📘 Lesson F9 – PWM (Pulse Width Modulation) and Signal Basics

🎯 Learning Objectives

After completing this lesson, students will be able to:

✅ Understand what PWM is

✅ Understand why PWM is needed

✅ Identify PWM pins on Arduino UNO

✅ Understand Duty Cycle

✅ Understand Frequency

✅ Control LED brightness using PWM

✅ Control motor speed using PWM

✅ Understand the difference between Digital Output and PWM Output

✅ Use PWM in real-world projects

1. Introduction

In previous lessons, we learned that digital pins can generate only two states:

HIGH (5V)

or

LOW (0V)

But many electronic devices require something between ON and OFF.

Examples:

• LED Brightness Control
• Motor Speed Control
• Fan Speed Control
• Servo Signal Generation
• Light Dimming Systems

Now the question is:

How can Arduino generate intermediate values if digital pins only understand HIGH and LOW?

The answer is:

PWM (Pulse Width Modulation)

2. What is PWM?

PWM stands for:

Pulse Width Modulation

PWM is a technique used to simulate analog output using digital signals.

Instead of producing a constant voltage, Arduino rapidly switches a pin:

ON

↓

OFF

↓

ON

↓

OFF

↓

ON

↓

OFF

thousands of times every second.

Because switching happens very fast, electronic devices behave as if they are receiving a lower voltage.

3. Understanding PWM with a Fan Example

Imagine a table fan.

Method 1

Power OFF

Fan Stops

Method 2

Power ON

Fan Runs Full Speed

But in real life we have:

• Speed 1
• Speed 2
• Speed 3
• Speed 4

PWM allows Arduino to achieve similar speed control.

4. Why PWM is Needed

Without PWM:

Digital Pins can only provide:

OFF

or

ON

With PWM:

Arduino can simulate:

• 10% Power
• 20% Power
• 50% Power
• 75% Power
• 100% Power

This makes smooth control possible.

5. PWM Pins on Arduino UNO

Not all digital pins support PWM.

Arduino UNO PWM Pins are:

These pins are marked with:

~

symbol on the board.

6. How PWM Works

Consider a digital signal:

HIGH HIGH HIGH HIGH HIGH

Output:

100% Power

Now consider:

HIGH LOW HIGH LOW HIGH LOW

Output:

50% Power

Now consider:

HIGH LOW LOW LOW HIGH LOW LOW LOW

Output:

25% Power

The percentage of ON time determines output power.

This percentage is called:

Duty Cycle

7. What is Duty Cycle?

Duty Cycle represents:

Percentage of ON Time

during one PWM cycle.

Formula

Duty Cycle(%)=ON TimeTotal Time×100Duty\ Cycle(\%)=\frac{ON\ Time}{Total\ Time}\times100Duty Cycle(%)=Total TimeON Time​×100

Examples

0% Duty Cycle

Pin always OFF

Output = 0V

25% Duty Cycle

Pin ON 25% of the time

Output appears low

50% Duty Cycle

Pin ON half the time

Medium power

75% Duty Cycle

Pin ON most of the time

High power

100% Duty Cycle

Pin always ON

Full power

8. Visualizing Duty Cycle

0%

__________

Always OFF

25%

|_|____|_|____

50%

|__|__|__|__

75%

|____|_|____|_

100%

||||||||||||||

Always ON

9. What is Frequency?

PWM signals consist of repeated cycles.

The number of cycles per second is called:

Frequency

Unit:

Hertz (Hz)

Example

1 Cycle per Second

=

1 Hz

1000 Cycles per Second

=

1000 Hz

Arduino UNO PWM frequency is typically around:

490 Hz

for most PWM pins.

10. PWM and Voltage Relationship

PWM does not actually change voltage.

Arduino still outputs:

0V

or

5V

Only.

However the average voltage changes.

This creates the illusion of analog voltage.

11. analogWrite() Function

Arduino uses:

analogWrite(pin,value);

for PWM output.

Syntax

analogWrite(pin,value);

Parameters

pin

PWM pin number

value

PWM value

Range:

0 to 255

Why 255?

Arduino uses:

8-bit PWM Resolution

Values:

0 – 255

Total:

256 Levels

PWM Value Examples

12. LED Brightness Control Example

void setup()
{
}

void loop()
{
analogWrite(9,50);
}

LED glows dimly.

analogWrite(9,128);

Medium brightness.

analogWrite(9,255);

Maximum brightness.

13. Motor Speed Control

PWM is commonly used for motors.

Example:

25% Duty Cycle

Motor rotates slowly.

50% Duty Cycle

Medium speed.

100% Duty Cycle

Maximum speed.

Why Motors Need PWM

Without PWM:

Motor only:

• Start
• Stop

With PWM:

Motor speed becomes controllable.

14. Applications of PWM

PWM is used everywhere.

LED Brightness Control

Adjust light intensity.

DC Motor Speed Control

Robot movement control.

Fan Speed Control

Airflow regulation.

Light Dimmers

Home automation systems.

Audio Signal Generation

Simple sound generation.

Robotics

Precise motion control.

15. PWM vs Digital Output

16. Common Beginner Mistakes

Mistake 1

Using analogWrite() on non-PWM pins.

Result:

PWM may not work.

Mistake 2

Using values above 255.

Wrong:

analogWrite(9,500);

Correct:

analogWrite(9,255);

Mistake 3

Expecting true analog voltage.

PWM is only a simulation.

Mistake 4

Confusing Analog Pins with PWM Pins.

PWM uses specific digital pins.

17. Real World Example

Line Follower Robot

Robot detects black line.

Arduino calculates correction.

PWM controls:

Left Motor Speed

Right Motor Speed

Result:

Smooth turning.

Without PWM:

Robot movement would be jerky.

18. Industrial Applications

PWM is widely used in:

• CNC Machines
• Industrial Robots
• Electric Vehicles
• Motor Drives
• Solar Systems
• Smart Home Devices

📊 Summary

In this lesson, we learned:

✅ What PWM is

✅ Why PWM is required

✅ PWM pins on Arduino UNO

✅ Duty Cycle

✅ Frequency

✅ analogWrite()

✅ LED brightness control

✅ Motor speed control

✅ Real-world applications

PWM is one of the most important concepts in embedded systems because it allows digital devices to behave like analog control systems.

📖 Key Terms

PWM

Pulse Width Modulation

Duty Cycle

Percentage of ON time in one cycle

Frequency

Number of cycles per second

Hertz

Unit of frequency

analogWrite()

Function used to generate PWM signals

PWM Pin

Pin capable of generating PWM output

🎯 Quiz

1. What does PWM stand for?

A. Pulse Width Modulation ✅

B. Power Wave Management

C. Pulse Wave Monitoring

D. Power Width Modulation

2. Which pins support PWM on Arduino UNO?

A. D3, D5, D6, D9, D10, D11 ✅

B. A0–A5

C. D0–D5

D. All pins

3. What is the range of analogWrite() values?

A. 0–100

B. 0–255 ✅

C. 0–512

D. 0–1023

4. Which function is used for PWM output?

A. digitalWrite()

B. analogRead()

C. analogWrite() ✅

D. digitalRead()

5. What is the duty cycle at PWM value 255?

A. 25%

B. 50%

C. 75%

D. 100% ✅

🏠 Assignment

Task 1

List all PWM pins available on Arduino UNO.

Task 2

Create a table showing PWM values and corresponding brightness percentages.

Task 3

Explain duty cycle with diagrams.

Task 4

Research three industrial applications of PWM.

Task 5

Write a simple Arduino program that gradually increases LED brightness using PWM.