Smart Fan Speed Controller: PWM with Temperature Sensor

A smart fan speed controller that automatically adjusts fan speed based on room temperature is one of the most practical home automation projects for Indian homes. During harsh summers in cities like Nagpur, Hyderabad, or Jaisalmer, a fan running at full speed all night disturbs sleep, while a fan that slows down as the room cools overnight gives perfect comfort. This project uses PWM (pulse width modulation) and a DHT22 or NTC thermistor to create a fully automatic temperature-controlled fan — controllable via Alexa, Google Home, or Home Assistant.

We cover two variants: a low-voltage DC fan controller (for 12V computer fans and exhaust fans) and a 230V AC ceiling fan controller using TRIAC dimming — the most common scenario in Indian households.

PWM vs TRIAC: Which to Use?

Understanding the difference is essential before building:

DC Fan PWM Controller (12V)

For computer case fans, exhaust fans, or 12V blowers, PWM control is simple and safe. Use an N-channel MOSFET (IRLZ44N) as the switch:

Components:

• ESP32 or ESP8266 NodeMCU
• IRLZ44N or 2N7000 N-channel MOSFET (logic-level gate, works with 3.3V)
• 10k gate resistor, 10k pull-down resistor
• Flyback diode (1N4007) across motor terminals
• DHT22 temperature sensor
• 12V power supply for fan, 5V for ESP

const int FAN_PIN = 16;     // GPIO16 for PWM
const int DHT_PIN = 4;      // DHT22 data pin

// Temperature to fan speed mapping
int tempToSpeed(float temp) {
  if (temp < 26) return 0;      // Off below 26 degrees C
  if (temp < 28) return 64;     // 25% speed 26-28 C
  if (temp < 30) return 128;    // 50% speed 28-30 C
  if (temp < 33) return 192;    // 75% speed 30-33 C
  return 255;                    // 100% above 33 C
}

void setup() {
  ledcAttachPin(FAN_PIN, 0);
  ledcSetup(0, 25000, 8);  // 25 kHz, 8-bit resolution
}

void loop() {
  float temp = dht.readTemperature();
  int speed = tempToSpeed(temp);
  ledcWrite(0, speed);
  delay(10000);
}

The 25 kHz PWM frequency is above human hearing range — fans run silently without the buzzing noise that lower PWM frequencies cause. ESP32’s LEDC peripheral is ideal for this application.

AC Ceiling Fan TRIAC Controller

Indian ceiling fans run on 230V AC at 50 Hz. Speed control requires a TRIAC that fires at a variable phase angle — the later in the AC cycle it fires, the less power reaches the fan motor and the slower it spins. This is identical to the mechanism used in traditional rotary fan regulators.

Components:

• BT136 or BT137 TRIAC (600V, 4A rating)
• MOC3021 or MOC3041 optocoupler (for isolation between ESP and mains)
• Zero-crossing detector (4N25 optocoupler + 22k resistor)
• 330-ohm gate resistor
• Snubber network (100-ohm + 100nF capacitor) for inductive fan loads

// Zero-crossing interrupt for TRIAC control
volatile bool zeroCross = false;
const int ZC_PIN = 2;      // Zero crossing input
const int TRIAC_PIN = 3;   // TRIAC gate output

int fanSpeed = 50;  // 0-100%, maps to phase angle

void IRAM_ATTR zeroCrossISR() {
  zeroCross = true;
}

void setup() {
  pinMode(ZC_PIN, INPUT_PULLUP);
  pinMode(TRIAC_PIN, OUTPUT);
  attachInterrupt(digitalPinToInterrupt(ZC_PIN),
                  zeroCrossISR, RISING);
}

void loop() {
  if (zeroCross) {
    zeroCross = false;
    // Delay proportional to (100 - speed)
    // At 50 Hz, half-cycle = 10000 microseconds
    int delayUs = (100 - fanSpeed) * 100;
    delayMicroseconds(delayUs);
    digitalWrite(TRIAC_PIN, HIGH);
    delayMicroseconds(50);
    digitalWrite(TRIAC_PIN, LOW);
  }
}

Temperature Sensing with DHT22

Place the DHT22 sensor away from the fan airstream — measure ambient room temperature, not the air the fan is pushing. Mount it at the opposite wall from the fan, at sitting/sleeping height (1-1.5 metres from floor):

#include <DHT.h>

DHT dht(4, DHT22);

void setup() {
  dht.begin();
}

float getAveragedTemp() {
  float sum = 0;
  int count = 0;
  for (int i = 0; i < 5; i++) {
    float t = dht.readTemperature();
    if (!isnan(t)) { sum += t; count++; }
    delay(200);
  }
  return count > 0 ? sum / count : 25.0;
}

Averaging 5 readings eliminates sensor noise and false triggers. DHT22 accuracy is +/- 0.5 degrees C — sufficient for comfort-based fan control where 1-degree precision is acceptable.

