volatile bool mydelay   = false;
volatile int myvalue    = 5000;
uint8_t last_CH1_state  = 0;

const int PinDiac       = 3;
// const int PinLed        = 11;

enum { LedOn = HIGH, LedOff = LOW };

// -----------------------------------------------------------------------------
void setup () {
  Serial.begin (9600);

  PCICR  |= (1 << PCIE0);   //enable PCMSK0 scan
  PCMSK0 |= (1 << PCINT0);  //Set pin D8 trigger an interrupt on state change. Input from optocoupler

  pinMode       (PinDiac, OUTPUT); //Define D3 as output for the DIAC pulse
  pinMode       (LED_BUILTIN,  OUTPUT);
  digitalWrite  (PinDiac, HIGH);
  digitalWrite  (LED_BUILTIN, LedOff);
}

// -----------------------------------------------------------------------------
void loop () {
  if (Serial.available()) {
    char rec = Serial.read ();

      case 181:
        digitalWrite (LED_BUILTIN, LedOff);
        break;

      case 182:
        digitalWrite (LED_BUILTIN, LedOn);
        break;

      default:
        myvalue = map (rec, 0, 180, 6700, 10);
        Serial.print   ("  myvalue ");
        Serial.println (myvalue);
        break;
    }
  }

  if (mydelay) {
    delayMicroseconds (myvalue); //This delay controls the power
    digitalWrite      (PinDiac, HIGH);
    delayMicroseconds (100);
    digitalWrite      (PinDiac, LOW);
    mydelay = false;
  }
}

// -----------------------------------------------------------------------------
ISR (PCINT0_vect) {
  uint8_t currentState = PINB & B00000001;
  if (currentState != last_CH1_state) {
    last_CH1_state = currentState;
    mydelay = true;
  }
}