📘 Digital Input & Output

🎯 Lesson Objective

By the end of this lesson, students will:

• Understand digital signals (HIGH / LOW)

• Learn how to use pinMode()

• Learn how to use digitalWrite()

• Learn how to use digitalRead()

• Control an LED

• Read a button input

• Build simple decision-based hardware logic

1️⃣ What is Digital Signal?

Digital signal has only two states:

• HIGH → 1 → 3.3V

• LOW → 0 → 0V

Unlike analog (continuous values), digital is binary.

In ESP32:

HIGH = 3.3V
LOW = 0V

Digital signals are used for:

• LEDs

• Relays

• Buttons

• PIR sensors

• Buzzers

2️⃣ What is GPIO?

GPIO = General Purpose Input Output

These are pins on ESP32 that can:

• Send signals (Output)

• Receive signals (Input)

Example:

GPIO 2
GPIO 4
GPIO 5

These pins connect to real hardware.

3️⃣ Digital Output – Controlling LED

🔹 Step 1 – Circuit

• Connect LED positive to GPIO 2 (via resistor)

• Connect LED negative to GND

🔹 Step 2 – Code

int ledPin = 2;

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

void loop() {
digitalWrite(ledPin, HIGH);
delay(1000);
digitalWrite(ledPin, LOW);
delay(1000);
}

🔹 Explanation

pinMode(ledPin, OUTPUT);
→ Tells ESP32 this pin will send signal.

digitalWrite(ledPin, HIGH);
→ Sends 3.3V → LED ON

digitalWrite(ledPin, LOW);
→ Sends 0V → LED OFF

This is basic digital output control.

4️⃣ What is pinMode()?

Syntax:

Modes:

• OUTPUT

• INPUT

• INPUT_PULLUP

Example:

Always define pin mode inside setup().

5️⃣ Digital Input – Reading a Button

🔹 Circuit

• Connect one side of button to GPIO 4

• Other side to GND

• Enable internal pull-up

🔹 Code

int buttonPin = 4;

void setup() {
Serial.begin(115200);
pinMode(buttonPin, INPUT_PULLUP);
}

void loop() {
int buttonState = digitalRead(buttonPin);

Serial.println(buttonState);
delay(500);
}

🔹 Explanation

digitalRead(buttonPin);
→ Reads current state (HIGH or LOW)

With INPUT_PULLUP:

• Button not pressed → HIGH

• Button pressed → LOW

Why?

Because internal pull-up resistor keeps pin HIGH by default.

6️⃣ Controlling LED with Button

Now combine input and output.

int ledPin = 2;
int buttonPin = 4;

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

void loop() {
int buttonState = digitalRead(buttonPin);

if (buttonState == LOW) {
digitalWrite(ledPin, HIGH);
} else {
digitalWrite(ledPin, LOW);
}
}

Logic:

Button pressed → LED ON
Button released → LED OFF

This is basic automation logic.

7️⃣ Understanding INPUT_PULLUP

Without pull-up resistor:

Pin floats → unstable readings.

INPUT_PULLUP:

• Activates internal resistor

• Keeps pin stable

This is best practice for buttons.

8️⃣ Real IoT Example – Relay Control

Relay works same as LED.

int relayPin = 26;

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

void loop() {
digitalWrite(relayPin, HIGH); // Turn ON AC
}

Relay control = Digital output control.

9️⃣ Reading Digital Sensors

Digital sensors output HIGH or LOW.

Example:

PIR sensor:

int pirPin = 27;

void setup() {
Serial.begin(115200);
pinMode(pirPin, INPUT);
}

void loop() {
int motion = digitalRead(pirPin);

if (motion == HIGH) {
Serial.println("Motion Detected");
}
}

Digital input reading builds security systems.

🔟 Common Beginner Mistakes

❌ Forgetting pinMode()
❌ Using wrong pin number
❌ No resistor with LED
❌ Confusing HIGH/LOW logic
❌ Forgetting INPUT_PULLUP

Always double-check connections.

1️⃣1️⃣ Digital I/O vs Analog I/O

Digital = Binary
Analog = Range values

1️⃣2️⃣ Professional Coding Style

Better structure:

const int ledPin = 2;
const int buttonPin = 4;

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

void loop() {
controlLED();
}

void controlLED() {
if (digitalRead(buttonPin) == LOW) {
digitalWrite(ledPin, HIGH);
} else {
digitalWrite(ledPin, LOW);
}
}

This uses modular programming.

📌 Lesson Summary

In this lesson, we learned:

• What digital signals are

• What GPIO pins are

• How to use pinMode()

• How to use digitalWrite()

• How to use digitalRead()

• How to use INPUT_PULLUP

• Controlling LED

• Reading button

• Basic automation logic

You now understand how programming controls real hardware.