Motor speed control with PWM in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

PWM Signal Frequency Calculation

What is the frequency of the PWM signal generated by analogWrite() on an Arduino Uno pin 9 by default?

A Approximately 490 Hz

B Approximately 31 kHz

C Approximately 1 kHz

D Approximately 60 Hz

💡 Hint

Arduino Uno uses timers with default PWM frequencies around a few hundred Hz on most pins.

On Arduino Uno, analogWrite() on pins 3, 9, 10, and 11 uses Timer1 with default PWM frequency about 490 Hz.

❓ Predict Output

intermediate

Motor Speed Control Code Output

What will be the motor speed value printed by this Arduino code snippet?

int motorPin = 9;
int speed = 128;

void setup() {
  Serial.begin(9600);
  analogWrite(motorPin, speed);
  Serial.println(speed);
}

void loop() {}

Arduino

int motorPin = 9;
int speed = 128;

void setup() {
  Serial.begin(9600);
  analogWrite(motorPin, speed);
  Serial.println(speed);
}

void loop() {}

A 64

B 255

C 0

D 128

💡 Hint

The analogWrite() function sets the PWM duty cycle but does not change the variable.

The code prints the value of speed which is 128. This value sets the PWM duty cycle to about 50%.

🔧 Debug

advanced

Identify the Error in PWM Motor Control Code

What error will this Arduino code cause when trying to control motor speed with PWM?

int motorPin = 9;

void setup() {
  pinMode(motorPin, INPUT);
}

void loop() {
  analogWrite(motorPin, 200);
  delay(1000);
}

Arduino

int motorPin = 9;

void setup() {
  pinMode(motorPin, INPUT);
}

void loop() {
  analogWrite(motorPin, 200);
  delay(1000);
}

A Arduino will reset continuously

B No output, motor will not run because pin is set as INPUT

C Motor runs at full speed ignoring PWM value

D Syntax error due to missing semicolon

💡 Hint

Check the pin mode before using analogWrite().

Setting the pin as INPUT disables PWM output. The motor will not receive the PWM signal and will not run.

🧠 Conceptual

advanced

PWM Duty Cycle Effect on Motor Speed

Which statement best describes how PWM duty cycle affects the speed of a DC motor?

A Higher duty cycle means higher average voltage, increasing motor speed

B Lower duty cycle increases motor speed by reducing voltage

C Duty cycle does not affect motor speed, only direction

D PWM frequency controls speed, duty cycle controls torque

💡 Hint

Think about how average power to the motor changes with duty cycle.

Increasing the PWM duty cycle increases the average voltage applied to the motor, which increases its speed.

❓ Predict Output

expert

PWM Value After Analog Write and Read Back

Consider this Arduino code snippet. What will be the output on the Serial Monitor?

int motorPin = 9;
int pwmValue = 180;

void setup() {
  Serial.begin(9600);
  pinMode(motorPin, OUTPUT);
  analogWrite(motorPin, pwmValue);
  int readValue = analogRead(motorPin);
  Serial.println(readValue);
}

void loop() {}

Arduino

int motorPin = 9;
int pwmValue = 180;

void setup() {
  Serial.begin(9600);
  pinMode(motorPin, OUTPUT);
  analogWrite(motorPin, pwmValue);
  int readValue = analogRead(motorPin);
  Serial.println(readValue);
}

void loop() {}

A 1023

B 180

C A value between 0 and 1023 depending on noise, usually near 0

D Compilation error due to analogRead on PWM pin

💡 Hint

Remember that analogRead() reads voltage on analog input pins, not PWM output pins.

Reading an analog value from a digital PWM output pin returns unpredictable values, often near 0, because PWM pins are not analog inputs.

Practice

1. What does PWM do in motor speed control on an Arduino?

easy

A. It changes the motor's voltage permanently.

B. It switches power on and off quickly to control speed.

C. It changes the motor's direction.

D. It measures the motor's speed.

Solution

1. Step 1: Understand PWM function

   PWM (Pulse Width Modulation) controls speed by rapidly switching power on and off.

2. Step 2: Compare options

   Only It switches power on and off quickly to control speed. correctly describes PWM's role in speed control, others describe different functions.

