E03. 3.3 V Logic, Power Rails and Level Shifting

What you will do

Learn the one rule that protects every ESP32: no 5 V into the pins. Then wire a button and an LED the safe way.

Sketch 1 reads the button and prints pressed or open. Sketch 2 lights the LED while you hold the button down.

Never put 5 V into an ESP32 pin

Uno pins use 5 V. ESP32 pins use 3.3 V. A 5 V signal - from an Uno pin or from many sensor modules - can damage an ESP32 pin for good. If a part sends out 5 V, bring it down to 3.3 V first, or buy the 3.3 V version of the part.

The power pins

USB powers the board while you are learning. The 3V3 pin gives out 3.3 V for small parts such as sensors and potentiometers. Never connect 5 V to the 3V3 pin, and never power motors or long LED strips from it. Always connect the GND of every part to the ESP32 GND.

Bringing 5 V down with two resistors

Two resistors in a row make a voltage divider. The point between them sits at a lower voltage, which is safe for the ESP32.

Course example: 2.2 k ohm from the 5 V signal, 3.3 k ohm to GND. The middle point is about 3 V. The HC-SR04 distance sensor's Echo pin is a common 5 V signal that needs this.

// Vout = Vin x R2 / (R1 + R2)
// 5 V x 3.3k / (2.2k + 3.3k) = 3.0 V  - safe for the ESP32

Level shifter modules

A divider only works one way (5 V in, 3 V out). For wires that carry signals both ways, such as I2C, buy a small level shifter module instead.

Why the button is on GPIO 4

GPIO 0 is checked by the ESP32 at start-up. A button there can stop the board starting normally. GPIO 4 is a safe choice, so the course uses it for buttons.

Wiring and safe build sequence

1. ESP32 GPIO 4 -> one side of the push-button; other side -> GND
2. Sketch 2: GPIO 2 -> 220-330 ohm -> LED long leg; LED short leg -> GND (or use the on-board LED)
3. USB power; Serial Monitor 115200

What this sketch is for: Press and release the button and watch the Serial Monitor.

const int buttonPin = 4;                // the button is between GPIO 4 and GND

void setup() {                           // runs once
  pinMode(buttonPin, INPUT_PULLUP);      // the pin reads HIGH until the button is pressed
  Serial.begin(115200);                  // start Serial at the ESP32 speed
}

void loop() {                            // runs over and over
  if (digitalRead(buttonPin) == LOW) {   // LOW means the button is pressed
    Serial.println("pressed");           // say so
  } else {                               // otherwise the pin reads HIGH
    Serial.println("open");              // the button is not pressed
  }
  delay(200);                            // slow the printing down
}

What this sketch is for: Add the LED, then hold the button down. Match the breadboard layout below before upload.

const int buttonPin = 4;                // the button is between GPIO 4 and GND
const int ledPin = 2;                   // the LED (with its resistor) is on GPIO 2

void setup() {                          // runs once
  pinMode(buttonPin, INPUT_PULLUP);     // the pin reads HIGH until the button is pressed
  pinMode(ledPin, OUTPUT);              // the LED pin is an output
}

void loop() {                           // runs over and over
  if (digitalRead(buttonPin) == LOW) {  // is the button pressed?
    digitalWrite(ledPin, HIGH);         // yes - LED on
  } else {                              // no
    digitalWrite(ledPin, LOW);          // LED off
  }
}