MAX6675 K-Type Thermocouple Thermometer

Read temperatures from a glowing soldering iron to a sub-freezing freezer with one chip and a few wires — the MAX6675 turns a fragile K-type thermocouple into a clean 12-bit SPI reading.

Difficulty: Breadboard. Estimated build time: about 30 minutes. Estimated parts cost: about US$9.50. 5-line bill of materials. Compare supplier offers when available. A digital thermometer that reads from a K-type thermocouple over a range of 0 to about 1024 degC, accurate to roughly +/-2 degC across most of that span.

Wire the SPI pins

Connect VCC to 5 V (or 3.3 V — the chip works on either), GND to ground, SCK to the SPI clock pin on your MCU, CS (chip select) to any free GPIO, and SO (serial out) to the SPI MISO pin. The MAX6675 is read-only over SPI — there is no MOSI line because the chip never receives commands; you just clock data out of it. The thermocouple wires go to the T+ and T- screw terminals on the breakout (yellow on red for K-type, by convention).

Read 16 bits

To take a reading: pull CS low, clock out 16 bits MSB-first, raise CS. Bit 15 is a dummy zero. Bits 14-3 are the 12-bit temperature reading. Bit 2 is the open-thermocouple flag (set if the probe is broken or disconnected). Bits 1-0 are unused. The temperature in degC is the 12-bit value divided by 4 — every count is 0.25 degC. A reading of 400 means 100.00 degC.

Print and sanity-check

Print the reading to your serial monitor once a second. Hold the probe tip against a glass of ice water — it should read close to 0 degC after a few seconds. Pinch the tip between your fingers — body heat gives you 32-35 degC. Hover the tip 1 cm above a soldering iron set to 350 degC and you should see the reading climb steadily into the 200-300 degC range (it won't quite hit 350 because the probe isn't touching). If the open-thermocouple bit fires, check that both leads are seated in the screw terminals.