Thermocouple interface

Thermocouples are a great choice for measuring the temperature at a probe, and work over a wide range; for example, the widely-used K-type thermocouple has a range of -200°C to +1350°C.

This post describes a simple thermocouple interface using uLisp, based on a low-cost MAX6675 Thermocouple Board, widely available on eBay and Chinese sites such as Banggood, often with a sensor included [1]:

MAX6675.jpg

The MAX6675 has a range of 0°C to 1024°C, which is ideal for many applications such as a cooking thermometer, or for measuring soldering temperatures [2]. Although it has been superseded by the MAX31855, which offers a wider temperature range of -270°C to 1800°C, the MAX6675 has the advantages of low cost, and the fact that it will work at 5V, whereas the MAX31855 is 3.3V maximum.

uLisp interface

The MAX6675 needs to be connected to the Arduino via five cables: 5V, GND, and the three SPI lines SO, CS, and CLK. I used an Arduino Uno, but any board should be suitable.

Here's the uLisp interface:

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

; Initialise pins
(defun tci ()
  (pinmode so nil)
  (pinmode cs t)
  (pinmode clk t)
  (digitalwrite cs 1))

; Read temperature in units of 0.25 degrees C
(defun tco ()
  (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 (tci) to initialise the I/O pins. Then call (tci) to read the temperature. This returns an integer in units of quarters of a degree Centigrade. For example:

> (tco)
89

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

What you do with the temperature depends on your application. For example, if you're making a sugar thermometer for sweet-making you could make it display the caramel stage; for example:

(let ((tmp (/ (tco) 4)))
  (cond
   ((>= 116 tmp 112) "Soft Ball")
   ...
   ((>= 130 tmp 121) "Hard Ball"))

  1. ^ MAX6675 Sensor Module Thermocouple on Banggood.
  2. ^ MAX6675 datasheet on Maxim Integrated.