20×4 I2C LCD with NodeMCU ESP8266 — level-shifted wiring

A 20×4 LCD gives a NodeMCU four full lines — enough for a tank level, two temperatures and the Wi-Fi status at once. It is a 5 V part, though, and the ESP8266 is not.

The clean way: power the LCD from VIN (5 V) and pass SDA (D2) and SCL (D1) through a small BSS138 level shifter, so the 5 V pull-ups on the backpack never reach the NodeMCU’s pins.

Wiring

Board quirks

• The PCF8574 backpack has 4.7 kΩ pull-ups to its own VCC, so wired straight in it pulls the NodeMCU’s SDA/SCL to 5 V. The ESP8266 isn’t specified for 5 V on its pins; the shifter keeps the board side at 3.3 V.
• Running the LCD from 3V3 instead avoids the shifter, but a standard 5 V LCD then shows faint or no characters. Only “3.3 V” LCD modules work that way.
• VIN is 5 V only while the NodeMCU runs from USB (or a 5 V supply on VIN). The board’s 3.3 V regulator has no spare for the backlight — don’t power the LCD from 3V3.
• I2C stays on the default D2 (SDA) and D1 (SCL). The backpack’s pull-ups must never end up on D8 (GPIO15 must be LOW at boot) or the board won’t start.
• Writing to an I2C LCD is slow (each character is several I2C transfers). Update only the line that changes, as the sketch does, rather than clearing the whole screen every second.

Code

// 20x4 character LCD with a PCF8574 I2C backpack on a NodeMCU (ESP8266).
// VCC → VIN (5 V), GND → GND, SDA → D2 and SCL → D1 through a BSS138 level shifter
// (LV = 3V3, HV = VIN). Address 0x27 (some backpacks: 0x3F).
#include <Wire.h>
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 20, 4);

void setup() {
  Serial.begin(115200);
  Serial.println();
  Wire.begin();  // SDA = D2 (GPIO4), SCL = D1 (GPIO5)
  lcd.init();
  lcd.backlight();
  lcd.setCursor(0, 0);
  lcd.print("FlashReady");
  lcd.setCursor(0, 1);
  lcd.print("NodeMCU ESP8266");
  lcd.setCursor(0, 2);
  lcd.print("Chip ID ");
  lcd.print(ESP.getChipId(), HEX);
  Serial.println("LCD ready - if it stays blank, turn the contrast trimmer");
}

void loop() {
  char line[21];
  snprintf(line, sizeof line, "Up %5lus Heap %5u", millis() / 1000, ESP.getFreeHeap());
  lcd.setCursor(0, 3);
  lcd.print(line);
  Serial.println(line);
  delay(1000);
}

// 20x4 character LCD with a PCF8574 I2C backpack on a NodeMCU (ESP8266).
// VCC → VIN (5 V), GND → GND, SDA → D2 and SCL → D1 through a BSS138 level shifter
// (LV = 3V3, HV = VIN). Address 0x27 (some backpacks: 0x3F).
#include <Wire.h>
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 20, 4);

void setup() {
  Serial.begin(115200);
  Serial.println();
  Wire.begin();  // SDA = D2 (GPIO4), SCL = D1 (GPIO5)
  lcd.init();
  lcd.backlight();
  lcd.setCursor(0, 0);
  lcd.print("FlashReady");
  lcd.setCursor(0, 1);
  lcd.print("NodeMCU ESP8266");
  lcd.setCursor(0, 2);
  lcd.print("Chip ID ");
  lcd.print(ESP.getChipId(), HEX);
  Serial.println("LCD ready - if it stays blank, turn the contrast trimmer");
}

void loop() {
  char line[21];
  snprintf(line, sizeof line, "Up %5lus Heap %5u", millis() / 1000, ESP.getFreeHeap());
  lcd.setCursor(0, 3);
  lcd.print(line);
  Serial.println(line);
  delay(1000);
}

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

Expected serial output

LCD ready - if it stays blank, turn the contrast trimmer
Up     0s Heap 51288
Up     1s Heap 51288
Up     2s Heap 51288

LCD ready - if it stays blank, turn the contrast trimmer
Up     0s Heap 51288
Up     1s Heap 51288
Up     2s Heap 51288

Common errors