MAX31865 - Pt100 RTD module

In this video I show you how to use the MAX31865 platinum thermometer module with an Arduino. I tell you some general information about the platinum-based thermometers, how they work, I demonstrate some of the principles, then I show you a simple Arduino code. This thermometer is probably the most precise and accurate one among those which I have introduced on my channel so far.

MAX31865 datasheet

Analog Devices AN-709 document

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Wiring diagram

This circuit is very simple, it consists of three components. The 128x64 OLED display is connected to the Arduino via i2C (A4 = SDA, A5 = SCL). The Adafruit MAX31865 module is connected to the Arduino via SPI (CS =10, MOSI = 11, MISO = 12, SCK = 13). Both devices are using the 3.3 V as supply voltage.

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Soruce code

#include <Wire.h> //Arduino Uno; SDA = A4, SCL = A5
#include <SPI.h>
//CS => CS //Arduino 10
//MISO => SDO //Arduino 12
//MOSI => SDI //Arduino 11
//SCK => SCK //Arduino 13

//Variables for the PT100 boards
double resistance;
uint8_t reg1, reg2; //reg1 holds MSB, reg2 holds LSB for RTD
uint16_t fullreg; //fullreg holds the combined reg1 and reg2
double temperature;
//Variables and parameters for the R - T conversion
double Z1, Z2, Z3, Z4, Rt;
double RTDa = 3.9083e-3;
double RTDb = -5.775e-7;
double rpoly = 0;

//--Display---------------------------------------------
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

#define SCREEN_WIDTH 128 // OLED display width, in pixels
#define SCREEN_HEIGHT 64 // OLED display height, in pixels
#define OLED_RESET     4 // Reset pin 
Adafruit_SSD1306 display(OLED_RESET);

const int chipSelectPin = 10;

void setup()
{
  SPI.begin();
  Serial.begin(115200); //Start serial

  pinMode(chipSelectPin, OUTPUT); //because CS is manually switched  

  display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
  display.setTextSize(3);
  display.setTextColor(SSD1306_WHITE);
}

void loop()
{
  readRegister();
  convertToTemperature();
  printDisplay();
}

void printDisplay()
{
  display.clearDisplay();     // Clear display
  display.setCursor(0, 0);    // Start at top-left corner
  display.print(temperature);
  display.display();
}

void convertToTemperature()
{
  Rt = resistance;
  Rt /= 32768;
  Rt *= 430; //This is now the real resistance in Ohms

  Z1 = -RTDa;
  Z2 = RTDa * RTDa - (4 * RTDb);
  Z3 = (4 * RTDb) / 100;
  Z4 = 2 * RTDb;

  temperature = Z2 + (Z3 * Rt);
  temperature = (sqrt(temperature) + Z1) / Z4;

  if (temperature >= 0)
  {
    Serial.print("Temperature: ");
    Serial.println(temperature); //Temperature in Celsius degrees
    return; //exit
  }
  else
  {
    Rt /= 100;
    Rt *= 100; // normalize to 100 ohm

    rpoly = Rt;

    temperature = -242.02;
    temperature += 2.2228 * rpoly;
    rpoly *= Rt; // square
    temperature += 2.5859e-3 * rpoly;
    rpoly *= Rt; // ^3
    temperature -= 4.8260e-6 * rpoly;
    rpoly *= Rt; // ^4
    temperature -= 2.8183e-8 * rpoly;
    rpoly *= Rt; // ^5
    temperature += 1.5243e-10 * rpoly;

    Serial.print("Temperature: ");
    Serial.println(temperature); //Temperature in Celsius degrees
  }
  //Note: all formulas can be found in the AN-709 application note from Analog Devices
}


void readRegister()
{
  SPI.beginTransaction(SPISettings(500000, MSBFIRST, SPI_MODE1));
  digitalWrite(chipSelectPin, LOW);

  SPI.transfer(0x80); //80h = 128 - config register
  SPI.transfer(0xB0); //B0h = 176 - 10110000: bias ON, 1-shot, start 1-shot, 3-wire, rest are 0
  digitalWrite(chipSelectPin, HIGH);

  digitalWrite(chipSelectPin, LOW);
  SPI.transfer(1);
  reg1 = SPI.transfer(0xFF);
  reg2 = SPI.transfer(0xFF);
  digitalWrite(chipSelectPin, HIGH);

  fullreg = reg1; //read MSB
  fullreg <<= 8;  //Shift to the MSB part
  fullreg |= reg2; //read LSB and combine it with MSB
  fullreg >>= 1; //Shift D0 out.
  resistance = fullreg; //pass the value to the resistance variable
  //note: this is not yet the resistance of the RTD!

  digitalWrite(chipSelectPin, LOW);

  SPI.transfer(0x80); //80h = 128
  SPI.transfer(144); //144 = 10010000
  SPI.endTransaction();
  digitalWrite(chipSelectPin, HIGH);

  Serial.print("Resistance: ");
  Serial.println(resistance);
}