Circuit Documentation

Summary

This circuit is designed to interface with a PT100 temperature sensor through the Adafruit MAX31865 RTD Sensor Breakout. It controls a 1 Channel 5V Relay Module based on temperature thresholds and displays the temperature readings on an I2C LCD display. The Arduino Mega 2560 serves as the central microcontroller unit, managing the sensor data, controlling the relay, and communicating with the LCD display.

Component List

Arduino Mega 2560

• Microcontroller board based on the ATmega2560
• It has 54 digital input/output pins, 16 analog inputs, 4 UARTs, a 16 MHz crystal oscillator, a USB connection, a power jack, an ICSP header, and a reset button.

Adafruit MAX31865 RTD Sensor Breakout

• An interface board for reading resistance temperature detectors (RTDs)
• Communicates with the Arduino via SPI and provides a digital reading of temperature.

PT100

• A platinum resistance temperature detector (RTD) with high accuracy
• It changes resistance with temperature and is interfaced with the MAX31865 breakout board.

1 Channel 5V Relay Module

• A relay module that allows for controlling high power devices with a low power signal
• It has one channel and can be driven by 5V from the Arduino.

LCD I2C Display

• A liquid crystal display that uses the I2C communication protocol
• It can display text on two lines with 16 characters each.

Wiring Details

Arduino Mega 2560

• 5V to LCD I2C Display VCC, Adafruit MAX31865 RTD Sensor Breakout VIN, and 1 Channel 5V Relay Module VCC+
• GND to LCD I2C Display GND, Adafruit MAX31865 RTD Sensor Breakout GND, and 1 Channel 5V Relay Module VCC- (GND)
• D21/SCL to LCD I2C Display SCL
• D20/SDA to LCD I2C Display SDA
• D52 to Adafruit MAX31865 RTD Sensor Breakout SCLK
• D50 to Adafruit MAX31865 RTD Sensor Breakout SDO
• D48 to Adafruit MAX31865 RTD Sensor Breakout DRDY
• D53 to Adafruit MAX31865 RTD Sensor Breakout CS
• D51 to Adafruit MAX31865 RTD Sensor Breakout SDI
• D35 to 1 Channel 5V Relay Module IN

Adafruit MAX31865 RTD Sensor Breakout

• RTDIN+ to PT100 -
• RTDIN- to PT100 +

PT100

• - to Adafruit MAX31865 RTD Sensor Breakout RTDIN+
• + to Adafruit MAX31865 RTD Sensor Breakout RTDIN-

1 Channel 5V Relay Module

• IN to Arduino Mega 2560 D35
• VCC+ to Arduino Mega 2560 5V
• VCC- (GND) to Arduino Mega 2560 GND

LCD I2C Display

• VCC to Arduino Mega 2560 5V
• GND to Arduino Mega 2560 GND
• SCL to Arduino Mega 2560 D21/SCL
• SDA to Arduino Mega 2560 D20/SDA

Documented Code

Arduino Mega 2560 Code

/*
 * This Arduino sketch interfaces with a PT100 temperature sensor via the
 * MAX31865 RTD Sensor Breakout, controls a relay module based on temperature
 * thresholds, and displays information on an I2C LCD display.
 */

#include <SPI.h>
#include <Adafruit_MAX31865.h>
#include <Wire.h>
#include <LiquidCrystal_I2C.h>

// MAX31865 pins
#define MAX31865_CS 53
#define MAX31865_DRDY 48

// Relay pin
#define RELAY_PIN 35

// Create the RTD sensor object
Adafruit_MAX31865 rtd = Adafruit_MAX31865(MAX31865_CS);

// The value of the Rref resistor
#define RREF 420.0

// Temperature threshold
float tempThreshold = 0.0;

// Create the LCD object
LiquidCrystal_I2C lcd(0x27, 16, 2);

void setup() {
  Serial.begin(9600);
  while (!Serial) delay(10); // Wait for serial monitor

  Serial.println("Enter temperature threshold:");
  while (Serial.available() == 0) delay(10); // Wait for input
  tempThreshold = Serial.parseFloat();
  Serial.print("Threshold set to: ");
  Serial.println(tempThreshold);

  // Initialize the MAX31865
  rtd.begin(MAX31865_2WIRE);

  // Set relay pin as output
  pinMode(RELAY_PIN, OUTPUT);
  digitalWrite(RELAY_PIN, LOW); // Ensure relay is off

  // Initialize the LCD
  lcd.begin();
  lcd.backlight();
  lcd.print("Temp: ");
}

void loop() {
  uint16_t rtdRaw = rtd.readRTD();
  float ratio = rtdRaw;
  ratio /= 32768;
  float rtdResistance = ratio * RREF;
  float temperature = (rtdResistance - 100) / 0.385;

  Serial.print("Temperature: ");
  Serial.println(temperature);

  lcd.setCursor(6, 0);
  lcd.print(temperature);
  lcd.print(" C");

  if (temperature >= tempThreshold) {
    digitalWrite(RELAY_PIN, HIGH); // Turn on relay
  } else {
    digitalWrite(RELAY_PIN, LOW); // Turn off relay
  }

  delay(1000); // Wait 1 second before next read
}

Adafruit MAX31865 RTD Sensor Breakout Code

The Adafruit MAX31865 RTD Sensor Breakout does not have separate code as it is controlled by the Arduino Mega 2560 through the SPI interface. The relevant code is included in the Arduino Mega 2560 sketch above.