const int latchPin = 5;  // Latch pin of 74HC595 is connected to Digital pin 5
const int clockPin = 6;  // Clock pin of 74HC595 is connected to Digital pin 6
const int dataPin = 4;   // Data pin of 74HC595 is connected to Digital pin 4

byte leds = 0;  // Variable to hold the pattern of which LEDs are currently turned on or off

void setup() {
  // Set all the pins that control 74HC595 as OUTPUT
  pinMode(latchPin, OUTPUT);
  pinMode(dataPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
  Serial.begin(9600);
  resetAllRelays();
}

void loop() {
  if (Serial.available() > 0) {
    int number = Serial.parseInt();
    Serial.print("Relay ");
    Serial.print(number);
    Serial.print(" is now ");
    if (bitRead(leds, number) == 0) {
      Serial.println("on");
      bitWrite(leds, number, 1);
    } else {
      Serial.println("off");
      bitWrite(leds, number, 0);
    }
    updateShiftRegister();
  }
}

void updateShiftRegister() {

  digitalWrite(latchPin, LOW);
  shiftOut(dataPin, clockPin, LSBFIRST, ~leds);
  digitalWrite(latchPin, HIGH);
}

void resetAllRelays() {
  digitalWrite(latchPin, LOW);
  shiftOut(dataPin, clockPin, LSBFIRST, 0b11111111);
  digitalWrite(latchPin, HIGH);
}