const byte LIGHT_PIN = A9;
const float REFERENCE_V = 3.3;
void setup() { Serial.begin(9600); }
void loop() {
  int light = analogRead(LIGHT_PIN);
  float volts = light * (REFERENCE_V / 1023.0);
  Serial.print("light:"); Serial.print(light);
  Serial.print(",volts:"); Serial.println(volts, 3);
  delay(100);
}