i2c LCD on Arduino: wiring, address & code

Step 1 · Wiring

i2c LCD wiring: four wires instead of sixteen

A bare 1602 (16×2) or 2004 (20×4) LCD uses the HD44780 controller and needs ~12 pins. The little PCF8574 board soldered to the back — the "i2c backpack" — converts that to four: GND, VCC, SDA, SCL. Power VCC from 5 V (the LCD glass needs it), then on an Uno or Nano wire SDA to A4 and SCL to A5:

Worth knowing: that backpack is just a general-purpose i2c GPIO expander wired to the LCD's pins. The same chip — or the roomier MCP23017 — gives you 8 or 16 free pins for buttons and LEDs on any project.

LCD backpack     Arduino Uno/Nano
  GND      ---->   GND
  VCC      ---->   5V
  SDA      ---->   A4
  SCL      ---->   A5

ESP32? Default SDA/SCL are GPIO21/22 — see the voltage note in the FAQ.

Step 2 · Address

Find the i2c address (0x27 or 0x3F)

Backpacks come at one of two addresses depending on the chip: 0x27 for the PCF8574, 0x3F for the PCF8574A. Don't guess — run a scanner and read it off:

#include <Wire.h>
void setup(){ Wire.begin(); Serial.begin(115200);
  for(byte a=8;a<120;a++){ Wire.beginTransmission(a);
    if(!Wire.endTransmission()){ Serial.print("found 0x"); Serial.println(a,HEX); } } }
void loop(){}

Whatever it prints is what you pass to the library next. (Both addresses are in our address list, and yes — 0x3F is the same PCF8574A address that can clash with other parts.)

Step 3 · Code

LiquidCrystal_I2C Arduino code: hello world

Install LiquidCrystal_I2C from the Library Manager, then:

#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 16, 2);  // address, columns, rows

void setup() {
  lcd.init();          // start the LCD
  lcd.backlight();     // turn the backlight on
  lcd.setCursor(0, 0); // column 0, row 0
  lcd.print("Hello, I2C!");
  lcd.setCursor(0, 1);
  lcd.print("i2c.net");
}

void loop() {}

Change 0x27 to whatever the scanner found, and 16, 2 to 20, 4 for a 2004 display. That's a complete working sketch.

Step 4 · The fix nobody mentions

i2c LCD blank screen? Turn the contrast pot

By far the most common "my i2c LCD doesn't work" is a display that's powered, scanning at the right address, running correct code — and showing nothing, or a row of solid blocks. The cause is contrast. There's a small blue potentiometer on the backpack; turn it slowly with a screwdriver and the characters fade into view. If the scan finds nothing at all instead, it's wiring or power — work the troubleshooting checklist, and confirm VCC is 5 V.