PWM frequency and duty cycle relationship in Raspberry Pi - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the PWM frequency to 1000 Hz.

Raspberry Pi

pwm = GPIO.PWM(pin, [1])

Drag options to blanks, or click blank then click option'

A500

B1000

C2000

D50

Attempts:

3 left

💡 Hint

Common Mistakes

Using a frequency too low or too high for the device.

Confusing frequency with duty cycle.

The frequency of PWM is set by the second argument in GPIO.PWM(). Here, 1000 Hz is the correct frequency.

2fill in blank

medium

Complete the code to set the PWM duty cycle to 75%.

Raspberry Pi

pwm.ChangeDutyCycle([1])

Drag options to blanks, or click blank then click option'

A50

B100

C25

D75

Attempts:

3 left

💡 Hint

Common Mistakes

Using values outside 0-100 range.

Confusing duty cycle with frequency.

The duty cycle is the percentage of time the signal is ON. 75 means 75% ON time.

3fill in blank

hard

Fix the error in setting the PWM frequency to 0 Hz (which is invalid).

Raspberry Pi

pwm = GPIO.PWM(pin, [1])

Drag options to blanks, or click blank then click option'

A100

B-50

C0

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Setting frequency to zero or negative values.

Passing None instead of a number.

Frequency cannot be zero or negative. 100 Hz is a valid frequency.

4fill in blank

hard

Fill both blanks to create a dictionary mapping duty cycle percentages to their ON time in milliseconds for a 1 kHz PWM signal.

Raspberry Pi

on_time_ms = {dc: dc / 100 [1] 1000 / freq for dc in duty_cycles if dc [2] 0}

Drag options to blanks, or click blank then click option'

A*

B/

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using division instead of multiplication for ON time.

Including duty cycles less than or equal to zero.

The ON time in milliseconds is calculated as (duty cycle / 100) * 1000 / frequency. We only include duty cycles greater than 0.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps duty cycle percentages to their OFF time in milliseconds for a given frequency.

Raspberry Pi

off_time_ms = {dc: (100 - dc) / 100 [1] 1000 [2] freq [3] 0}

Drag options to blanks, or click blank then click option'

A*

B/

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect operators for multiplication or division.

Including duty cycles less than or equal to zero.

The OFF time is calculated as (100 - duty cycle) / 100 * 1000 / frequency. We include only duty cycles greater than 0.

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