

Important Notes

Higher frequency means faster pulses, which can make motors run smoother or LEDs flicker less.

Changing duty cycle changes how much power the device gets without changing frequency.

Always clean up GPIO pins to avoid warnings or errors on next run.

Summary

PWM frequency controls how fast the power pulses repeat.

Duty cycle controls how long the power stays ON in each pulse.

Changing frequency or duty cycle changes device behavior like brightness or speed.

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