#02 Beginner ⏱ 20 min

ESP32: Button turns on LED

The first digital input. Press a button → the LED lights. You learn what a pull-up resistor is and why the pin reads "1" when nothing is pressed.

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What you learn

INPUT_PULLUP and why it is neededdigitalRead and inverted logicPlacing a button across the grooveTwo GND rails

You need

ESP32 DevKit V1 — microcontroller with WiFi and Bluetooth
Breadboard — wiring without soldering
Jumper wires — M–M, M–F, F–F
LED — light-emitting diode 5 mm
Resistor — 220 Ω, 1 kΩ, 10 kΩ…
Push button — tactile switch 6×6 mm

How it works

The button sits across the groove of the breadboard. Inside, the two legs on the same side are permanently joined; pressing joins both sides too. By wiring diagonally opposite legs (25 top, 27 bottom) we are sure to pass through the switching contact.

The problem with an input: if the pin is connected to nothing, it "floats" and reads a random 0 or 1. A pull-up resistor (built into the ESP32, ~45 kΩ) gently pulls the pin to 3.3 V → a stable HIGH. The button connects it to GND and "beats" the resistor → LOW. That is why the logic is inverted: if (digitalRead(BTN) == LOW) = pressed.

Wiring

From	To	Why
ESP32 · D4 (GPIO 4)	Button, leg in column 25 (top)	The pin is an input with the internal pull-up on: without a press it reads HIGH (1).
Button, diagonal leg (column 27, bottom)	− rail	When you press, the button connects GPIO 4 directly to GND → the pin reads LOW (0).
ESP32 · D2	LED → 220 Ω → − rail	Same as in project 1 — the output we control.
ESP32 · GND (both pins)	− rails top and bottom	The top and bottom rails are not joined — so we use both GND pins.

Step by step

LED (anode 19c, cathode 20c) and resistor 20a → − rail, as in project 1.
The button: place it so it straddles rows e and f (across the groove), legs in columns 25 and 27.
Wire D4 → 25a (top half).
Short wire 27j → bottom − rail.
GND → top − rail, GND (second pin) → bottom − rail.
Upload the code. The LED lights while you hold the button. Open the Serial Monitor (115200) and watch the values.

Code (Arduino IDE)

// Project 2: Button turns on LED (with internal pull-up)
const int LED_PIN = 2;
const int BTN_PIN = 4;

void setup() {
  pinMode(LED_PIN, OUTPUT);
  pinMode(BTN_PIN, INPUT_PULLUP);  // internal resistor to 3.3 V
  Serial.begin(115200);
}

void loop() {
  int state = digitalRead(BTN_PIN); // 1 = released, 0 = pressed

  if (state == LOW) {
    digitalWrite(LED_PIN, HIGH);
    Serial.println("Pressed!");
  } else {
    digitalWrite(LED_PIN, LOW);
  }
  delay(20); // a little "settling" (debounce)
}

Typical problems

The button "works" without pressing? Your legs are on the same pair — move one leg to another column.

Flicker in the Serial Monitor on press = bouncing. Fix: delay(20–50) or a check with millis().

GPIO 34–39 will not work for this — no pull-up on them.

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