Software PWM with RPi.GPIO in Raspberry Pi - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is PWM and why is it useful in controlling devices with Raspberry Pi?

PWM stands for Pulse Width Modulation. It lets you control the power delivered to devices like LEDs or motors by switching the power on and off very quickly. This way, you can make an LED dim or a motor run at different speeds without changing the voltage.

beginner

How do you set up a GPIO pin for software PWM using RPi.GPIO?

First, import RPi.GPIO and set the mode (e.g., BCM). Then, set the pin as an output using GPIO.setup(pin, GPIO.OUT). Finally, create a PWM object with GPIO.PWM(pin, frequency) where frequency is in Hertz.

beginner

What does the start() method do in RPi.GPIO PWM?

The start() method begins the PWM signal on the pin with a given duty cycle (percentage of time the signal is ON). For example, start(50) means the signal is ON half the time and OFF half the time.

beginner

How can you change the brightness of an LED using software PWM?

You change the duty cycle of the PWM signal. A higher duty cycle means the LED is ON longer in each cycle, making it brighter. A lower duty cycle means it is ON less, making it dimmer.

beginner

Why should you call stop() and cleanup() when you finish using PWM with RPi.GPIO?

stop() stops the PWM signal on the pin, and cleanup() resets all GPIO pins to their default state. This prevents unwanted signals and frees the pins for other uses.

What does PWM stand for?

A Power Wave Management

B Power Width Modulation

C Pulse Wire Method

D Pulse Width Modulation

PWM means Pulse Width Modulation, a technique to control power by switching on and off quickly.

Which RPi.GPIO method starts the PWM signal on a pin?

A pwm.start()

B GPIO.start()

C GPIO.begin()

D pwm.begin()

The start() method is called on the PWM object to begin the signal.

What does a 75% duty cycle mean for an LED controlled by PWM?

A LED is on 75% of the time

B LED brightness is 25%

C LED blinks 75 times per second

D LED is off 75% of the time

A 75% duty cycle means the LED is ON 75% of the time, making it brighter.

Why do you call GPIO.cleanup() after using PWM?

A To start PWM again

B To reset pins and avoid conflicts

C To increase PWM frequency

D To save power

cleanup() resets GPIO pins to default, preventing conflicts with other programs.

Which frequency unit is used when creating a PWM object in RPi.GPIO?

A Seconds

B Milliseconds

C Hertz

D Volts

Frequency for PWM is given in Hertz (cycles per second).

Explain how to set up and start software PWM on a Raspberry Pi GPIO pin using RPi.GPIO.

Think about the steps from preparing the pin to starting the PWM signal.

Describe how changing the duty cycle affects an LED's brightness when using software PWM.

Relate duty cycle to how long the LED is powered in each cycle.

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