int LAMP = 3;
int dimming = 1;
int i = 0;

String str;

void setup()
{
Serial.begin(9600);
delay(300);

  pinMode(LAMP, OUTPUT);

  attachInterrupt(0, zero_crosss_int, RISING);
}

void loop()
{
       Serial.println(dimming);
    if(Serial.available() > 0)
    {
      str = Serial.readStringUntil('\n');
      dimming = str.toInt();
      Serial.println(dimming);
    }
      delay(100);
}

void zero_crosss_int()
{
  int dimtime = (65 * dimming);
  delayMicroseconds(dimtime);
  digitalWrite(LAMP, HIGH);
  delayMicroseconds(8.33);
  digitalWrite(LAMP, LOW);
}