16x2 I2C LCD with ESP32-C3 SuperMini — level-shifted

The blue 16x2 I2C LCD is a 5 V part; the SuperMini is a 3.3 V board. Powering the LCD from 3V3 gives a dim, low-contrast screen, and powering it from 5V puts 5 V on the I2C lines through the backpack’s pull-ups.

The clean fix is a cheap BSS138 level shifter between them: the LCD runs at 5 V, the C3 sees 3.3 V on GPIO5 (SDA) and GPIO6 (SCL). The sketch also shows the one software trap on this board — calling Wire.begin(5, 6) before lcd.init().

Wiring

Board quirks

• The PCF8574 backpack has pull-up resistors to its own VCC. At 5 V that puts 5 V on SDA/SCL, and the C3’s pins are not 5 V tolerant — hence the level shifter.
• lcd.init() calls Wire.begin() with no pins, which on a generic C3 means GPIO8/9 (the LED and BOOT strapping pins). Calling Wire.begin(5, 6) first keeps the bus on GPIO5/6 — the core just keeps the bus that is already running.
• If the screen is lit but blank, turn the blue contrast pot on the backpack — at 5 V it is usually set right, at 3.3 V it rarely is.
• The backpack address is 0x27 (PCF8574T) or 0x3F (PCF8574AT). The sketch checks 0x27 and tells you if nothing answers.

Code

// 16x2 I2C LCD (PCF8574 backpack) on an ESP32-C3 SuperMini.
// VCC → 5V, GND → GND, SDA → GPIO5 and SCL → GPIO6 through a BSS138 level shifter
// (LV side 3V3, HV side 5V). The backpack pulls SDA/SCL up to 5 V; the C3's pins are 3.3 V only.
#include <Wire.h>
#include <LiquidCrystal_I2C.h>

constexpr int SDA_PIN = 5;
constexpr int SCL_PIN = 6;
LiquidCrystal_I2C lcd(0x27, 16, 2);  // many backpacks are 0x3F instead

void setup() {
  Serial.begin(115200);
  delay(1500);  // let the native USB serial port enumerate
  // Start Wire on our pins FIRST. lcd.init() calls Wire.begin() again with no pins;
  // the core keeps the bus that is already running, so GPIO5/6 stay in use.
  Wire.begin(SDA_PIN, SCL_PIN);
  Wire.beginTransmission(0x27);
  if (Wire.endTransmission() != 0) Serial.println("Nothing at 0x27 — try 0x3F in the sketch");
  lcd.init();
  lcd.backlight();
  lcd.setCursor(0, 0);
  lcd.print("ESP32-C3 Mini");
  Serial.println("LCD ready");
}

void loop() {
  lcd.setCursor(0, 1);
  lcd.printf("Up %5lu s", millis() / 1000);
  delay(1000);
}

// 16x2 I2C LCD (PCF8574 backpack) on an ESP32-C3 SuperMini.
// VCC → 5V, GND → GND, SDA → GPIO5 and SCL → GPIO6 through a BSS138 level shifter
// (LV side 3V3, HV side 5V). The backpack pulls SDA/SCL up to 5 V; the C3's pins are 3.3 V only.
#include <Wire.h>
#include <LiquidCrystal_I2C.h>

constexpr int SDA_PIN = 5;
constexpr int SCL_PIN = 6;
LiquidCrystal_I2C lcd(0x27, 16, 2);  // many backpacks are 0x3F instead

void setup() {
  Serial.begin(115200);
  delay(1500);  // let the native USB serial port enumerate
  // Start Wire on our pins FIRST. lcd.init() calls Wire.begin() again with no pins;
  // the core keeps the bus that is already running, so GPIO5/6 stay in use.
  Wire.begin(SDA_PIN, SCL_PIN);
  Wire.beginTransmission(0x27);
  if (Wire.endTransmission() != 0) Serial.println("Nothing at 0x27 — try 0x3F in the sketch");
  lcd.init();
  lcd.backlight();
  lcd.setCursor(0, 0);
  lcd.print("ESP32-C3 Mini");
  Serial.println("LCD ready");
}

void loop() {
  lcd.setCursor(0, 1);
  lcd.printf("Up %5lu s", millis() / 1000);
  delay(1000);
}

Library: LiquidCrystal I2C (Frank de Brabander) — compiled with LiquidCrystal I2C 1.1.2.

Expected serial output

LCD ready

LCD ready

Common errors