Temperature-Controlled Fan with Thermistor and Transistor

A little fan that spins up only when things get hot — and spins faster the hotter it gets. Great for cooling a project box or a router shelf.

What you’ll build

A 10K NTC thermistor senses temperature; the Nano converts the reading to °C and drives a DC motor (your fan) with PWM through a BC547 transistor — off below 28°C, then speed ramping up smoothly to full blast at 40°C.

Components you’ll need

Step-by-step guide

1. Build the sensing divider: 5V → thermistor → A0, and from A0 a 10K resistor to GND.
2. Wire the driver: Nano D9 → 1K resistor → BC547 base; emitter to GND; motor between 5V and the collector.
3. Clip the 1N4007 flyback diode across the motor terminals, striped end to 5V — it protects the transistor from voltage spikes.
4. Attach a small propeller or fan blade to the DC motor.
5. Upload the sketch; warm the thermistor between your fingers and the fan should start, then accelerate.

The code

The Steinhart-Hart approximation turns the raw reading into real degrees, then the temperature maps to a PWM duty cycle.

const int therm=A0, fan=9;
float readC(){
  int v = analogRead(therm);
  float r = 10000.0 * v / (1023 - v);          // thermistor resistance
  float t = 1.0 / (log(r/10000.0)/3950.0 + 1.0/298.15) - 273.15;
  return t;
}
void setup(){ pinMode(fan,OUTPUT); Serial.begin(9600); }
void loop(){
  float t = readC();
  Serial.println(t);
  int speed = t <= 28 ? 0 : constrain(map(t*10, 280, 400, 80, 255), 0, 255);
  analogWrite(fan, speed);
  delay(1000);
}