Temperature Controlled Relay: AC Fan and Heater Switch

A temperature controlled relay automatically switches AC-powered fans, heaters, or other loads based on temperature readings. This is the foundation for thermostat projects, incubators, fermentation chambers, and HVAC control. This guide covers building a safe, reliable temperature-controlled relay switch for both cooling and heating applications.

What Is a Temperature Controlled Relay?

A temperature controlled relay combines a temperature sensor (DS18B20 or DHT22), a microcontroller (Arduino), and a relay module to switch AC mains loads. When temperature exceeds a threshold, the relay activates a fan for cooling. When temperature drops below another threshold, the relay activates a heater. With proper hysteresis, this provides stable, automatic temperature control.

Components and Circuit Design

• Arduino Uno or Nano
• DS18B20 temperature sensor
• Relay module (5V, rated for your AC load)
• AC fan or heater (the load being controlled)
• 4.7kΩ pull-up resistor (for DS18B20)
• Enclosure rated for mains voltage
• Mains-rated wire (1.5mm² minimum for loads up to 10A)

For loads above 10A: Use an SSR (Solid State Relay) instead of a mechanical relay. SSRs handle higher currents, switch faster, and have no contact wear.

Wiring for AC Fan Control

AC fan cooling circuit:

• DS18B20 → Arduino pin 2 (with 4.7kΩ pull-up)
• Arduino pin 7 → Relay module IN
• AC mains Live → Relay COM (Common)
• Relay NO (Normally Open) → AC fan Live input
• AC mains Neutral → AC fan Neutral

When temperature rises above threshold, Arduino sets pin 7 HIGH, relay closes, fan runs. When temperature drops below threshold minus hysteresis, relay opens, fan stops.

IMPORTANT: The relay only switches the Live wire. Neutral connects directly to the load. This is a safety requirement — never switch Neutral while Live remains connected.

Wiring for AC Heater Control

AC heater control circuit: Same wiring as fan, but the logic is reversed — relay closes when temperature drops below the set point, and opens when temperature rises above the set point.

For a combined heating/cooling system (like an incubator), use two relays: one for the heater (activated below set point – hysteresis) and one for the fan/cooler (activated above set point + hysteresis). The dead zone between heater-off and cooler-on prevents both running simultaneously.

Arduino Code with Hysteresis

#include <OneWire.h>
#include <DallasTemperature.h>

#define SENSOR_PIN 2
#define FAN_RELAY 7    // Cooling relay
#define HEATER_RELAY 8 // Heating relay
#define SETPOINT 37.5  // Target temperature
#define HYST 1.0       // Hysteresis (±1°C)

OneWire oneWire(SENSOR_PIN);
DallasTemperature sensors(&oneWire);

void setup() {
  Serial.begin(9600);
  sensors.begin();
  pinMode(FAN_RELAY, OUTPUT);
  pinMode(HEATER_RELAY, OUTPUT);
  digitalWrite(FAN_RELAY, LOW);
  digitalWrite(HEATER_RELAY, LOW);
}

void loop() {
  sensors.requestTemperatures();
  float temp = sensors.getTempCByIndex(0);
  
  // Cooling: ON above setpoint + hysteresis
  if (temp >= SETPOINT + HYST) {
    digitalWrite(FAN_RELAY, HIGH);  // Fan ON
  } else if (temp <= SETPOINT) {
    digitalWrite(FAN_RELAY, LOW);   // Fan OFF
  }
  
  // Heating: ON below setpoint - hysteresis
  if (temp = SETPOINT) {
    digitalWrite(HEATER_RELAY, LOW);  // Heater OFF
  }
  
  Serial.print("Temp: "); Serial.print(temp);
  Serial.print(" Fan: "); Serial.print(digitalRead(FAN_RELAY));
  Serial.print(" Heater: "); Serial.println(digitalRead(HEATER_RELAY));
  delay(2000);
}

Safety Considerations for Mains Switching

WARNING: This circuit switches AC mains voltage (230V in India). Improper wiring can cause electrocution or fire.

• Use relay modules rated for at least 1.5x your load current at 250V AC
• Use mains-rated wire (1.5mm² minimum, ISI marked)
• Enclose ALL mains connections in an insulated box — no exposed terminals
• Use a proper mains plug with earth connection
• Add a fuse rated for your load current in the Live line
• Test with a multimeter before connecting the load
• Keep low-voltage (Arduino) and high-voltage (mains) wiring physically separated
• Consider using an SSR for mains switching — no mechanical contacts to arc and weld

Recommended Components

Common Applications

• Egg incubator: Maintain 37.5°C with heater and fan relays
• Mushroom growing chamber: Maintain 20-25°C with humidifier and exhaust fan
• Fermenter: Maintain lager temp 10-14°C or ale temp 18-22°C
• Server room backup: Emergency fan activation if main AC fails
• Greenhouse: Heating at night, ventilation during day
• Aquarium: Heater control for tropical fish (25-28°C)