BH1750 light sensor with Raspberry Pi Pico W — lux in MicroPython

The BH1750 is a digital light meter: it reports lux directly over I2C. On the Pico W that takes two commands — power on, continuous high-resolution mode — and then a two-byte read divided by 1.2.

Wire it to 3V3 and I2C0 (GP4/GP5). The script prints lux once a second with a rough description of what that light level means.

Wiring

Every connection
Raspberry Pi Pico W	BH1750 lux sensor (GY-302)	Wire	Notes
3V3	VCC	red	3V3 is pin 36.
GND	GND	black
GP5	SCL	yellow	GP5 is physical pin 7 (I2C0 SCL).
GP4	SDA	blue	GP4 is physical pin 6 (I2C0 SDA). ADDR left open = 0x23.

3–5 V (on-board regulator) part, 3.3 V board. The GY-302 has a 3.3 V regulator and accepts 3–5 V on VCC; its pull-ups suit both 3.3 V and 5 V boards.

Board quirks

Unlike an LDR, the BH1750 gives calibrated lux, so readings compare between sensors — useful for grow lights and auto-brightness.
The sensor needs up to 180 ms per high-resolution reading; reading faster just returns the previous value.
It saturates at about 54,000 lx. Indian midday sun can reach 100,000 lx, so outdoors it will clip unless you shade it or change the measurement time register.

Code

main.pyMicroPython

# BH1750 (GY-302) light sensor in lux on a Raspberry Pi Pico W (MicroPython, no library).
# Wiring: VCC -> 3V3 (pin 36), GND -> GND, SCL -> GP5 (pin 7), SDA -> GP4 (pin 6). I2C0.
# ADDR unconnected/LOW = 0x23, HIGH = 0x5C.
from machine import Pin, I2C
import time

i2c = I2C(0, sda=Pin(4), scl=Pin(5), freq=400_000)
found = i2c.scan()
addr = 0x23 if 0x23 in found else (0x5C if 0x5C in found else None)
if addr is None:
    raise SystemExit("No BH1750 at 0x23/0x5C - found {}".format([hex(a) for a in found]))

i2c.writeto(addr, b"\x01")  # power on
i2c.writeto(addr, b"\x10")  # continuous high-resolution mode (1 lx steps, up to 180 ms per reading)
time.sleep_ms(180)
print("BH1750 at", hex(addr))


def lux():
    hi, lo = i2c.readfrom(addr, 2)
    return ((hi << 8) | lo) / 1.2  # datasheet: count / 1.2 = lux


def describe(lx):
    if lx < 10:
        return "dark"
    if lx < 200:
        return "dim room"
    if lx < 1000:
        return "bright room / office"
    if lx < 10000:
        return "daylight, shade"
    return "direct sun"


while True:
    lx = lux()
    print("{:8.1f} lx  {}".format(lx, describe(lx)))
    time.sleep(1)

# BH1750 (GY-302) light sensor in lux on a Raspberry Pi Pico W (MicroPython, no library).
# Wiring: VCC -> 3V3 (pin 36), GND -> GND, SCL -> GP5 (pin 7), SDA -> GP4 (pin 6). I2C0.
# ADDR unconnected/LOW = 0x23, HIGH = 0x5C.
from machine import Pin, I2C
import time

i2c = I2C(0, sda=Pin(4), scl=Pin(5), freq=400_000)
found = i2c.scan()
addr = 0x23 if 0x23 in found else (0x5C if 0x5C in found else None)
if addr is None:
    raise SystemExit("No BH1750 at 0x23/0x5C - found {}".format([hex(a) for a in found]))

i2c.writeto(addr, b"\x01")  # power on
i2c.writeto(addr, b"\x10")  # continuous high-resolution mode (1 lx steps, up to 180 ms per reading)
time.sleep_ms(180)
print("BH1750 at", hex(addr))


def lux():
    hi, lo = i2c.readfrom(addr, 2)
    return ((hi << 8) | lo) / 1.2  # datasheet: count / 1.2 = lux


def describe(lx):
    if lx < 10:
        return "dark"
    if lx < 200:
        return "dim room"
    if lx < 1000:
        return "bright room / office"
    if lx < 10000:
        return "daylight, shade"
    return "direct sun"


while True:
    lx = lux()
    print("{:8.1f} lx  {}".format(lx, describe(lx)))
    time.sleep(1)

Expected serial output

REPL / Thonny shellOutput

BH1750 at 0x23
   312.5 lx  bright room / office
   318.3 lx  bright room / office
    45.8 lx  dim room

BH1750 at 0x23
   312.5 lx  bright room / office
   318.3 lx  bright room / office
    45.8 lx  dim room

Common errors

No BH1750 at 0x23/0x5C

Check SDA/SCL (GP4 is pin 6, GP5 is pin 7) and 3V3. If ADDR is wired HIGH, the script finds 0x5C on its own.

Always 0.0 lx

The power-on or mode command did not arrive. Re-run; if it persists, the module is faulty.