Activity 4.2.3 — Pulse Width Modulation (PWM)¶

Learning Objectives¶

By the end of this lesson, students will be able to:

Explain what PWM is and how it controls average power
Calculate duty cycle and understand its relationship to output power
Use analogWrite() to control LED brightness
Control DC motor speed using PWM
Position servo motors using the Servo library

Vocabulary¶

Vocabulary (click to expand)

Term	Definition
PWM	Pulse Width Modulation - rapidly switching a signal on and off
Duty Cycle	Percentage of time a PWM signal is HIGH
Frequency	How often the PWM cycle repeats (Hz)
analogWrite()	Arduino function to output PWM signal
Servo Motor	A motor with precise angular position control

Part 1: What is PWM?¶

Definition¶

Pulse Width Modulation (PWM) is a technique that rapidly turns a signal on and off to control the average amount of power delivered to a device.

How It Works¶

Instead of varying the voltage, PWM switches between fully ON and fully OFF very quickly:

100% Duty Cycle (Always ON):
          ────────────────────────
          │                       │
          │                       │
──────────┘                       ┘

50% Duty Cycle (Half ON, Half OFF):
          ┌─────────┐             ┌─────────┐
          │         │             │         │
          │         │             │         │
──────────┘         └─────────────┘         └────

25% Duty Cycle (Mostly OFF):
          ┌──────┐                     ┌──────┐
          │      │                     │      │
          │      │                     │      │
──────────┘      └─────────────────────┘      └────

0% Duty Cycle (Always OFF):
          ────────────────────────────────
          │                               │
          │                               │
────────────────────────────────────────

Key Terms¶

Term	Definition
Period	Total time for one PWM cycle (frequency = 1/period)
Pulse Width	Time the signal is HIGH during each cycle
Duty Cycle	(Pulse Width / Period) × 100%

Calculating Duty Cycle¶

If a PWM signal is HIGH for 2ms and LOW for 2ms (period = 4ms):

Duty Cycle = (2ms / 4ms) × 100% = 50%

Why PWM Works¶

Our eyes cannot see flickering at high frequencies. The eye averages the light, seeing: - 0% duty cycle = OFF (0V average) - 50% duty cycle = DIM (2.5V average) - 100% duty cycle = FULL (5V average)

Part 2: PWM on Arduino¶

PWM Pins¶

Not all Arduino pins support PWM. Only pins marked with ~ can output PWM:

Arduino Uno: Pins 3, 5, 6, 9, 10, 11
These pins are controlled by timer registers

analogWrite() Function¶

Arduino provides analogWrite() to output PWM:

analogWrite(pin, value);

pin: PWM-capable pin number
value: 0 to 255 (0 = always off, 255 = always on)

Value	Duty Cycle	Average Voltage
0	0%	0V
64	25%	1.25V
128	50%	2.5V
191	75%	3.75V
255	100%	5V

Default PWM Frequency¶

Arduino PWM runs at approximately 490 Hz or 980 Hz (pins 5 and 6).

This is fast enough that: - LEDs appear to dim smoothly - Motors run smoothly - Humans don't hear the switching

Part 3: Project - LED Brightness Control¶

Circuit¶

Arduino Pin 9 ──[220Ω resistor]─── LED ── GND

Pin 9 is a PWM pin (marked with ~).

Code - Dimming an LED¶

int ledPin = 9;        // PWM-capable pin
int brightness = 0;    // Initial brightness (0-255)
int fadeAmount = 5;    // How much to change each step

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

void loop() {
  // Set LED brightness
  analogWrite(ledPin, brightness);

  // Change brightness for next iteration
  brightness = brightness + fadeAmount;

  // Reverse direction at ends
  if (brightness <= 0 || brightness >= 255) {
    fadeAmount = -fadeAmount;
  }

  delay(30);  // Wait 30ms before changing again
}

How It Works¶

Start with brightness = 0 (LED off)
Gradually increase brightness
When reaches 255, reverse direction
Gradually decrease back to 0
Repeat forever

The result: LED pulses smoothly from dim to bright and back.

Practice: Different Fade Speeds¶

Try changing these values: - fadeAmount = 1 - Very slow fade - fadeAmount = 20 - Fast pulse - delay(10) - Faster overall cycle - delay(100) - Slower overall cycle

Part 4: Project - DC Motor Speed Control¶

Circuit¶

Important: Motors need more current than Arduino can provide. Use a motor driver!

