Code Along - Chapter 19

Switching a Fan (MOSFET Driver)

Optional practice: switch a fan on and off through a MOSFET, set its speed with PWM, give it a soft start, and add a thermostat that does not flicker on and off.

Estimated time 30-45 min, optionalExercises passed 0 / 4

Optional practice: a fan on the virtual Uno

This practice is optional. Do it if you have time. Nothing in class depends on it.

The virtual board has a DC fan switched by a logic-level MOSFET on D9, wired like the lesson. The pin only drives the MOSFET's gate - the fan's current comes from its own supply. So the code is exactly what you would write for an LED on D9:

digitalWrite(fanPin, HIGH) turns the fan on at full power, LOW turns it off
analogWrite(fanPin, 0 to 255) sets the speed with PWM, as in Lesson 15

What the simulator does not model: the external supply, the gate resistors, the flyback diode, a MOSFET getting hot, or a fan that will not start at low PWM. Those are what the bench is for.

Worked example

The lesson's sketch

Press Run and watch the fan speed up from off to full, stop for a second, then start again.

const byte loadPin = 9;

void setup() {
  pinMode(loadPin, OUTPUT);
}

void loop() {
  for (int power = 0; power <= 255; power++) {
    analogWrite(loadPin, power);
    delay(10);
  }
  analogWrite(loadPin, 0);
  delay(1000);
}

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Each step of the loop raises the power by 1 and waits 10 ms, so the ramp takes about 2.5 s.
A real fan may not start turning until the PWM value is quite high. That is normal.

Exercise 19.1

Fan on, fan off

Not started

Turn the fan on for 2 seconds, then off for 1 second, over and over.

Use digitalWrite - on is HIGH, off is LOW.

const byte fanPin = 9;

void setup() {
  pinMode(fanPin, OUTPUT);
}

void loop() {
  // Fan on, wait 2 seconds, fan off, wait 1 second

}

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Exercise 19.2

Half speed, then full speed

Not started

Now set the speed with analogWrite. Each time round loop():

run the fan at half power (128) for 2 seconds
then at full power (255) for 2 seconds
then off (0) for 1 second

const byte fanPin = 9;

void setup() {
  pinMode(fanPin, OUTPUT);
}

void loop() {
  // Half power for 2 s, full power for 2 s, off for 1 s

}

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Exercise 19.3

Soft start, then hold

Not started

Starting a fan at full power all at once pulls a big surge of current. A soft start brings the power up in small steps instead.

Each time round loop():

use a for loop to raise the power from 0 to 255, one step at a time, waiting 10 ms after each step
hold full power for 2 seconds
then turn the fan off for 1 second

It is the lesson's sketch with a 2 second hold added.

const byte fanPin = 9;

void setup() {
  pinMode(fanPin, OUTPUT);
}

void loop() {
  // for loop: power from 0 to 255, analogWrite each step, then delay(10)

  // hold full power for 2 seconds

  // off for 1 second

}

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Exercise 19.4

A thermostat that does not flicker

Not started

The board now has a temperature sensor on A0 (move its slider while the sketch runs). Switch the fan on when the room is hot.

A single switch point causes trouble: if the fan turns on above 600 and off below 600, a reading that wobbles around 600 switches the fan on and off many times a second. The fix is a gap between the two switch points, called hysteresis:

reading above 600: fan on (HIGH)
reading below 500: fan off (LOW)
in between: do nothing - the fan stays as it was

Read the sensor every 50 ms. The test warms the room up, lets it cool a little, cools it right down, then warms it again - and checks the fan only changes at the right points.

const byte fanPin = 9;
const byte sensorPin = A0;

void setup() {
  pinMode(fanPin, OUTPUT);
}

void loop() {
  int reading = analogRead(sensorPin);
  // Above 600: fan on. Below 500: fan off. In between: leave the fan alone.

  delay(50);
}

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Try it on a real Uno

Build the MOSFET fan circuit from Lesson 16 - gate resistor, pull-down, flyback diode and common GND - and upload Half speed, then full speed. Measure VDS at full power: well under 1 V means the MOSFET is fully on.

For the thermostat, a potentiometer on A0 stands in for the temperature sensor, as in the lesson's challenge.