Room Temperature Controller: Relay and Fan Automation

A room temperature controller with relay and fan automation maintains comfortable room temperatures by automatically switching fans, coolers, or ACs based on sensor readings. This practical Arduino project is perfect for Indian homes and offices where temperature management directly affects comfort and productivity.

Project Overview

This project uses an Arduino or ESP32 to read room temperature, display it on an LCD, and control a relay that switches an AC fan or cooler. The system turns cooling on when temperature exceeds the set point and off when it drops below — with hysteresis to prevent rapid cycling.

Components Required

• Arduino Uno (₹193) or ESP32 (₹329 for WiFi)
• DHT22 temperature and humidity sensor (₹91)
• 16×2 I2C LCD display for local readout
• Relay module — 5V for fan, SSR for AC unit
• Pushbuttons (2) for set point adjustment
• Buzzer for alerts
• 5V power supply or USB power

Circuit Design

• DHT22: VCC→5V, GND→GND, Data→Arduino pin 2
• LCD I2C: SDA→A4, SCL→A5
• Relay IN→Arduino pin 7
• Button UP→pin 3, Button DOWN→pin 4 (with internal pull-ups)
• Buzzer→pin 8

Arduino Code with LCD Display

#include <DHT.h>
#include <LiquidCrystal_I2C.h>

#define DHT_PIN 2
#define RELAY_PIN 7
#define BTN_UP 3
#define BTN_DOWN 4
#define BUZZER 8
#define HYST 2.0

DHT dht(DHT_PIN, DHT22);
LiquidCrystal_I2C lcd(0x27, 16, 2);
float setpoint = 28.0;
bool fanOn = false;

void setup() {
  dht.begin();
  lcd.init(); lcd.backlight();
  pinMode(RELAY_PIN, OUTPUT);
  pinMode(BTN_UP, INPUT_PULLUP);
  pinMode(BTN_DOWN, INPUT_PULLUP);
  pinMode(BUZZER, OUTPUT);
}

void loop() {
  if (!digitalRead(BTN_UP)) { setpoint += 0.5; delay(200); }
  if (!digitalRead(BTN_DOWN)) { setpoint -= 0.5; delay(200); }
  setpoint = constrain(setpoint, 18, 35);
  
  float temp = dht.readTemperature();
  float hum = dht.readHumidity();
  
  if (temp >= setpoint + HYST && !fanOn) {
    digitalWrite(RELAY_PIN, HIGH); fanOn = true;
  } else if (temp = 40) tone(BUZZER, 2000, 500); // High temp alarm
  
  lcd.setCursor(0, 0);
  lcd.print("T:"); lcd.print(temp,1); lcd.print("C ");
  lcd.print("H:"); lcd.print(hum,0); lcd.print("%");
  lcd.setCursor(0, 1);
  lcd.print("Set:"); lcd.print(setpoint,1);
  lcd.print(fanOn ? " FAN ON " : " FAN OFF");
  delay(2000);
}

Multi-Zone Control

For multi-room control, add more DHT22 sensors and relay channels. Each zone gets its own set point. Use an Arduino Mega (more pins) or ESP32 with I2C expander for 4+ zones.

Adding WiFi with ESP32

Upgrade to an ESP32 to add WiFi control, smartphone app (via Blynk or web server), temperature logging to InfluxDB, and Telegram alerts. The core control logic remains identical — only the communication layer changes.

Recommended Components

Extending the Project

• Add a real-time clock (DS3231) for scheduled temperature profiles (cooler at night, warmer during day)
• Add an SD card for data logging — analyse temperature patterns over weeks
• Integrate with home automation (MQTT to Home Assistant)
• Add occupancy detection (PIR sensor) — turn off cooling when room is empty