ESP32 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 ESP32 DevKit 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 GPIO 21 (D21) and SCL to GPIO 22 (D22). The display needs 5 V, so on the 3.3 V ESP32 it is powered from VIN (5 V from USB), and the I2C lines need care on a real bench, as the wiring notes explain.
- 3.3 V logic
- Library: LiquidCrystal_I2C.h
- I2C address 0x27
- Powered from VIN (5 V from USB)
Wiring the I2C LCD to the ESP32
Every connection in the circuit above, with the board’s own pin names.
| I2C LCD pin | ESP32 pin | Why |
|---|---|---|
| VCC | VIN (5 V from USB) | The LCD glass needs 5 V for its contrast; at 3.3 V most modules show nothing. |
| GND | GND | Common ground. |
| SDA | GPIO 21 (D21) | I2C data. |
| SCL | GPIO 22 (D22) | I2C clock. |
On an ESP32 DevKit, Wire.begin() with no arguments puts SDA on GPIO 21 and SCL on GPIO 22, labeled D21 and D22. The backpack runs from 5 V and carries 4.7 k pull-ups on SDA and SCL, so on a real ESP32 those lines idle at 5 V. Either remove the backpack’s two pull-up resistors and add 4.7 k pull-ups to 3V3, or put a two-channel level shifter between them. The simulator does not punish it; a real ESP32 (Xtensa LX6) may.
Pins to leave alone on the ESP32
GPIO 34, 35, 36 (VP) and 39 (VN) are input only: they cannot drive a pin or take an internal pull-up. GPIO 0, 2, 5, 12 and 15 are strapping pins, read at power-on, and GPIO 6 to 11 are the flash chip.
On a real bench
The thirty-pin DevKit straddles a breadboard with one row of holes free on each side, which is where the I2C LCD’s jumpers go. Many ESP32 boards are wider and cover both rows; then use two breadboards side by side. Hold BOOT while uploading if the IDE cannot connect.
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 GPIO 21 (D21), SCL GPIO 22 (D22).
LiquidCrystal_I2C lcd(0x27, 16, 2);
void setup() {
Serial.begin(115200);
lcd.init();
lcd.backlight();
lcd.setCursor(0, 0);
lcd.print("ESP32");
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(115200) opens the serial monitor. Serial is UART0 on the USB bridge. ESP32 sketches usually run it at 115200 baud, which this one does.
Common mistakes with the I2C LCD on the ESP32
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.
Powering it from 3V3
The backlight comes on, so it looks wired, but the liquid crystal needs about 5 V of drive for contrast and the characters stay invisible. Use VIN (5 V from USB).
Questions and answers
Can I simulate a 16x2 I2C LCD with an ESP32 DevKit?
Yes. The circuit at the top of this page is an ESP32 DevKit with a 16x2 I2C LCD, running in your browser on a simulated ESP32 (Xtensa LX6). 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 ESP32?
On an ESP32 DevKit, Wire.begin() with no arguments puts SDA on GPIO 21 and SCL on GPIO 22, labeled D21 and D22.
Is a 5 V I2C LCD safe on the ESP32?
Power is fine from VIN (5 V from USB). The I2C lines are the risk: the backpack pulls them up to 5 V. On a real board use a level shifter, or move the pull-ups to 3.3 V.
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.