unsigned char AC_LOAD = 4;
unsigned char dimming = 3;
unsigned char i = 55;

void setup() {
  // put your setup code here, to run once:
  pinMode(AC_LOAD, OUTPUT);
  pinMode(3, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(3), zero_cross_int, RISING);

}

void zero_cross_int() {
  int dimtime = (100 * i);
  delayMicroseconds(dimtime);
  digitalWrite(AC_LOAD, HIGH);
  delayMicroseconds(10);
  digitalWrite(AC_LOAD, LOW);
  EIFR = 1; EIFR = 2;
}

void loop() {
  // put your main code here, to run repeatedly:

}