DS18B20 with Raspberry Pi Pico W — MicroPython 1-Wire code

The DS18B20 is the go-to sensor for water tanks, aquariums and freezers: sealed, accurate to ±0.5 °C, and many of them share one GPIO. MicroPython for the Pico W ships the onewire and ds18x20 drivers, so this is a single short file.

Wire the probe to 3V3, GP22 and GND — three neighbouring pins on the right-hand header — with a 4.7 kΩ resistor from DQ to 3V3.

Wiring

Also needed: 4.7 kΩ resistor.

Every connection
Raspberry Pi Pico W	DS18B20 (1-Wire)	Wire	Notes
3V3	VDD	red	Red wire on the probe. 3V3 is physical pin 36.
GP22	DQ	yellow	Yellow (or white) wire. GP22 is physical pin 29.
3V3	DQ	red	Via 4.7 kΩ — The 1-Wire bus needs this pull-up — waterproof probes too. Some breakout boards already have it.
GND	GND	black	Black wire on the probe. GND pin 28 sits right next to GP22.

3.0–5.5 V part, 3.3 V board. Power it from your board's logic voltage (3.3 V on ESP32/Pico, 5 V on Uno) so DQ never goes above the GPIO limit. DQ needs a 4.7 kΩ pull-up to that same supply.

Board quirks

onewire and ds18x20 are frozen into MicroPython for the Pico W — nothing to copy to the board.
Several probes can hang on the same three wires; scan() finds each one by its 64-bit ROM code, printed at start-up so you can label them.
Use 3V3, not VBUS: with a 5 V supply the pull-up would put 5 V on GP22, and the Pico’s pins are not 5 V tolerant.
For long runs (water tank, compost pit) use 3-core cable and drop the pull-up to 2.2 kΩ beyond about 5 m.

Code

main.pyMicroPython

# DS18B20 temperature probe(s) on a Raspberry Pi Pico W (MicroPython, built-in onewire + ds18x20).
# Wiring: VDD (red) -> 3V3 (pin 36), DQ (yellow) -> GP22 (pin 29), GND (black) -> GND,
# plus a 4.7 kOhm pull-up from DQ to 3V3. Several probes can share the same three wires.
from machine import Pin
import onewire
import ds18x20
import time

bus = ds18x20.DS18X20(onewire.OneWire(Pin(22)))
roms = bus.scan()
print("Found {} DS18B20 sensor(s)".format(len(roms)))
for rom in roms:
    print("  ROM", rom.hex())

while True:
    if not roms:
        print("No sensor - check the 4.7 kOhm pull-up and the DQ wire on GP22")
        time.sleep(5)
        roms = bus.scan()
        continue
    bus.convert_temp()
    time.sleep_ms(750)  # 12-bit conversion time
    for i, rom in enumerate(roms):
        t = bus.read_temp(rom)
        if t == 85.0:
            print("Sensor {}: 85.0 C = power-on value, conversion not finished".format(i))
        else:
            print("Sensor {}: {:.2f} C".format(i, t))
    time.sleep(2)

# DS18B20 temperature probe(s) on a Raspberry Pi Pico W (MicroPython, built-in onewire + ds18x20).
# Wiring: VDD (red) -> 3V3 (pin 36), DQ (yellow) -> GP22 (pin 29), GND (black) -> GND,
# plus a 4.7 kOhm pull-up from DQ to 3V3. Several probes can share the same three wires.
from machine import Pin
import onewire
import ds18x20
import time

bus = ds18x20.DS18X20(onewire.OneWire(Pin(22)))
roms = bus.scan()
print("Found {} DS18B20 sensor(s)".format(len(roms)))
for rom in roms:
    print("  ROM", rom.hex())

while True:
    if not roms:
        print("No sensor - check the 4.7 kOhm pull-up and the DQ wire on GP22")
        time.sleep(5)
        roms = bus.scan()
        continue
    bus.convert_temp()
    time.sleep_ms(750)  # 12-bit conversion time
    for i, rom in enumerate(roms):
        t = bus.read_temp(rom)
        if t == 85.0:
            print("Sensor {}: 85.0 C = power-on value, conversion not finished".format(i))
        else:
            print("Sensor {}: {:.2f} C".format(i, t))
    time.sleep(2)

Expected serial output

REPL / Thonny shellOutput

Found 2 DS18B20 sensor(s)
  ROM 28ff6a1d61160452
  ROM 28ff3c7a6016043e
Sensor 0: 27.44 C
Sensor 1: 18.06 C

Found 2 DS18B20 sensor(s)
  ROM 28ff6a1d61160452
  ROM 28ff3c7a6016043e
Sensor 0: 27.44 C
Sensor 1: 18.06 C

Common errors

Found 0 DS18B20 sensor(s)

No pull-up, or DQ is on the wrong pin. Check the 4.7 kΩ from DQ to 3V3 and that the yellow wire goes to GP22 (physical pin 29).

Reads 85.0 C

85 °C is the power-on value: the conversion did not finish. Keep the 750 ms wait after convert_temp() and check the supply.

onewire.OneWireError

The bus is shorted or a probe dropped off mid-read — usually a loose screw terminal or water in a cheap probe’s joint.