int stcp_Pin = 4;
int shcp_Pin = 3;
int ds_Pin = 2;
int sig_Pin = A0;

void setup() {
  pinMode(stcp_Pin, OUTPUT);
  pinMode(shcp_Pin, OUTPUT);
  pinMode(ds_Pin, OUTPUT);
  pinMode(sig_Pin, INPUT);
}

void loop() {
  int rotor = analogRead(sig_Pin);
  byte LED_a = map(rotor, 0, 1023, 0, 8);
  byte data = 0B00000000;

  for (byte i = 0; i < LED_a; i++) {
    data |= (1 << i);
  }

  digitalWrite(stcp_Pin, LOW);
  shiftOut(ds_Pin, shcp_Pin, LSBFIRST, data);
  digitalWrite(stcp_Pin, HIGH);
}