ESPHome Fan Entity

For DC fan control with ESPHome, the fan platform creates a speed-controllable fan entity in Home Assistant:

esphome:
  name: smart-fan
  platform: ESP32
  board: esp32dev

wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password

api:
ota:
logger:

sensor:
  - platform: dht
    pin: GPIO4
    model: DHT22
    temperature:
      name: "Room Temperature"
      id: room_temp
    humidity:
      name: "Room Humidity"
    update_interval: 30s

output:
  - platform: ledc
    pin: GPIO16
    frequency: 25000
    id: fan_output

fan:
  - platform: speed
    output: fan_output
    name: "Smart Ceiling Fan"
    speed_count: 5  # 5 speed steps like a real fan regulator

The fan entity appears in Home Assistant with speed slider 1-5, on/off toggle, and can be controlled by voice (“Alexa, set the bedroom fan to speed 3”).

PID Temperature Control

A basic step-function speed mapping is simple but may cause uncomfortable oscillation — fan suddenly jumps from 25% to 75% speed as temperature crosses a threshold. A PID (proportional-integral-derivative) controller provides smooth, gradual speed adjustment:

// Simple P-controller for fan speed
float kP = 15.0;         // Proportional gain
float targetTemp = 27.0; // Target room temperature

int calculateFanSpeed(float currentTemp) {
  float error = currentTemp - targetTemp;

  if (error <= 0) return 0;  // At or below target: off

  int speed = (int)(kP * error);
  return constrain(speed, 0, 100);
}

This proportional controller increases fan speed smoothly as room temperature rises above the target. At 27°C (setpoint), fan is off. At 28°C (1 degree above), fan runs at 15%. At 30°C, fan runs at 45%. At 33°C (6 degrees above), fan runs at 90%.

Home Assistant Dashboard

Build an automation to enforce temperature-based fan speed in Home Assistant:

automation:
  - alias: "Auto Fan Speed by Temperature"
    trigger:
      - platform: state
        entity_id: sensor.room_temperature
    action:
      - choose:
          - conditions:
              - condition: numeric_state
                entity_id: sensor.room_temperature
                below: 26
            sequence:
              - service: fan.turn_off
                target:
                  entity_id: fan.smart_ceiling_fan
          - conditions:
              - condition: numeric_state
                entity_id: sensor.room_temperature
                above: 30
            sequence:
              - service: fan.set_percentage
                target:
                  entity_id: fan.smart_ceiling_fan
                data:
                  percentage: 100

Frequently Asked Questions

Can I control an existing Usha or Orient ceiling fan with this?

Yes. Indian ceiling fans (Usha, Orient, Crompton, Bajaj, Havells) all run on 230V AC single-phase. The TRIAC controller works with any capacitor-run induction motor ceiling fan. Ensure the TRIAC is rated for at least twice the fan’s current draw (typically 0.4-0.6 A for a 75W fan, so use a 2A TRIAC minimum).

Why does the fan make a buzzing noise at low TRIAC speeds?

Buzzing at low speeds is normal with TRIAC control on inductive motors — phase angle cutting creates harmonic distortion that the motor coil vibrates at. Add a proper snubber network (100-ohm resistor + 100 nF capacitor in series across the TRIAC) to reduce this. Setting a minimum speed of 30-40% also helps.

What is the minimum temperature the fan should turn on?

For Indian comfort standards (ASHRAE adaptive comfort model for warm-humid climates), fans should run above 26-27°C. Below 26°C, fan air movement feels cold and uncomfortable. The automation should turn off the fan when temperature drops below this threshold.

Can the DHT22 be mounted inside the switch box?

No. Switch boxes are too small and get warm from wiring heat. Mount the DHT22 sensor in open air, away from direct sunlight, the fan airstream, and AC vents. A small white plastic enclosure on the wall at sitting height works well.

Will this work with BLDC inverter fans?

BLDC fans (like Crompton Energion, Atomberg Efficio) have their own electronics and speed control — they cannot be controlled with a TRIAC. Use a relay for on/off control only, and use the fan’s IR remote (with an ESP32 IR blaster) for speed changes.