Servo motor control with PWM in Raspberry Pi - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the PWMOutputDevice class from the gpiozero library.

Raspberry Pi

from gpiozero import [1]

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

APWM

BPWMOutputDevice

CServo

DLED

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Servo' instead of 'PWMOutputDevice' because Servo is a different class.

Using 'LED' which is unrelated to PWM control.

The PWMOutputDevice class is used to control pulse width modulation signals on Raspberry Pi GPIO pins.

2fill in blank

medium

Complete the code to create a PWM object on GPIO pin 17.

Raspberry Pi

servo = PWMOutputDevice([1], frequency=50)

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

A18

B27

C22

D17

Attempts:

3 left

💡 Hint

Common Mistakes

Using pin 18 which is often used for PWM but not the one specified here.

Using pin 22 or 27 which are valid GPIO pins but not the correct one.

GPIO pin 17 is commonly used for PWM control of servos on Raspberry Pi.

3fill in blank

hard

Fix the error in setting the servo pulse width to 1.5 milliseconds.

Raspberry Pi

servo.value = [1]

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

A0.075

B1.5

C0.5

D1500

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1.5 directly which is out of range.

Using 1500 which is a pulse width in microseconds, not a fraction.

The servo.value expects a duty cycle value between 0 and 1. At 50Hz (20ms period), 1.5ms pulse width corresponds to 1.5/20 = 0.075.

4fill in blank

hard

Fill both blanks to create a dictionary mapping angles to PWM values for 0 and 90 degrees.

Raspberry Pi

angle_to_pwm = {0: [1], 90: [2]

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

A0.0

B0.075

C0.05

D1.0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0.0 for 0 degrees which gives no pulse.

Using 1.0 for 90 degrees which is maximum duty cycle.

For servo control at 50Hz, 0.05 corresponds to ~1ms pulse (0 degrees) and 0.075 to ~1.5ms pulse (90 degrees) duty cycle.

5fill in blank

hard

Fill all three blanks to complete the code that sets the servo to 45 degrees using the angle_to_pwm dictionary.

Raspberry Pi

angle = 45
pwm_value = (angle_to_pwm[0] + angle_to_pwm[90]) / 2
servo.value = [1]

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

Apwm_value

Bangle

C0.25

Dservo.value

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'angle' which is just the angle, not the PWM value.

Using 'servo.value' which is the property, not a value.

The variable pwm_value holds the PWM value for 45 degrees, so we assign it to servo.value.

Practice

1. What does PWM control in a servo motor connected to a Raspberry Pi?

easy

A. The temperature of the servo motor

B. The angle position of the servo arm

C. The brightness of an LED

D. The speed of the Raspberry Pi processor

Solution

1. Step 1: Understand PWM role in servo control

   PWM (Pulse Width Modulation) sends signals to set the servo arm's angle by changing the pulse width.

2. Step 2: Identify what PWM affects

   It does not control speed, brightness, or temperature but the position of the servo arm.

3. Final Answer:

   The angle position of the servo arm -> Option B

4. Quick Check:

   PWM controls servo angle = D [OK]

Hint: PWM changes pulse width to set servo angle [OK]

Common Mistakes:

• Confusing PWM with speed control
• Thinking PWM controls LED brightness here
• Assuming PWM changes temperature

2. Which of the following is the correct way to start PWM on GPIO pin 18 at 50 Hz in Python using RPi.GPIO?

easy

A. pwm = GPIO.PWM(18, 50); pwm.start(7.5)

B. pwm = GPIO.PWM(50, 18); pwm.start(7.5)

C. pwm = GPIO.PWM(18); pwm.start(50)

D. pwm = GPIO.PWM(7.5, 18); pwm.start(50)

Solution

1. Step 1: Check PWM initialization syntax

   GPIO.PWM(pin, frequency) initializes PWM on the specified pin and frequency.

2. Step 2: Verify correct parameters

   Pin 18 and frequency 50 Hz are correct; start() takes duty cycle like 7.5.

3. Final Answer:

   pwm = GPIO.PWM(18, 50); pwm.start(7.5) -> Option A

4. Quick Check:

   GPIO.PWM(pin, freq) with start(duty) = B [OK]

Hint: Remember GPIO.PWM(pin, freq) then start(duty) [OK]

Common Mistakes:

• Swapping pin and frequency order
• Missing frequency argument
• Passing duty cycle in PWM() instead of start()

3. What will be the output angle of the servo if the duty cycle is set to 12.5% using PWM at 50 Hz?

medium

A. 0 degrees

B. 90 degrees

C. 270 degrees

D. 180 degrees

Solution

1. Step 1: Understand duty cycle to angle mapping

   At 50 Hz, about 2.5% duty cycle corresponds to 0°, 7.5% to 90°, and 12.5% to 180°.

2. Step 2: Match 12.5% duty cycle to angle

   12.5% duty cycle sets the servo to its maximum angle, 180°.

3. Final Answer:

   180 degrees -> Option D

4. Quick Check:

   12.5% duty cycle = 180° [OK]

Hint: Max duty cycle ~12.5% means 180° angle [OK]

Common Mistakes:

• Confusing duty cycle values with angles
• Assuming 12.5% is 90 degrees
• Thinking servo can rotate beyond 180 degrees

4. Given this code snippet, what is the error?

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 50)
pwm.start(7.5)
# Missing cleanup

medium

A. GPIO pins are not cleaned up after use

B. PWM frequency is incorrect

C. Duty cycle value is out of range

D. GPIO mode is not set

Solution

1. Step 1: Review code for setup and cleanup

   The code sets mode, pin, and starts PWM correctly but does not call GPIO.cleanup().

2. Step 2: Identify missing cleanup importance

   Not cleaning up GPIO pins can cause warnings or errors on next runs.

3. Final Answer:

   GPIO pins are not cleaned up after use -> Option A

4. Quick Check:

   Missing GPIO.cleanup() = A [OK]

Hint: Always call GPIO.cleanup() after PWM use [OK]

Common Mistakes:

• Ignoring GPIO.cleanup()
• Thinking frequency or duty cycle is wrong here
• Forgetting to set GPIO mode

5. You want to sweep a servo motor from 0° to 180° smoothly using PWM on Raspberry Pi. Which approach is best?

hard

A. Use a fixed duty cycle of 12.5% without delay

B. Set duty cycle to 7.5% and wait for 5 seconds

C. Change duty cycle gradually from 2.5% to 12.5% in small steps with delays

D. Change frequency from 50 Hz to 100 Hz rapidly

Solution

1. Step 1: Understand servo sweeping method

   Sweeping means moving smoothly from 0° to 180°, which requires changing duty cycle gradually.

2. Step 2: Identify correct PWM control

   Duty cycle must change from about 2.5% (0°) to 12.5% (180%) in small steps with short delays for smooth motion.

3. Final Answer:

   Change duty cycle gradually from 2.5% to 12.5% in small steps with delays -> Option C

4. Quick Check:

   Smooth sweep = gradual duty cycle change [OK]

Hint: Sweep by small duty cycle steps with delays [OK]

Common Mistakes:

• Using fixed duty cycle for sweeping
• Changing frequency instead of duty cycle
• Skipping delays causing jerky motion