BH1750: ambient light in lux over I2C

// Lux night lamp: a BH1750 light meter on I2C, and an LED that gets brighter
// as the room gets darker.
//
//   A4, A5  SDA and SCL to the BH1750 (GY-302 module, address 0x23)
//   pin 9   through 220 ohm to the LED's anode; its cathode to GND
//   5V/GND  the module (it has its own 3.3 V regulator and pull-ups)
//
// An LDR in a divider gives a number that depends on the resistor and the
// cell. The BH1750 gives lux: its count is the light times 1.2, so the
// library's readLightLevel() is count / 1.2 and nothing else. A classroom is
// a few hundred lux, a cloudy day outdoors a few thousand, full sun past the
// 54612 the chip can count to.
//
// # The lamp
//
// Under 20 lux the lamp is full on; over 500 it is off; in between it fades,
// in whole numbers:
//
//   brightness = 255 x (500 - lux) / 480
//
// readLightLevelCentis() gives the lux in hundredths, because this board has
// no floating point to do the sum with.

#include <Wire.h>
#include <BH1750.h>

const int LAMP = 9;
const long DARK = 20;
const long BRIGHT = 500;

BH1750 lightMeter;

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: BH1750 lux night lamp");
  Wire.begin();
  if (!lightMeter.begin()) {
    Serial.println("no BH1750: check SDA, SCL and power");
  }
  pinMode(LAMP, OUTPUT);
}

void loop() {
  if (lightMeter.measurementReady()) {
    long lux = lightMeter.readLightLevelCentis() / 100;
    int level;
    if (lux <= DARK) {
      level = 255;
    } else if (lux >= BRIGHT) {
      level = 0;
    } else {
      level = (int)(255 * (BRIGHT - lux) / (BRIGHT - DARK));
    }
    analogWrite(LAMP, level);

    Serial.print("light ");
    Serial.print(lux);
    Serial.print(" lx   lamp ");
    Serial.println(level);
  }
  delay(200);
}