Simple PWM fan controller for Arduino Nano

pwm-fan.ino

#include <EEPROM.h>

// 25 kHz PWM Fan Controller with Button Speed Steps
// Arduino Nano (ATmega328P)
// PWM on D9 (OC1A), Button on D2
// Onboard LED (D13) pulses reflect fan duty
// Last setting is stored in EEPROM

const byte PIN_BUTTON = 2;
const byte PIN_LED = 13;  // Onboard LED
const byte PIN_FAN_PWM = 9;
const int LED_PULSE_PERIOD = 500;  // Period for the pulsing LED indicator
const int EEPROM_ADDR_STEP = 0;    // EEPROM address to store last step
float SPEED_STEPS[] = { 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 };

unsigned long lastLedPeriod = 0;

const int numSteps = sizeof(SPEED_STEPS) / sizeof(SPEED_STEPS[0]);
int currentStep = 0;

bool lastButtonState = HIGH;
unsigned long lastDebounce = 0;

void setup() {
  pinMode(PIN_FAN_PWM, OUTPUT);       // PWM output
  pinMode(PIN_BUTTON, INPUT_PULLUP);  // Button input
  pinMode(PIN_LED, OUTPUT);           // Onboard LED

  // Timer1 → 25 kHz PWM on D9
  TCCR1A = 0;
  TCCR1B = 0;
  ICR1 = 639;  // TOP → 25 kHz
  TCCR1B |= (1 << WGM13) | (1 << WGM12);
  TCCR1A |= (1 << WGM11);
  TCCR1A |= (1 << COM1A1);  // non-inverting mode
  TCCR1B |= (1 << CS10);    // prescaler = 1

  // Load last step from EEPROM
  int storedStep = EEPROM.read(EEPROM_ADDR_STEP);
  if (storedStep >= 0 && storedStep < numSteps) {
    currentStep = storedStep;
  } else {
    currentStep = 0;
  }
}

void loop() {
  // ------------------- Button Handling -------------------
  bool reading = digitalRead(PIN_BUTTON);
  if (reading != lastButtonState) {
    if (reading == LOW && lastButtonState == HIGH && millis() - lastDebounce > 50) {
      currentStep++;
      if (currentStep >= numSteps) {
        currentStep = 0;
      }

      EEPROM.update(EEPROM_ADDR_STEP, currentStep);
    }
    lastDebounce = millis();
    lastButtonState = reading;
  }

  // ------------------- Apply Fan PWM -------------------
  float duty = SPEED_STEPS[currentStep];
  OCR1A = (uint16_t)(duty * ICR1);

  // ------------------- LED Pulse -------------------
  // Scale duty 0.0–1.0 to pulse length of onboard LED
  if (millis() - lastLedPeriod >= LED_PULSE_PERIOD * duty) {
    digitalWrite(PIN_LED, LOW);
  }
  if (millis() - lastLedPeriod >= LED_PULSE_PERIOD) {
    if (duty > 0) {
      digitalWrite(PIN_LED, HIGH);
    }
    lastLedPeriod = millis();
  }
}