A simple Arudino controller for triggering 2-wire dry contact heater with a 5v relay. It uses 1602 LCD and a rotary encoder to adjust the temperature.

heater-controller.ino

#include <LiquidCrystal.h>
#include <DHT11.h>

// ===== Pin Definitions =====
#define DHTPIN    8        // DHT11 data pin
DHT11 dht11(DHTPIN);

// LCD pins: RS, E, D4, D5, D6, D7
const int rsPin = 12;
const int enPin = 11;
const int d4Pin = 4;
const int d5Pin = 5;
const int d6Pin = 6;
const int d7Pin = 7;
LiquidCrystal lcd(rsPin, enPin, d4Pin, d5Pin, d6Pin, d7Pin);

// Rotary encoder pins
const int pinEncoderCLK = 2;  
const int pinEncoderDT  = 3;  

// Relay control pin
const int pinRelay = 10;      

// ===== Control Parameters =====
float targetTemp    = 20.0;    // Desired temperature (°C)
float smoothedTemp  = 20.0;    // Smoothed measured temperature
const float alpha       = 0.8; // Smoothing factor (0 < α < 1)
const float hysteresis  = 0.5; // Hysteresis band ±0.5°C

// ===== State Variables =====
bool relayOn        = false;   // Relay state
volatile bool encA_last = LOW; // Last state of encoder CLK
volatile bool encB_last = LOW; // Last state of encoder DT

void setup() {
  // Initialize relay pin
  pinMode(pinRelay, OUTPUT);
  digitalWrite(pinRelay, LOW); // Relay off

  // Initialize LCD (16 cols × 2 rows)
  lcd.begin(16, 2);
  lcd.clear();

  // Initialize encoder pins with pullups
  pinMode(pinEncoderCLK, INPUT_PULLUP);
  pinMode(pinEncoderDT,  INPUT_PULLUP);
  encA_last = digitalRead(pinEncoderCLK);
  encB_last = digitalRead(pinEncoderDT);
  // Attach interrupt on CLK pin
  attachInterrupt(digitalPinToInterrupt(pinEncoderCLK), handleEncoder, CHANGE);
}

void loop() {
  // 1. Read temperature & humidity from DHT11
  float newTemp = dht11.readTemperature();    // °C
  float hum     = dht11.readHumidity();       // %

  // If reading failed, skip update
  if (!isnan(newTemp)) {
    // 2. Exponential smoothing of temperature
    smoothedTemp = alpha * newTemp + (1 - alpha) * smoothedTemp;
  }

  // 3. Hysteresis control for relay
  if (!relayOn && smoothedTemp < targetTemp - hysteresis) {
    relayOn = true;
    digitalWrite(pinRelay, LOW);
  } else if (relayOn && smoothedTemp > targetTemp + hysteresis) {
    relayOn = false;
    digitalWrite(pinRelay, HIGH);
  }

  // 4. Update LCD display
  // lcd.clear();
  // Line 1: Setpoint and Humidity
  lcd.setCursor(0, 0);
  lcd.print("Set:");
  lcd.print(targetTemp, 1);
  lcd.print((char)223); // degree symbol
  lcd.print("C H:");
  if (!isnan(hum)) {
    lcd.print(hum, 0);
  } else {
    lcd.print("--");
  }
  lcd.print("%");

  // Line 2: Current temperature and relay state
  lcd.setCursor(0, 1);
  lcd.print("Cur:");
  lcd.print(smoothedTemp, 1);
  lcd.print((char)223);
  lcd.print("C ");
  lcd.print(relayOn ? "ON " : "OFF");

  delay(1000); // Update every second
}

// Interrupt service routine for rotary encoder
void handleEncoder() {
  bool encA = digitalRead(pinEncoderCLK);
  bool encB = digitalRead(pinEncoderDT);
  // Only act on state change of CLK
  if (encA != encA_last) {
    // Determine rotation direction
    if (encA != encB) {
      targetTemp -= 0.5;  // Clockwise → increase
    } else {
      targetTemp += 0.5;  // Counter-clockwise → decrease
    }
    // Clamp targetTemp within reasonable bounds
    if (targetTemp < 5.0)  targetTemp = 5.0;
    if (targetTemp > 30.0) targetTemp = 30.0;
  }
  encA_last = encA;
  encB_last = encB;
}