```
// Pins (CONFIRMED by schematic)
#define GATE1 8    // → MOC3023M (OK1) → Triac U1 (FA)
#define GATE2 9    // → MOC3023M (OK2) → Triac U8 (FB)
#define GATE3 10   // → MOC3023M (OK3) → Triac U9 (FC)
#define ZC1 2      // ← PC817 (OP1) phase FA
#define ZC2 3      // ← PC817 (OP2) phase FB
#define ZC3 4      // ← PC817 (OP3) phase FC
#define POT A0     // Potentiometer input

volatile bool ZC_flag[3] = {0, 0, 0};
volatile unsigned long ZC_time[3];
unsigned long alpha = 0;
bool fired[3] = {0, 0, 0};

void handleZC(int idx) {
  ZC_flag[idx] = true;
  ZC_time[idx] = micros();
  fired[idx] = false;
}

void setup() {
  // Triac control pins
  pinMode(GATE1, OUTPUT);
  pinMode(GATE2, OUTPUT);
  pinMode(GATE3, OUTPUT);
  digitalWrite(GATE1, LOW);
  digitalWrite(GATE2, LOW);
  digitalWrite(GATE3, LOW);

  // Zero-crossing inputs (with pullups)
  pinMode(ZC1, INPUT_PULLUP);
  pinMode(ZC2, INPUT_PULLUP);
  pinMode(ZC3, INPUT_PULLUP);

  // Interrupts for phases FA & FB (FALLING edge)
  attachInterrupt(digitalPinToInterrupt(ZC1), []{ handleZC(0); }, FALLING);
  attachInterrupt(digitalPinToInterrupt(ZC2), []{ handleZC(1); }, FALLING);

  // Pin-change interrupt for phase FC
  PCICR |= (1 << PCIE2);     // Enable PCINT2 group
  PCMSK2 |= (1 << PCINT20);  // Enable for D4 (PCINT20)
}

// Interrupt for phase FC
ISR(PCINT2_vect) {
  static uint8_t lastState = HIGH;
  uint8_t currentState = digitalRead(ZC3);
  if (lastState == HIGH && currentState == LOW) {
    handleZC(2);  // Falling edge detected
  }
  lastState = currentState;
}

void loop() {
  unsigned long now = micros();

  // Process all three phases
  for (int i = 0; i < 3; i++) {
    if (ZC_flag[i] && !fired[i] && (now - ZC_time[i] >= alpha)) {
      digitalWrite(GATE1 + i, HIGH);
      delayMicroseconds(200);  // 200µs trigger pulse
      digitalWrite(GATE1 + i, LOW);
      fired[i] = true;
    }
  }

  // Update alpha from pot every 100ms
  static unsigned long lastUpdate;
  if (millis() - lastUpdate > 100) {
    int potValue = analogRead(POT);
    alpha = (1023 - potValue) * 10;  // 0-10230 µs
    alpha = min(alpha, 9500);         // Cap at 95% of half-cycle
    lastUpdate = millis();
  }
}```