A 5 V I²C screen on an ESP32

An LCD1602 with an I²C backpack wants 5 V, and its pull-up resistors put that 5 V on the bus. A 2CH board between it and an ESP32 takes three power wires and two signal wires, and a scan proves it works in ten seconds.

Updated 2026-09-116 min readlogic level converter

Why this screen needs a converter

The LCD1602 needs 5 V to show anything, and its I²C backpack has pull-up resistors to that same 5 V. So SDA and SCL sit at 5 V whenever the bus is idle. Your sketch does not send that 5 V. The screen puts it there by itself.

The screen's pull-ups decide the voltage, not your code

through the 2CH board

Between the ESP32 and the screen

ESP32 SDA at rest

0.0 V

ESP32 pin rated to

3.6 V

Screen's bus at rest

0.0 V

Each side idles at its own voltage. The screen still sees a 5 V bus, the ESP32 sees 3.3 V, and the 2CH board copies every LOW across in both directions. Two wires in, two wires out, plus LV, HV and GND.

What goes where

From	To
ESP32 3V3	2CH LV
ESP32 5V	2CH HV, and backpack VCC
ESP32 GND	2CH GND, and backpack GND
ESP32 SDA	2CH A1
ESP32 SCL	2CH A2
2CH B1	backpack SDA
2CH B2	backpack SCL

Which GPIOs are SDA and SCL depends on your ESP32. The sketch below prints them.

The 2-channel converter at an angle, with A1, LV, GND and A2 down the edge nearest the camera and B1, HV, GND and B2 down the far edge, two transistors between them. — A1 and A2 face the ESP32, B1 and B2 face the screen. LV, HV and GND are the middle pins on each side.

Scan the bus

Upload the sketch and open the serial monitor at 115200. It prints the two pins, then every address that answers, every three seconds. An LCD backpack normally answers at 0x27 or 0x3F.

Finding the address proves more than it seems. Each answer is the screen pulling a line LOW back through the converter, so both directions work.

Then print something

Any I²C LCD library from the Library Manager will take the address you just found. If the screen lights but shows only blocks, turn the contrast potentiometer on the back of the backpack.

The code

scan_through_converter.ino

A bus scan, and nothing else. It prints which GPIOs your ESP32 uses for SDA and SCL, so you know where A1 and A2 go, then lists every address that answers. An LCD backpack usually answers at 0x27 or 0x3F.

// Converter wiring for this sketch (2CH board).
//
//   ESP32 3V3  -> LV
//   ESP32 5V   -> HV, and the LCD backpack's VCC
//   ESP32 GND  -> GND, and the backpack's GND
//   ESP32 SDA  -> A1      B1 -> backpack SDA
//   ESP32 SCL  -> A2      B2 -> backpack SCL
//
// Arduino IDE: any ESP32 board. Tools settings as you normally use them —
// this sketch needs no library and no special options.

#include <Wire.h>

void setup() {
  Serial.begin(115200);
  delay(500);
  Wire.begin();
  // SDA and SCL are your board's default I2C pins. Wire A1 and A2 to these.
  Serial.printf("SDA is GPIO %d, SCL is GPIO %d\n", SDA, SCL);
}

void loop() {
  int found = 0;
  for (uint8_t addr = 1; addr < 127; addr++) {
    Wire.beginTransmission(addr);
    if (Wire.endTransmission() == 0) {
      Serial.printf("found a device at 0x%02X\n", addr);
      found++;
    }
  }
  if (found == 0) Serial.println("nothing answered - check LV, HV and GND first");
  Serial.println();
  delay(3000);
}

If it prints “nothing answered”, the fault is the wiring, not the sketch — check LV, HV and GND with a meter before anything else. If it finds an address, the converter is working in both directions: the scan only succeeds when the screen can pull a line LOW back through it.

When it does not work

The scan finds nothing

Check in this order: LV at 3.3 V and HV at 5 V with a meter, GND shared, then SDA and SCL not swapped between A1 and A2. Make sure it is a MOSFET board or a TXS — a TXB will not pass I²C. And confirm the backpack is getting 5 V on its VCC pin.

It finds 0x27, but the screen shows a row of blocks or nothing at all

The bus is fine; that is the screen's contrast. Turn the small potentiometer on the back of the backpack slowly with a screwdriver until characters appear. It is the commonest problem with these screens and has nothing to do with the converter.

The address appears on some scans and not others

An intermittent bus is almost always a loose ground or a long wire. Check that the GND wire really connects both boards, and keep the I²C wires short. On a breadboard, press the converter's pins firmly home.

Do I need to add pull-up resistors?

No. The backpack has pull-ups on the 5 V side, and the 2CH board has a 10 kΩ pull-up on each side of both channels.

Alongside this page

What the scan is actually doing on the wire Where an I²C address comes from, and 0x27 versus 0x3F

Where this goes next

The other common build: one channel of a TXB0108 and a rainbow.

An LED strip from a 3.3 V pin →

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