//www.elegoo.com
//2016.12.9

int lightPin = 0;
int latchPin = 11;
int clockPin = 9;
int dataPin = 12;

int leds = 0;

void setup() 
{
  pinMode(latchPin, OUTPUT);
  pinMode(dataPin, OUTPUT);  
  pinMode(clockPin, OUTPUT);
}
/*   The most common method of using 74CH595
 *   lctchPin->LOW : Begin transmitting signals.
 *   shiftOut(dataPin, clockPin, bitOrder, value)
 *   dataPin: the pin on which to output each bit. Allowed data types: int.
 *   clockPin: the pin to toggle once the dataPin has been set to the correct value. Allowed data types: int.
 *   bitOrder: which order to shift out the bits; either MSBFIRST or LSBFIRST. (Most Significant Bit First, or, Least Significant Bit First).
 *   value: the data to shift out. Allowed data types: byte. 
 *   lctchPin->HIch : The end of the transmission signal.
*/
void updateShiftRegister()
{
   digitalWrite(latchPin, LOW);
   shiftOut(dataPin, clockPin, LSBFIRST, leds);
   digitalWrite(latchPin, HIGH);
}
void loop() 
{
  int reading  = analogRead(lightPin);
  int numLEDSLit = reading / 57;  //1023 / 9 / 2
  if (numLEDSLit > 8) numLEDSLit = 8;
  leds = 0;   // no LEDs lit to start
  for (int i = 0; i < numLEDSLit; i++)
  {
    leds = leds + (1 << i);  // sets the i'th bit
  }
  updateShiftRegister();
}