36. 3.3 V Logic, Power Rails and Level Shifting

Project: Safe panel acknowledge

Build a 3.3 V-safe front-panel control: press to acknowledge / arm a status LED. That is the same habit you need on any ESP32 machine panel - buttons to GND with INPUT_PULLUP, no 5 V on GPIO, modest LED current.

Sketch 1 proves the button and prints the 3.3 V rule. Sketch 2 is the useful panel: edge-detect press, debounce with millis, toggle a status LED on GPIO 2.

Never put 5 V into ESP32 GPIO

ESP32 pins are 3.3 V logic. A 5 V signal from an Uno or many sensor modules can damage the pin. If a module outputs 5 V on a data line, add level shifting or use a 3.3 V-safe variant.

USB, VIN and 3V3 rails

USB powers most DevKits. VIN/5V may accept external 5 V within board limits. 3V3 feeds the ESP32 and 3.3 V peripherals. Do not inject 5 V into 3V3. Do not draw strip/motor current from the tiny regulator. Share GND.

Voltage divider for a 5 V sensor echo

HC-SR04 Echo is about 5 V - use a divider before ESP32 GPIO. Course example: R1 = 2.2 k ohm from Echo, R2 = 3.3 k ohm to GND, mid-point to GPIO. Vout = Vin * R2 / (R1 + R2) -> about 3.0 V from 5 V. Trig can often stay 3.3 V from ESP32.

// Vout = Vin * R2 / (R1 + R2)
// Example: 5 V * 3.3k / (2.2k + 3.3k) = 3.0 V

MOSFET bidirectional shifters

Dividers are fine for one-way 5 V -> 3.3 V inputs. For I2C or bidirectional lines use a proper MOSFET shifter module.

GPIO 0 caution; this lesson uses GPIO 4

GPIO 0 is a strapping pin. A button to GND there can confuse boot. Use GPIO 4 with INPUT_PULLUP for the panel button.

Wiring and safe build sequence

1. ESP32 GPIO 4 -> one side of push-button; other side -> GND
2. Sketch 2: GPIO 2 -> 220-330 ohm -> LED anode; cathode -> GND (or on-board LED)
3. USB power; Serial Monitor 115200
4. Optional HC-SR04 divider on Echo as described in theory

What this sketch is for: Discovery: INPUT_PULLUP button on GPIO 4 prints pressed/open and reminds you ESP32 is 3.3 V logic.

const int btnPin = 4;

void setup() {
  pinMode(btnPin, INPUT_PULLUP);
  Serial.begin(115200);
  delay(300);
  Serial.println("ESP32 is 3.3 V logic - never feed 5 V into GPIO");
}

void loop() {
  int pressed = (digitalRead(btnPin) == LOW);
  Serial.println(pressed ? "button pressed" : "button open");
  delay(200);
}

What this sketch is for: Useful panel control: each clean button edge toggles a status LED (armed/acknowledged). millis debounce stops bounce from flipping twice. Match the breadboard layout below before upload.

const int btnPin = 4;
const int ledPin = 2;
const unsigned long debounceMs = 40;

bool ledOn = false;
bool lastStable = HIGH;
bool lastRead = HIGH;
unsigned long lastEdge = 0;

void setup() {
  pinMode(btnPin, INPUT_PULLUP);
  pinMode(ledPin, OUTPUT);
  digitalWrite(ledPin, LOW);
  Serial.begin(115200);
  Serial.println("Panel ACK - press to toggle status LED");
}

void loop() {
  bool reading = digitalRead(btnPin);
  if (reading != lastRead) {
    lastEdge = millis();
    lastRead = reading;
  }

  if ((millis() - lastEdge) > debounceMs && reading != lastStable) {
    lastStable = reading;
    if (lastStable == LOW) {
      ledOn = !ledOn;
      digitalWrite(ledPin, ledOn ? HIGH : LOW);
      Serial.println(ledOn ? "ACK / armed" : "cleared");
    }
  }
}