DC motor with transistor driver in Arduino

Introduction

We use a transistor to control a DC motor because the motor needs more power than the Arduino can provide directly. The transistor acts like a switch that turns the motor on and off safely.

When you want to turn a DC motor on or off using an Arduino.

When the motor needs more current than the Arduino pin can supply.

When you want to control the motor speed using PWM signals.

When you want to protect your Arduino from high current damage.

When building simple robots or fans controlled by Arduino.

Syntax

Arduino

int motorPin = 9;  // Arduino pin connected to transistor base

void setup() {
  pinMode(motorPin, OUTPUT);  // Set pin as output
}

void loop() {
  digitalWrite(motorPin, HIGH);  // Turn motor ON
  delay(1000);                   // Wait 1 second
  digitalWrite(motorPin, LOW);   // Turn motor OFF
  delay(1000);                   // Wait 1 second
}

The transistor base is connected to an Arduino digital pin through a resistor.

Use a diode across the motor terminals to protect from voltage spikes.

Examples

Simple ON/OFF control of the motor using digital signals.

Arduino

digitalWrite(motorPin, HIGH);  // Motor ON

digitalWrite(motorPin, LOW);   // Motor OFF

Use PWM to control motor speed by changing the signal's duty cycle.

Arduino

analogWrite(motorPin, 128);  // Motor runs at half speed (PWM)

Always set the pin mode to OUTPUT to control the transistor properly.

Arduino

pinMode(motorPin, OUTPUT);  // Set pin as output before controlling motor

Sample Program

This program turns the motor on for 2 seconds, then off for 2 seconds, repeatedly. The transistor acts as a switch controlled by the Arduino pin.

Arduino

int motorPin = 9;  // Pin connected to transistor base

void setup() {
  pinMode(motorPin, OUTPUT);  // Set motorPin as output
}

void loop() {
  digitalWrite(motorPin, HIGH);  // Turn motor ON
  delay(2000);                   // Run motor for 2 seconds
  digitalWrite(motorPin, LOW);   // Turn motor OFF
  delay(2000);                   // Wait for 2 seconds
}

Important Notes

Always connect a diode (called a flyback diode) across the motor terminals to protect the transistor from voltage spikes.

Use a resistor (around 1k ohm) between Arduino pin and transistor base to limit current.

Make sure your transistor can handle the motor's current.

Summary

A transistor lets Arduino safely control a DC motor by acting as a switch.

Use digitalWrite to turn the motor on or off through the transistor.

Protect your circuit with a diode and resistor for safe operation.

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