DC motor with transistor driver in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Arduino sketch controlling a DC motor speed?

Consider this Arduino code snippet that uses PWM to control a DC motor speed through a transistor driver. What will be the motor speed value printed on the serial monitor?

Arduino

const int motorPin = 9;
int speed = 128;

void setup() {
  pinMode(motorPin, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  analogWrite(motorPin, speed);
  Serial.println(speed);
  delay(1000);
}

A 64

B 255

C 128

D 0

💡 Hint

Look at the value assigned to speed and what is printed.

The variable speed is set to 128 and printed directly. The motor speed is controlled by PWM with this value.

🧠 Conceptual

intermediate

Which component protects the transistor from voltage spikes when the motor stops?

In a DC motor driver circuit using a transistor, which component is essential to protect the transistor from voltage spikes generated by the motor's inductive load?

A Capacitor

B LED

C Resistor

D Flyback diode

💡 Hint

Think about what happens when the motor suddenly stops and the transistor switches off.

The flyback diode provides a path for the current generated by the motor's inductance, preventing high voltage spikes that can damage the transistor.

🔧 Debug

advanced

Why does this Arduino code fail to run the motor at full speed?

Analyze the following Arduino code intended to run a DC motor at full speed using a transistor driver. Identify the reason the motor does not run at full speed.

Arduino

const int motorPin = 9;

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

void loop() {
  analogWrite(motorPin, 256);
  delay(1000);
}

A PWM value 256 is invalid; max is 255

B Pin 9 cannot output PWM

C Missing digitalWrite to turn motor on

D Delay is too short to see effect

💡 Hint

Check the valid range for PWM values in Arduino.

Arduino PWM values must be between 0 and 255. Using 256 causes unexpected behavior or no output.

📝 Syntax

advanced

Which option fixes the syntax error in this transistor driver control code?

Identify the correct syntax to set the transistor control pin as output and turn the motor on.

Arduino

int motorPin = 8

void setup() {
  pinMode(motorPin, OUTPUT)
  digitalWrite(motorPin, HIGH);
}

A Add semicolons after variable declaration and pinMode line

B Change pinMode to pinmode and add semicolon after digitalWrite

C Replace int with const int and remove digitalWrite line

D Add parentheses after motorPin declaration

💡 Hint

Look carefully at missing punctuation in Arduino C++ code.

Arduino code requires semicolons after statements. Missing semicolons cause syntax errors.

🚀 Application

expert

How many PWM steps are possible controlling the motor speed with analogWrite on Arduino?

Using analogWrite() on an Arduino to control a transistor-driven DC motor, how many distinct speed levels can you set?

A 1024

B 256

C 128

D 512

💡 Hint

Recall the resolution of Arduino's 8-bit PWM.

Arduino's analogWrite() uses 8-bit PWM, allowing 256 distinct levels (0 to 255).

Practice

1. What is the main role of a transistor when controlling a DC motor with an Arduino?

easy

A. To act as a switch that controls the motor safely

B. To increase the motor speed directly

C. To power the Arduino board

D. To measure the motor's temperature

Solution

1. Step 1: Understand transistor function

   A transistor acts like a switch or amplifier in circuits.

2. Step 2: Apply to motor control

   In Arduino motor control, the transistor switches the motor on/off safely without damaging the board.

3. Final Answer:

   To act as a switch that controls the motor safely -> Option A

4. Quick Check:

   Transistor = switch for motor control [OK]

Hint: Think of transistor as a safe on/off switch for the motor [OK]

Common Mistakes:

• Confusing transistor with motor speed controller
• Thinking transistor powers the Arduino
• Assuming transistor measures anything

2. Which Arduino code snippet correctly sets a transistor pin to control a DC motor?

easy

A. pinMode(9, INPUT); digitalRead(9);

B. pinMode(9, INPUT); digitalWrite(9, HIGH);

C. pinMode(9, OUTPUT); digitalRead(9);

D. pinMode(9, OUTPUT); digitalWrite(9, HIGH);

Solution

1. Step 1: Set pin as output for controlling transistor

   To control a transistor, the Arduino pin must be OUTPUT.

