rgb_speed_lum.ino

#define DATA_PIN 7
#define CLOCK_PIN 12
#define LATCH_PIN 9

uint8_t k = 0;
float speed_msecs = 0;
int lum = 0;

void setup() {
  pinMode(DATA_PIN, OUTPUT);
  pinMode(CLOCK_PIN, OUTPUT);
  pinMode(LATCH_PIN, OUTPUT);

  digitalWrite(DATA_PIN, LOW);
  digitalWrite(CLOCK_PIN, LOW);
  digitalWrite(LATCH_PIN, LOW);
}

void loop() {
  lum = analogRead(A1);
  
  if (lum < 5)
    lum = 5;

  speed_msecs = (1. / lum) * 10000;

  digitalWrite(LATCH_PIN, LOW);
  shiftOut(DATA_PIN, CLOCK_PIN, MSBFIRST, k);
  digitalWrite(LATCH_PIN, HIGH);
  
  delay((int)speed_msecs);
  
  if (++k >= 3) k = 0;
}