Servo motor control with PWM in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PWM duty cycle print?

Consider this Python code controlling a servo motor's PWM on a Raspberry Pi. What will be printed?

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 50)  # 50Hz frequency
pwm.start(7.5)  # Neutral position

time.sleep(1)
print(f"Current duty cycle: {7.5}")
pwm.ChangeDutyCycle(12.5)
time.sleep(1)
print(f"Current duty cycle: {12.5}")
pwm.stop()
GPIO.cleanup()

A

Current duty cycle: 7.5
Current duty cycle: 12.5

B

Current duty cycle: 12.5
Current duty cycle: 7.5

C

Current duty cycle: 0
Current duty cycle: 100

D SyntaxError due to missing GPIO.cleanup()

💡 Hint

Look at the print statements and the values passed to ChangeDutyCycle.

The code prints the duty cycle values exactly as given in the print statements. The first print shows 7.5, the second shows 12.5. No syntax errors occur.

🧠 Conceptual

intermediate

What does a 50Hz PWM frequency mean for servo control?

In servo motor control using PWM on a Raspberry Pi, what does setting the PWM frequency to 50Hz imply?

A The PWM signal repeats 50 times per second, matching typical servo requirements

B The servo motor will rotate at 50 revolutions per second

C The duty cycle must always be 50%

D The PWM signal is too fast for any servo to respond

💡 Hint

Think about how often the PWM signal repeats per second.

50Hz means the PWM signal repeats 50 times each second, which is the standard frequency for many hobby servos to interpret position commands.

🔧 Debug

advanced

Why does this servo not move as expected?

This code is intended to move a servo to 0, 90, and 180 degrees but the servo stays still. What is the problem?

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 50)
pwm.start(0)

positions = [2.5, 7.5, 12.5]
for pos in positions:
    pwm.ChangeDutyCycle(pos)
    time.sleep(1)
pwm.stop()
GPIO.cleanup()

A The GPIO pin 18 is not set as input

B Starting PWM with 0 duty cycle causes servo not to respond; should start with 7.5

C The frequency 50Hz is too high for servo control

D The positions list values are out of range for duty cycle

💡 Hint

Think about the initial duty cycle value when starting PWM.

Starting PWM with 0 duty cycle means no signal is sent initially, so the servo may not respond. Starting with 7.5 (neutral) is standard to initialize the servo position.

📝 Syntax

advanced

Which option causes a SyntaxError in this PWM code?

Identify which code snippet will cause a syntax error when controlling a servo with PWM on Raspberry Pi.

A

pwm = GPIO.PWM(18, 50)
pwm.start(7.5)
pwm.ChangeDutyCycle(10)

B

pwm = GPIO.PWM(18, 50)
pwm.start(7.5)
pwm.ChangeDutyCycle(10.0)

C

pwm = GPIO.PWM(18, 50)
pwm.start(7.5)
pwm.ChangeDutyCycle(10

D

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

💡 Hint

Look carefully at parentheses in each option.

Option C is missing a closing parenthesis in ChangeDutyCycle call, causing a SyntaxError.

🚀 Application

expert

How many distinct positions can this servo code produce?

Given this code controlling a servo with PWM on Raspberry Pi, how many distinct servo positions will it produce?

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 50)
pwm.start(7.5)

for duty in range(25, 126, 25):  # duty cycles from 2.5 to 12.5 stepping by 2.5
    pwm.ChangeDutyCycle(duty / 10)
    time.sleep(0.5)
pwm.stop()
GPIO.cleanup()

A 6

B 4

C 3

D 5

💡 Hint

Count how many values the range produces and how they are used.

The range(25, 126, 25) produces 5 values: 25, 50, 75, 100, 125. Dividing by 10 gives 2.5, 5.0, 7.5, 10.0, 12.5 duty cycles, so 5 distinct positions.