3. Final Answer:

   It switches power on and off quickly to control speed. -> Option B

4. Quick Check:

   PWM controls speed by switching power on/off [OK]

Hint: PWM controls speed by fast power switching [OK]

Common Mistakes:

• Thinking PWM changes voltage permanently
• Confusing speed control with direction control
• Believing PWM measures speed

2. Which Arduino function is used to set motor speed with PWM?

easy

A. pinMode(pin, OUTPUT)

B. digitalWrite(pin, value)

C. analogWrite(pin, value)

D. delay(value)

Solution

1. Step 1: Identify PWM function

   analogWrite(pin, value) sets PWM signal on a pin with value 0-255 controlling speed.

2. Step 2: Check other functions

   digitalWrite sets HIGH/LOW only, pinMode sets pin mode, delay pauses code; none control PWM speed.

3. Final Answer:

   analogWrite(pin, value) -> Option C

4. Quick Check:

   Use analogWrite for PWM speed control [OK]

Hint: Use analogWrite to set PWM speed value [OK]

Common Mistakes:

• Using digitalWrite instead of analogWrite
• Confusing pinMode with speed control
• Trying to use delay to control speed

3. What will be the motor speed if the code runs

analogWrite(9, 128);

?

medium

A. Motor runs at full speed

B. Motor is off

C. Motor runs at double speed

D. Motor runs at about half speed

Solution

1. Step 1: Understand PWM value range

   analogWrite value ranges 0 (off) to 255 (full speed). 128 is about half of 255.

2. Step 2: Interpret speed from value

   Value 128 means roughly half the power, so motor runs at about half speed.

3. Final Answer:

   Motor runs at about half speed -> Option D

4. Quick Check:

   128/255 ≈ half speed [OK]

Hint: PWM 128 means about half speed [OK]

Common Mistakes:

• Thinking 128 means full speed
• Assuming motor is off at 128
• Believing speed can be doubled

4. Identify the error in this code snippet for motor speed control:

int speed = 300;
analogWrite(6, speed);

medium

A. Speed value is out of PWM range (0-255).

B. Pin 6 cannot be used for PWM.

C. analogWrite requires three arguments.

D. Variable speed must be declared as float.

Solution

1. Step 1: Check PWM value limits

   analogWrite accepts values from 0 to 255. 300 is outside this range.

2. Step 2: Verify other options

   Pin 6 supports PWM, analogWrite takes two arguments, and int is fine for speed.

3. Final Answer:

   Speed value is out of PWM range (0-255). -> Option A

4. Quick Check:

   PWM value must be 0-255 [OK]

Hint: PWM values must be 0 to 255 only [OK]

Common Mistakes:

• Using values above 255 for PWM
• Thinking analogWrite needs 3 arguments
• Believing PWM pins are limited incorrectly

5. You want to smoothly increase motor speed from 0 to full over 5 seconds using PWM on pin 10. Which code snippet achieves this?

hard

A. for(int speed=0; speed<=255; speed++) { analogWrite(10, speed); delay(20); }

B. analogWrite(10, 255); delay(5000); analogWrite(10, 0);

C. for(int speed=255; speed>=0; speed--) { analogWrite(10, speed); delay(20); }

D. digitalWrite(10, HIGH); delay(5000); digitalWrite(10, LOW);

Solution

1. Step 1: Understand smooth speed increase

   Gradually increasing PWM value from 0 to 255 in small steps with delay creates smooth speed change.

2. Step 2: Analyze code snippets

   for(int speed=0; speed<=255; speed++) { analogWrite(10, speed); delay(20); } uses a for loop from 0 to 255 with delay(20)ms, totaling about 5 seconds. Others either jump speed or use digitalWrite which is not PWM.

3. Final Answer:

   for(int speed=0; speed<=255; speed++) { analogWrite(10, speed); delay(20); } -> Option A

4. Quick Check:

   Gradual PWM increase with delay = smooth speed [OK]

Hint: Use for loop with analogWrite and delay for smooth speed [OK]

Common Mistakes:

• Using digitalWrite instead of analogWrite
• Jumping speed without gradual steps
• Not using delay to slow speed change