16x2 I2C LCD with Raspberry Pi Pico W — level-shifted wiring and MicroPython

A 16x2 LCD with an I2C backpack needs only four wires, but on a Pico W two of them need care: the backpack runs on 5 V and pulls the I2C lines up to 5 V. This guide powers it from VBUS and puts a BSS138 level shifter between the backpack and GP4/GP5.

MicroPython has no LCD driver built in, so the sketch includes a small one. The script prints “Hello, Pico W!”, then shows an uptime clock.

Wiring

Also needed: 4-channel BSS138 I2C level shifter.

Every connection
Raspberry Pi Pico W	16×2 LCD + I2C backpack	Wire	Notes
GND	GND	black
VBUS	VCC	red	VBUS (pin 40) is USB 5 V. At 3.3 V most of these LCDs show almost no contrast.
GP4	SDA	blue	Via level shifter — GP4 (pin 6) → shifter LV1 ↔ HV1 → SDA. Shifter LV = 3V3, HV = VBUS, GND = GND.
GP5	SCL	yellow	Via level shifter — GP5 (pin 7) → shifter LV2 ↔ HV2 → SCL.

5 V (3.3 V versions exist) part, 3.3 V board. Most of these LCDs need 5 V for good contrast, and the backpack pulls SDA/SCL up to its own VCC. On ESP32 or Pico, put a BSS138 level shifter between the board and the backpack instead of wiring SDA/SCL directly.

Board quirks

The backpack pulls SDA and SCL up to 5 V. Wired straight to a Pico, that 5 V sits on GP4/GP5 all the time — the BSS138 shifter keeps the Pico side at 3.3 V.
Copy lcd_i2c.py (a 60-line driver written for FlashReady, MIT) to the board next to main.py. The same driver handles a 20x4 LCD: LcdI2c(i2c, addr, 20, 4).
The script scans the bus and picks 0x27 (PCF8574T) or 0x3F (PCF8574AT) automatically.
Blank screen with the backlight on? Turn the blue contrast pot on the backpack before suspecting the code.

Code

main.pyMicroPython

# 16x2 I2C LCD (PCF8574 backpack) on a Raspberry Pi Pico W (MicroPython).
# The backpack runs on 5 V and pulls SDA/SCL up to 5 V, so they go through a BSS138 level shifter:
#   LCD VCC -> VBUS (pin 40, USB 5 V), LCD GND -> GND
#   LCD SDA -> shifter HV1 / LV1 -> GP4 (pin 6), LCD SCL -> shifter HV2 / LV2 -> GP5 (pin 7)
#   Shifter LV -> 3V3 (pin 36), HV -> VBUS, GND -> GND.
# Copy lcd_i2c.py to the board too.
from machine import Pin, I2C
import time
from lcd_i2c import LcdI2c

i2c = I2C(0, sda=Pin(4), scl=Pin(5), freq=100_000)
found = i2c.scan()
print("I2C devices:", [hex(a) for a in found])
addr = 0x27 if 0x27 in found else (0x3F if 0x3F in found else None)
if addr is None:
    raise SystemExit("No LCD backpack at 0x27/0x3F - check the level shifter and 5 V supply")

lcd = LcdI2c(i2c, addr, 16, 2)
lcd.line(0, "Hello, Pico W!")
lcd.line(1, "FlashReady.in")
print("LCD at", hex(addr))
time.sleep(2)

start = time.ticks_ms()
while True:
    up = time.ticks_diff(time.ticks_ms(), start) // 1000
    lcd.line(0, "Uptime")
    lcd.line(1, "{:02d}:{:02d}:{:02d}".format(up // 3600, up // 60 % 60, up % 60))
    print("Uptime", up, "s")
    time.sleep(1)

# 16x2 I2C LCD (PCF8574 backpack) on a Raspberry Pi Pico W (MicroPython).
# The backpack runs on 5 V and pulls SDA/SCL up to 5 V, so they go through a BSS138 level shifter:
#   LCD VCC -> VBUS (pin 40, USB 5 V), LCD GND -> GND
#   LCD SDA -> shifter HV1 / LV1 -> GP4 (pin 6), LCD SCL -> shifter HV2 / LV2 -> GP5 (pin 7)
#   Shifter LV -> 3V3 (pin 36), HV -> VBUS, GND -> GND.
# Copy lcd_i2c.py to the board too.
from machine import Pin, I2C
import time
from lcd_i2c import LcdI2c

i2c = I2C(0, sda=Pin(4), scl=Pin(5), freq=100_000)
found = i2c.scan()
print("I2C devices:", [hex(a) for a in found])
addr = 0x27 if 0x27 in found else (0x3F if 0x3F in found else None)
if addr is None:
    raise SystemExit("No LCD backpack at 0x27/0x3F - check the level shifter and 5 V supply")

lcd = LcdI2c(i2c, addr, 16, 2)
lcd.line(0, "Hello, Pico W!")
lcd.line(1, "FlashReady.in")
print("LCD at", hex(addr))
time.sleep(2)

start = time.ticks_ms()
while True:
    up = time.ticks_diff(time.ticks_ms(), start) // 1000
    lcd.line(0, "Uptime")
    lcd.line(1, "{:02d}:{:02d}:{:02d}".format(up // 3600, up // 60 % 60, up % 60))
    print("Uptime", up, "s")
    time.sleep(1)

Expected serial output

REPL / Thonny shellOutput

I2C devices: ['0x27']
LCD at 0x27
Uptime 0 s
Uptime 1 s
Uptime 2 s

I2C devices: ['0x27']
LCD at 0x27
Uptime 0 s
Uptime 1 s
Uptime 2 s

Common errors

No LCD backpack at 0x27/0x3F

Check the shifter: LV to 3V3, HV to VBUS, both grounds connected, and SDA/SCL on matching channels. Run on USB — VBUS is dead on battery.

Row of solid blocks, no text

The LCD has power but missed the initialise sequence. Press Ctrl-D in Thonny to soft-reset so LcdI2c() runs again.

ImportError: no module named 'lcd_i2c'

Save lcd_i2c.py to the Pico (Thonny: File → Save copy → Raspberry Pi Pico).