Arduino Pin 5 ──▶ L293D Input
Motor ──▶ L293D Output
+5V ──▶ L293D VCC2
GND ──▶ L293D GND

Code¶

int motorPin = 5;       // PWM pin connected to motor driver
int speed = 0;          // Motor speed (0-255)

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

void loop() {
  // Accelerate from stop to full speed
  for (speed = 0; speed <= 255; speed += 5) {
    analogWrite(motorPin, speed);
    delay(50);  // Gradual acceleration
  }

  // Hold at full speed
  delay(1000);

  // Decelerate to stop
  for (speed = 255; speed >= 0; speed -= 5) {
    analogWrite(motorPin, speed);
    delay(50);  // Gradual deceleration
  }

  delay(1000);  // Hold at stop
}

Controlling Direction¶

To control direction (forward/reverse), use an H-bridge like the L293D:

int enablePin = 5;    // Speed control (PWM)
int in1Pin = 4;       // Direction control
int in2Pin = 3;       // Direction control

void setup() {
  pinMode(enablePin, OUTPUT);
  pinMode(in1Pin, OUTPUT);
  pinMode(in2Pin, OUTPUT);
}

void setMotor(int speed, boolean reverse) {
  analogWrite(enablePin, speed);
  digitalWrite(in1Pin, !reverse);
  digitalWrite(in2Pin, reverse);
}

void loop() {
  setMotor(200, false);  // Forward at 78% speed
  delay(2000);
  setMotor(0, false);    // Stop
  delay(1000);
  setMotor(200, true);   // Reverse at 78% speed
  delay(2000);
  setMotor(0, false);    // Stop
  delay(1000);
}

Part 5: Servo Motors¶

What is a Servo?¶

A servo motor rotates to a specific angle (typically 0° to 180°) and holds that position.

Servo Control¶

Servos use a special PWM signal: - Pulse width determines angle - 20ms period (50 Hz refresh rate)

Pulse Width	Angle
1 ms	0°
1.5 ms	90°
2 ms	180°

Arduino Servo Library¶

Arduino includes a built-in Servo library:

#include <Servo.h>

Servo myServo;    // Create servo object
int pos = 0;      // Current position (angle)

void setup() {
  myServo.attach(9);  // Attach to pin 9
}

void loop() {
  // Sweep from 0 to 180 degrees
  for (pos = 0; pos <= 180; pos += 1) {
    myServo.write(pos);  // Move to position
    delay(15);           // Wait for servo to reach position
  }

  // Sweep back from 180 to 0 degrees
  for (pos = 180; pos >= 0; pos -= 1) {
    myServo.write(pos);
    delay(15);
  }
}

Key Servo Functions¶

Function	Description
`Servo.attach(pin)`	Connect servo to a pin
`Servo.write(angle)`	Move to angle (0-180)
`Servo.writeMicroseconds(us)`	Set pulse width in microseconds
`Servo.read()`	Read current angle

Part 6: Practice Problems¶

Problem 1: LED Breathing¶

Write a program that creates a "breathing" LED effect - starts at 0%, gradually increases to 100%, then back to 0%, repeating.

Show Solution

int ledPin = 9;
int brightness = 0;

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

void loop() {
  // Fade in
  for (int brightness = 0; brightness <= 255; brightness += 1) {
    analogWrite(ledPin, brightness);
    delay(10);
  }

  // Fade out
  for (int brightness = 255; brightness >= 0; brightness -= 1) {
    analogWrite(ledPin, brightness);
    delay(10);
  }

  delay(500);  // Pause at each end
}

Problem 2: Variable Speed Motor¶

Modify the motor speed control to use a potentiometer for speed control.

Show Solution

int motorPin = 5;
int potPin = A0;    // Potentiometer on analog pin 0
int speed = 0;

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

void loop() {
  // Read potentiometer (0-1023)
  int potValue = analogRead(potPin);

  // Map to motor speed (0-255)
  speed = map(potValue, 0, 1023, 0, 255);

  // Control motor
  analogWrite(motorPin, speed);

  delay(10);  // Small delay for stability
}

Problem 3: Servo Position Control¶

Write a program that moves a servo to 0°, 45°, 90°, 135°, 180° with a button press.

Show Solution

#include <Servo.h>

Servo myServo;
int servoPin = 9;
int buttonPin = 2;
int angles[] = {0, 45, 90, 135, 180};
int currentIndex = 0;

void setup() {
  myServo.attach(servoPin);
  pinMode(buttonPin, INPUT_PULLUP);
  myServo.write(angles[0]);  // Start at 0 degrees
}

void loop() {
  if (digitalRead(buttonPin) == LOW) {
    delay(50);  // Debounce
    currentIndex = (currentIndex + 1) % 5;  // Next angle
    myServo.write(angles[currentIndex]);

    // Wait for button release
    while (digitalRead(buttonPin) == LOW) {
      delay(10);
    }
    delay(50);  // Additional debounce
  }
}

Summary¶

PWM rapidly switches between ON and OFF to control average power
Duty cycle = percentage of time signal is HIGH
Arduino PWM: analogWrite(pin, value) where value = 0-255
LED dimming uses PWM (value controls brightness)
Motor speed uses PWM (value controls speed)
Servo motors use angle-based control via Servo library

Key Reminders¶

Only PWM pins (marked ~) support analogWrite()
Motors need motor drivers (L293D) - never connect directly to Arduino
Higher duty cycle = more power/brightness/speed
Servo library handles the special PWM timing automatically
0-255 maps to 0-100% duty cycle

Custom activity — adapted from PLTW Digital Electronics