Inputs & Outputs: Button → LED

By the end of this project, you will:

• Understand what a GPIO pin is and how the same pin can be an input or an output.
• Configure pins with pinMode() and know why that belongs in setup().
• Read a pushbutton with digitalRead() and drive an LED with digitalWrite().
• Explain why a floating input is a problem, and fix it with a pull-down resistor.
• Recognise contact bounce and handle it with a short debounce delay.
• Wire an LED safely, respecting polarity and using a current-limiting resistor.

pinMode(buttonPin, INPUT);   // GPIO 4: read the button
pinMode(ledPin,   OUTPUT);  // GPIO 5: drive the LED

buttonState = digitalRead(buttonPin);
if (buttonState == HIGH) digitalWrite(ledPin, HIGH); // pressed  -> LED on
else                     digitalWrite(ledPin, LOW);  // released -> LED off

The LED lights when the pin reads HIGH, so the button sends 3.3 V to GPIO 4 when pressed, and a 10 kΩ pull‑down holds the pin LOW when released.

Both pins are used digitally (HIGH/LOW). Use 3V3 (never 5V) for the button's HIGH side.

LED circuit (output, GPIO 5)

1. GPIO 5 → 220 Ω → LED anode (long leg, +)
2. LED cathode (short leg, −) → GND rail

Button circuit (input, GPIO 4)

3. One side of the button → 3.3 V rail
4. Other side → GPIO 4
5. From that same GPIO 4 node → 10 kΩ → GND rail

LONG leg  = Anode  (+)  -> toward the 220 Ω / GPIO 5 side
SHORT leg = Cathode (-)  -> toward GND   (flat notch on the rim = - side)

If the LED never lights, check first that it isn't reversed.

Let's walk through the sketch so you understand what each line does and why.

Step 1: Pin definitions & state variables

const int buttonPin = 4;  // the pushbutton pin (digital input)
const int ledPin    = 5;  // the LED pin        (digital output)

int buttonState = 0;       // this round's reading: HIGH or LOW
int lastButtonState = -1;  // the previous reading, so we can spot changes

Naming the pins with const int means the number appears once: rewire the LED to another GPIO and you change one line. lastButtonState starts at -1, a value digitalRead() can never return, so the first real reading always counts as a change.

Step 2: one-time preparation in setup()

void setup() {
  Serial.begin(115200);        // open the USB link to the computer
  delay(500);                  // let the Serial Monitor connect

  pinMode(buttonPin, INPUT);   // GPIO 4 becomes an input
  pinMode(ledPin, OUTPUT);     // GPIO 5 becomes an output
}

setup() runs once at power-up or reset. pinMode() decides the direction of a pin: an input listens for a voltage someone else supplies, an output drives the pin to 3.3 V or 0 V itself. Every GPIO can do either, so you must say which you want.

Step 3: loop() reads → compares → acts

void loop() {
  buttonState = digitalRead(buttonPin);      // 1. READ the button

  if (buttonState != lastButtonState) {      // 2. did it CHANGE?
    if (buttonState == HIGH) {
      digitalWrite(ledPin, HIGH);            // 3. pressed  -> LED on
      Serial.println("[EVENT] Button PRESSED  -> LED is ON");
    } else {
      digitalWrite(ledPin, LOW);             //    released -> LED off
      Serial.println("[EVENT] Button RELEASED -> LED is OFF");
    }
    lastButtonState = buttonState;           // 4. remember for next time
  }

  delay(50);                                 // 5. debounce
}

loop() runs again and again forever, thousands of times per second. The if comparing this reading with the last one is what keeps the Serial Monitor usable: without it, holding the button down would print the same line endlessly.

Key concepts: what each line really does

Expected output on the Serial Monitor

========================================
 Project 1: ESP32 Inputs & Outputs
========================================
Button (input)  -> GPIO 4
LED    (output) -> GPIO 5
Press the button to turn the LED ON.
Waiting for button presses...
----------------------------------------
[EVENT] Button PRESSED  -> LED is ON
[EVENT] Button RELEASED -> LED is OFF

Each press and release prints exactly one line. If a single press prints several, the debounce delay is too short for your button.

Wire the button between GPIO 4 and GND and enable the internal pull‑up. This inverts the logic:

pinMode(buttonPin, INPUT_PULLUP);   // reads HIGH when NOT pressed
if (buttonState == LOW) digitalWrite(ledPin, HIGH); // pressed  -> LED on
else                    digitalWrite(ledPin, LOW);  // released -> LED off

This is the smallest complete physical-computing program: read the world, decide, act on the world. Everything later in the kit is a variation on it. The key takeaways:

• Every GPIO can be an input or an output. pinMode() in setup() decides which.
• digitalRead() returns one of two values, HIGH or LOW, and digitalWrite() drives a pin to 3.3 V or 0 V. This is digital I/O, the opposite of the analog reading you meet in Project 2.
• An input pin connected to nothing floats and reads noise. A pull-down resistor gives it a definite LOW when the button is open; INPUT_PULLUP (§7) does the same job in software, with the logic inverted.
• Mechanical buttons bounce. A short delay() after a change is the simplest cure.
• Comparing each reading against the previous one turns a fast polling loop into clean events, which is what makes the serial log readable.
• LEDs are polarized and always need a series resistor. 220 Ω keeps the current near 6 mA, well inside the ESP32's safe range.