16x2 I2C LCD with Arduino Uno — wiring and code

A 16×2 LCD with an I2C “backpack” needs only four wires instead of the twelve a bare LCD takes. On the Uno everything is 5 V, so the backpack wires straight to the board.

The sketch prints a title on the first line and counts uptime on the second, overwriting old digits with spaces instead of clearing the screen — that avoids flicker.

Wiring

Board quirks

• The SDA/SCL pins next to AREF are the same signals as A4/A5 — use one pair, not both.
• Backlight on but a blank screen? Turn the blue contrast trimmer on the backpack until the characters appear.
• Backpacks answer at 0x27 or 0x3F. If nothing shows, run the I2C scanner and change the address in the sketch.

Code

// 16x2 LCD with a PCF8574 I2C backpack on an Arduino Uno R3.
// GND → GND, VCC → 5V, SDA → SDA (A4), SCL → SCL (A5). Address 0x27 (or 0x3F).
#include <Wire.h>
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 16, 2);

void setup() {
  Serial.begin(9600);
  lcd.init();
  lcd.backlight();
  lcd.setCursor(0, 0);
  lcd.print(F("FlashReady"));
  lcd.setCursor(0, 1);
  lcd.print(F("Uptime:"));
  Serial.println(F("LCD ready at 0x27"));
}

void loop() {
  lcd.setCursor(8, 1);
  lcd.print(millis() / 1000);
  lcd.print(F("s   "));  // trailing spaces clear old digits
  delay(1000);
}

// 16x2 LCD with a PCF8574 I2C backpack on an Arduino Uno R3.
// GND → GND, VCC → 5V, SDA → SDA (A4), SCL → SCL (A5). Address 0x27 (or 0x3F).
#include <Wire.h>
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 16, 2);

void setup() {
  Serial.begin(9600);
  lcd.init();
  lcd.backlight();
  lcd.setCursor(0, 0);
  lcd.print(F("FlashReady"));
  lcd.setCursor(0, 1);
  lcd.print(F("Uptime:"));
  Serial.println(F("LCD ready at 0x27"));
}

void loop() {
  lcd.setCursor(8, 1);
  lcd.print(millis() / 1000);
  lcd.print(F("s   "));  // trailing spaces clear old digits
  delay(1000);
}

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

Expected serial output

LCD ready at 0x27

LCD ready at 0x27

Common errors