2. Step 2: Write HIGH to turn motor on

   digitalWrite(9, HIGH) sends voltage to transistor base to switch motor on.

3. Final Answer:

   pinMode(9, OUTPUT); digitalWrite(9, HIGH); -> Option D

4. Quick Check:

   Output pin + HIGH = motor ON [OK]

Hint: Use OUTPUT mode and digitalWrite to control transistor [OK]

Common Mistakes:

• Setting pin as INPUT when controlling transistor
• Using digitalRead instead of digitalWrite
• Not setting pinMode before writing

3. What will be the effect of this Arduino code controlling a transistor connected to a DC motor?

int motorPin = 9;
void setup() {
  pinMode(motorPin, OUTPUT);
}
void loop() {
  analogWrite(motorPin, 128);
  delay(2000);
  analogWrite(motorPin, 0);
  delay(2000);
}

medium

A. Motor runs at half speed for 2 seconds, then stops for 2 seconds repeatedly

B. Motor runs at full speed continuously

C. Motor never runs because analogWrite is wrong

D. Motor runs at full speed for 2 seconds, then stops for 2 seconds repeatedly

Solution

1. Step 1: Understand analogWrite value

   analogWrite with 128 sets PWM to about 50% duty cycle, so motor runs at half speed.

2. Step 2: Analyze timing in loop

   Motor runs at half speed for 2 seconds, then analogWrite(0) stops motor for 2 seconds, repeating.

3. Final Answer:

   Motor runs at half speed for 2 seconds, then stops for 2 seconds repeatedly -> Option A

4. Quick Check:

   analogWrite(128) = half speed [OK]

Hint: PWM 128 means half speed, 0 means stop [OK]

Common Mistakes:

• Thinking analogWrite(128) is full speed
• Confusing analogWrite with digitalWrite
• Ignoring delay timing effects

4. Identify the error in this Arduino code controlling a transistor for a DC motor:

int motorPin = 9;
void setup() {
  pinMode(motorPin, INPUT);
}
void loop() {
  digitalWrite(motorPin, HIGH);
}

medium

A. Pin number 9 cannot be used for motor control

B. Pin is set as INPUT but digitalWrite is used

C. digitalWrite should be digitalRead

D. Missing delay in loop

Solution

1. Step 1: Check pinMode for transistor control

   Pin controlling transistor must be OUTPUT, not INPUT.

2. Step 2: Identify conflict with digitalWrite

   digitalWrite on an INPUT pin causes undefined behavior or no effect.

3. Final Answer:

   Pin is set as INPUT but digitalWrite is used -> Option B

4. Quick Check:

   OUTPUT needed for digitalWrite [OK]

Hint: Set pinMode to OUTPUT before digitalWrite [OK]

Common Mistakes:

• Using INPUT mode with digitalWrite
• Confusing digitalWrite with digitalRead
• Ignoring pinMode setup

5. You want to control a DC motor speed using PWM on pin 9 with a transistor driver. Which additional component is essential to protect your circuit from voltage spikes caused by the motor?

hard

A. An LED connected in parallel with the transistor

B. A capacitor connected to the Arduino 5V pin

C. A diode connected across the motor terminals

D. A resistor in series with the motor

Solution

1. Step 1: Understand motor voltage spikes

   When motor stops suddenly, it can create voltage spikes that damage components.

2. Step 2: Identify protection method

   A diode placed across motor terminals (flyback diode) safely redirects spikes, protecting transistor and Arduino.

3. Final Answer:

   A diode connected across the motor terminals -> Option C

4. Quick Check:

   Flyback diode protects circuit from spikes [OK]

Hint: Always add diode across motor for spike protection [OK]

Common Mistakes:

• Skipping diode and risking transistor damage
• Using resistor instead of diode for spike protection
• Confusing capacitor or LED as spike protection