uLisp Sensor Library - MAX6675 Thermocouple Amplifier

MAX6675 Thermocouple Amplifier

The MAX6675 ^[1] will measure K type thermocouples over a range of 0 °C to 1024 °C, which is ideal for many applications such as a cooking thermometer, or for measuring soldering temperatures. Although it has been superseded by the MAX31855, which offers a wider temperature range of -270 °C to 1800 °C, the MAX6675 has the advantages of low cost, and the fact that it will work at 5 V, whereas the MAX31855 is 3.3 V maximum.

Boards are available on eBay, Banggood, and AliExpress, often with a sensor included ^[2]:

Reading the thermocouple

The MAX6675 needs to be connected to the Arduino via five cables: 5V, GND, and the three SPI lines SO, CS, and CLK.

Here's the uLisp interface:

(defvar so 8)
(defvar cs 9)
(defvar clk 10)

; Initialise pins
(defun max6657-init ()
  (pinmode so nil)
  (pinmode cs t)
  (pinmode clk t)
  (digitalwrite cs 1))

; Read temperature in units of 0.25 degrees C
(defun max6657-temp ()
  (let ((dat 0))
    (digitalwrite cs 0)
    (dotimes (i 16)
      (digitalwrite clk 1)
      (when (digitalread so) (incf dat (ash 1 (- 15 i))))
      (digitalwrite clk 0))
    (digitalwrite cs 1)
    (ash dat -3)))

The I/O pins used to connect to the MAX6675 are defined by the three defvar statements, so change these if you want to use different pins.

First call (max6657-init) to initialise the I/O pins. Then call (max6657-temp) to read the temperature. This returns an integer in 1/4s of a degree Celsius. For example:

> (max6657-temp)
89

means that the temperature of the probe is 22.25 °C.