Engine Cooling, Reimagined by You

/* =========================================================================
 * DIY BMW E46 Cooling Fan Controller Firmware
 * Measures 12V 100Hz PWM from DME on Pin 2 via PC817 Optocoupler
 * Drives BTS7960 High-Current Driver on Pin 9
 * ========================================================================= */

#define PWM_IN_PIN 2     // Input pin from PC817 (ECU PWM)
#define PWM_OUT_PIN 9    // Output PWM pin to BTS7960 RPWM
#define FAILSAFE_MS 300  // No pulse timeout (300 milliseconds)

volatile unsigned long lastPulseTime = 0;
volatile unsigned long pulseHighDuration = 0;
volatile unsigned long pulsePeriod = 0;

void setup() {
  // Set input pin with internal pull-up (Optocoupler output pulls pin to GND)
  pinMode(PWM_IN_PIN, INPUT_PULLUP);
  pinMode(PWM_OUT_PIN, OUTPUT);
  
  // Attach interrupt to monitor signal state changes
  attachInterrupt(digitalPinToInterrupt(PWM_IN_PIN), handlePulseInterrupt, CHANGE);
}

void loop() {
  unsigned long currentTime = millis();
  unsigned long timeSinceLastPulse = currentTime - lastPulseTime;
  
  int targetSpeed = 0;

  // Check for Signal Loss (Fail-safe triggering)
  if (timeSinceLastPulse > FAILSAFE_MS) {
    // If ECU signal goes missing, drive fan at 100% capacity
    targetSpeed = 255; 
  } else {
    // Prevent zero-division errors
    if (pulsePeriod > 0) {
      // Calculate incoming duty cycle (0.0 to 1.0)
      // Note: Optocoupler inverts logic. Invert back in calculation.
      float dutyCycle = 1.0 - ((float)pulseHighDuration / (float)pulsePeriod);
      
      // Map DME 10% - 90% PWM range to 0 - 255 (BTS7960 Output range)
      if (dutyCycle < 0.10) {
        targetSpeed = 0; // Keep fan off
      } else if (dutyCycle > 0.90) {
        targetSpeed = 255; // Maximum cooling speed
      } else {
        // Linearly scale from 10% duty cycle (OFF) to 90% duty cycle (MAX)
        targetSpeed = map(dutyCycle * 100, 10, 90, 0, 255);
        targetSpeed = constrain(targetSpeed, 0, 255);
      }
    }
  }

  // Drive BTS7960 motor speed
  analogWrite(PWM_OUT_PIN, targetSpeed);
  
  // Add small loop smoothing delay
  delay(30);
}

// Interrupt Service Routine to capture pulse durations
void handlePulseInterrupt() {
  static unsigned long lastChange = 0;
  static unsigned long lastRising = 0;
  
  unsigned long now = micros();
  int pinVal = digitalRead(PWM_IN_PIN);
  
  lastPulseTime = millis(); // Reset fail-safe watchdog

  if (pinVal == HIGH) {
    // Pin went high (inverting optocoupler meant input LED went off)
    pulsePeriod = now - lastRising;
    lastRising = now;
  } else {
    // Pin went low (optocoupler LED turned on)
    pulseHighDuration = now - lastRising;
  }
}