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.