Arduino Nano I2C LCD Simulator: Wiring, Code and a Live Circuit
This is a 16 by 2 character LCD with a PCF8574 I2C backpack, wired to an Arduino Nano and running live: the first line names the board and the second counts the seconds since reset. It uses the LiquidCrystal_I2C library, so four wires replace the sixteen of a bare LCD.
SDA goes to pin A4 and SCL to pin A5. Everything runs at 5 V, the Arduino Nano’s own level.
- 5 V logic
- Library: LiquidCrystal_I2C.h
- I2C address 0x27
- Powered from 5V
Wiring the I2C LCD to the Arduino Nano
Every connection in the circuit above, with the board’s own pin names.
| I2C LCD pin | Arduino Nano pin | Why |
|---|---|---|
| VCC | 5V | The display and its backlight run from 5 V. |
| GND | GND | Common ground. |
| SDA | pin A4 | I2C data. |
| SCL | pin A5 | I2C clock. |
The Nano has the Uno’s chip, so I2C is A4 (SDA) and A5 (SCL), on the analog side of the board next to A6 and A7. The backpack’s own pull-ups hold both lines at 5 V, which is the Arduino Nano’s own logic level, so nothing else is needed.
Pins to leave alone on the Arduino Nano
A6 and A7 are analog only: pinMode(A6, OUTPUT) does nothing and digitalRead(A7) is not there to call. D0 and D1 are the USB serial port.
On a real bench
The Nano plugs into the breadboard itself, straddling the center channel, so the I2C LCD goes in a few columns along and every connection is a short jumper. The silkscreen labels its pins D2 to D13 and A0 to A7; in code D2 is simply 2. A Nano clone with a CH340 USB chip needs that driver on a real computer.
The code
The sketch the circuit above runs. Open it in the editor, change a line and press Run: it compiles in your browser.
#include <Wire.h>
#include <LiquidCrystal_I2C.h>
// A 16 x 2 display behind a PCF8574 backpack at 0x27.
// SDA pin A4, SCL pin A5.
LiquidCrystal_I2C lcd(0x27, 16, 2);
void setup() {
Serial.begin(9600);
lcd.init();
lcd.backlight();
lcd.setCursor(0, 0);
lcd.print("Arduino Nano");
Serial.println("LCD ready");
}
void loop() {
lcd.setCursor(0, 1);
lcd.print("Seconds: ");
lcd.print(millis() / 1000);
lcd.print(" "); // clears the tail of a shorter number
delay(250);
}
LiquidCrystal_I2C lcd(0x27, 16, 2) names the backpack’s address and the size of the display. init() wakes the HD44780 controller behind the backpack in four-bit mode, and backlight() turns the LED on, which is a bit in the backpack, not in the display.
setCursor(column, row) counts from zero, so (0, 1) is the start of the second line. print() writes text there and moves the cursor along, the way Serial.print() does.
The three spaces after the number overwrite whatever a longer number left behind. Calling clear() every time would do the same and make the display flicker, because clear() takes about 2 ms and blanks the whole panel. Serial.begin(9600) opens the serial monitor. Serial is the USB port, as on the Uno; a clone with a CH340 chip needs that driver on a real computer, which the simulator does not.
Common mistakes with the I2C LCD on the Arduino Nano
The wrong address
Backpacks with the PCF8574T chip answer at 0x27; those with the PCF8574AT answer at 0x3F. The screen lights but shows nothing when the address is wrong. Run an I2C scanner, or set the part’s address property in the editor to match yours.
Contrast turned all the way down
A row of solid blocks, or nothing at all with the backlight on, is usually the blue trimmer on the backpack. Turn it until the text appears. In the simulator the part has a contrast property for the same thing.
Clearing the screen in every loop
lcd.clear() takes about 2 milliseconds and blanks the panel, so calling it every pass makes the text flicker. Overwrite the line and pad it with spaces, as the sketch does.
Questions and answers
Can I simulate a 16x2 I2C LCD with an Arduino Nano?
Yes. The circuit at the top of this page is an Arduino Nano with a 16x2 I2C LCD, running in your browser on a simulated ATmega328P. The characters the sketch prints appear on the simulated panel, backlight and contrast included. Press Run it in the editor to change the wiring or the code; it is free and needs no account.
Where do SDA and SCL go on the Arduino Nano?
The Nano has the Uno’s chip, so I2C is A4 (SDA) and A5 (SCL), on the analog side of the board next to A6 and A7.
Do I need a level shifter?
No. The Arduino Nano and the backpack both run at 5 V, so SDA and SCL connect directly.
How do I find the LCD’s I2C address?
Upload an I2C scanner sketch: it calls Wire.beginTransmission() on every address and prints the ones that answer. The two you will almost always see are 0x27 and 0x3F.
Build your own I2C LCD project
Open this circuit in the editor, change the wiring or the code, and keep your version in a free account.