MAX6675 thermocouple with Raspberry Pi Pico — MicroPython high-temperature reading

A thermocouple measures far hotter than any digital sensor — up to about 1000 °C with a K-type probe — and the MAX6675 module turns its tiny voltage into a number. On the Raspberry Pi Pico it reads cleanly over SPI1: SCK on GP10, SO on GP12, CS on GP13.

Power it from 3V3, screw the thermocouple into T+ and T−, and the script prints the temperature twice a second.

Wiring

Every connection
Raspberry Pi Pico	MAX6675 K-type thermocouple module	Wire	Notes
—	T+	—	To K-type thermocouple + (non-magnetic leg) (not drawn)
—	T-	—	To K-type thermocouple − (magnetic leg) (not drawn)
GND	GND	black
3V3	VCC	red	3V3 is pin 36. SO then switches at 3.3 V.
GP10	SCK	yellow	SPI1 SCK, physical pin 14.
GP13	CS	orange	Physical pin 17.
GP12	SO	blue	SPI1 RX, physical pin 16.

3.0–5.5 V part, 3.3 V board. The MAX6675 runs from 3.0–5.5 V and its SO output follows VCC. Power it from 3.3 V on ESP32 and Pico, 5 V on Uno — then every pin matches your board.

Board quirks

The MAX6675 is read-only: two bytes over SPI, temperature = bits 15–3 × 0.25 °C. Bit 2 set means the thermocouple is disconnected — the script reports it and lights the GP25 LED.
Power it from 3V3, not VBUS: SO follows VCC, and the Pico’s pins are not 5 V tolerant.
A conversion takes about 220 ms, so read no faster than every 250 ms or you just get the previous value.
Uses for Indian workshops: tandoor or kiln temperature, soldering-iron checks, motor and transformer heating. Keep the electronics away from the heat — only the probe goes in.

Code

main.pyMicroPython

# MAX6675 + K-type thermocouple (0-1024 C) on a Raspberry Pi Pico (MicroPython, hardware SPI1).
# Wiring: VCC -> 3V3 (pin 36), GND -> GND, SCK -> GP10 (pin 14), SO -> GP12 (pin 16, SPI1 RX),
# CS -> GP13 (pin 17). Thermocouple + -> T+, - -> T-.
from machine import Pin, SPI
import time

spi = SPI(1, baudrate=1_000_000, polarity=0, phase=0, sck=Pin(10), mosi=Pin(11), miso=Pin(12))
cs = Pin(13, Pin.OUT, value=1)
led = Pin(25, Pin.OUT)


def read_c():
    cs.value(0)
    hi, lo = spi.read(2)
    cs.value(1)  # CS HIGH starts the next conversion (about 220 ms)
    raw = (hi << 8) | lo
    if raw & 0x04:  # D2 set = thermocouple input open
        return None
    return (raw >> 3) * 0.25


print("MAX6675 on SPI1 (SCK GP10, SO GP12, CS GP13)")
time.sleep_ms(500)
while True:
    t = read_c()
    if t is None:
        led.on()
        print("No thermocouple - check the T+ / T- screws")
    else:
        led.off()
        print("Thermocouple {:.2f} C".format(t))
    time.sleep_ms(500)  # never faster than 250 ms - it would return the old value

# MAX6675 + K-type thermocouple (0-1024 C) on a Raspberry Pi Pico (MicroPython, hardware SPI1).
# Wiring: VCC -> 3V3 (pin 36), GND -> GND, SCK -> GP10 (pin 14), SO -> GP12 (pin 16, SPI1 RX),
# CS -> GP13 (pin 17). Thermocouple + -> T+, - -> T-.
from machine import Pin, SPI
import time

spi = SPI(1, baudrate=1_000_000, polarity=0, phase=0, sck=Pin(10), mosi=Pin(11), miso=Pin(12))
cs = Pin(13, Pin.OUT, value=1)
led = Pin(25, Pin.OUT)


def read_c():
    cs.value(0)
    hi, lo = spi.read(2)
    cs.value(1)  # CS HIGH starts the next conversion (about 220 ms)
    raw = (hi << 8) | lo
    if raw & 0x04:  # D2 set = thermocouple input open
        return None
    return (raw >> 3) * 0.25


print("MAX6675 on SPI1 (SCK GP10, SO GP12, CS GP13)")
time.sleep_ms(500)
while True:
    t = read_c()
    if t is None:
        led.on()
        print("No thermocouple - check the T+ / T- screws")
    else:
        led.off()
        print("Thermocouple {:.2f} C".format(t))
    time.sleep_ms(500)  # never faster than 250 ms - it would return the old value

Expected serial output

REPL / Thonny shellOutput

MAX6675 on SPI1 (SCK GP10, SO GP12, CS GP13)
Thermocouple 29.75 C
Thermocouple 186.50 C
No thermocouple - check the T+ / T- screws

MAX6675 on SPI1 (SCK GP10, SO GP12, CS GP13)
Thermocouple 29.75 C
Thermocouple 186.50 C
No thermocouple - check the T+ / T- screws

Common errors

Temperature drops when you heat the probe

T+ and T− are swapped. The leg a magnet sticks to is T−.

Always “No thermocouple”

Loose screw terminals or a broken probe. Measure the probe with a multimeter: a few ohms is good, open circuit is broken.

Reads 0.00 C constantly

SO isn’t reaching GP12, or CS/SCK are swapped. Check the SPI1 pins.