PWM Explained Simply — Motor Control for ECE Students

The Problem: Digital Pins Are All or Nothing

A digital output pin on Arduino has exactly two states: 5V (HIGH) or 0V (LOW). On or off. There is no middle ground — no 2.5V, no 3V. It is a binary world.

But the real world is not binary. A motor should run at 50% speed for a careful turn, not full throttle. An LED should dim gracefully, not just blink. A servo motor needs to hold at exactly 90 degrees. How do you get variable output from a pin that only knows on and off?

What Is PWM?

PWM stands for Pulse Width Modulation. The key word is modulation — you are changing (modulating) the width of the ON pulse within each cycle to control how much power is effectively delivered.

Duty Cycle — The Core Concept

In each PWM cycle (which happens thousands of times per second), the pin is ON for some portion of the time and OFF for the rest. The percentage of time it is ON is called the duty cycle.

• 0% duty cycle — always OFF (0V average) — motor stopped, LED off
• 25% duty cycle — on for 1/4 of each cycle — motor at 25% speed
• 50% duty cycle — on half the time — motor at half speed, LED at half brightness
• 75% duty cycle — on for 3/4 of each cycle — motor at 75%
• 100% duty cycle — always ON (5V) — full speed, full brightness

The switching happens so fast (490–980 Hz on Arduino) that a motor or LED responds to the average voltage, not the individual pulses. The result is smooth, continuous-looking control.

PWM in Arduino Code

Arduino makes PWM simple with one function: analogWrite(pin, value). Despite the name, it does not output a true analog voltage — it outputs a PWM signal. The value goes from 0 (0% duty, always off) to 255 (100% duty, always on).

// LED with 220 ohm resistor on pin 9 (PWM pin ~)
const int LED_PIN = 9;

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

void loop() {
  // Fade in from off to full brightness
  for (int brightness = 0; brightness <= 255; brightness++) {
    analogWrite(LED_PIN, brightness);
    delay(8);  // 8ms × 256 steps = ~2 seconds to fade in
  }

  // Fade out from full brightness to off
  for (int brightness = 255; brightness >= 0; brightness--) {
    analogWrite(LED_PIN, brightness);
    delay(8);
  }
  // Loop repeats — LED fades in and out continuously
}

Controlling Motor Speed

This is where PWM becomes genuinely useful. DC motors draw more current than any microcontroller pin can supply — trying to drive a motor directly from an Arduino pin will likely damage the Arduino. You need a motor driver IC between the Arduino and the motor.

The most common motor driver for beginners is the L298N. It has an ENA pin (Enable A) and ENB pin (Enable B) — connect your PWM signal here to control speed. The IN1/IN2 pins control direction.

// L298N Motor Driver connections:
// ENA → Arduino pin 9 (PWM)
// IN1 → Arduino pin 7
// IN2 → Arduino pin 8
// Motor A terminals → your DC motor
// L298N VCC → 9V or 12V battery, GND to Arduino GND
// Potentiometer middle pin → A0

const int ENA = 9;  // PWM speed control
const int IN1 = 7;  // Direction control
const int IN2 = 8;
const int POT = A0;

void setup() {
  pinMode(ENA, OUTPUT);
  pinMode(IN1, OUTPUT);
  pinMode(IN2, OUTPUT);
  Serial.begin(9600);

  // Set motor direction: IN1 HIGH, IN2 LOW = forward
  digitalWrite(IN1, HIGH);
  digitalWrite(IN2, LOW);
}

void loop() {
  int potValue = analogRead(POT);           // 0 to 1023
  int speed = map(potValue, 0, 1023, 0, 255); // Map to PWM range

  analogWrite(ENA, speed);  // Set motor speed via PWM

  Serial.print("Speed: ");
  Serial.println(speed);

  delay(50);
}

Servo Motors and PWM

Servo motors are different from DC motors. Instead of controlling speed, PWM controls their angle. A standard servo rotates between 0° and 180° based on the pulse width it receives: a 1ms pulse → 0°, a 1.5ms pulse → 90°, a 2ms pulse → 180°.

You do not have to calculate pulse widths yourself. The Arduino Servo library handles it with a simple angle value.

#include <Servo.h>

Servo myServo;  // Create servo object

void setup() {
  myServo.attach(9);  // Servo signal wire → pin 9
  // Servo power: red → 5V, brown/black → GND
}

void loop() {
  // Sweep from 0 to 180 degrees
  for (int angle = 0; angle <= 180; angle++) {
    myServo.write(angle);
    delay(15);  // Give servo time to reach position
  }

  // Sweep back from 180 to 0 degrees
  for (int angle = 180; angle >= 0; angle--) {
    myServo.write(angle);
    delay(15);
  }
}