UV index meter on an Arduino Uno

// UV index meter: a GUVA-S12SD UV sensor module and the UV index on a 16x2
// I2C LCD, with the WHO's name for the band it is in.
//
//   A0      the module's OUT (its VCC on 5 V, GND on GND)
//   LCD     SDA -> A4    SCL -> A5    VCC -> 5 V    GND -> GND
//
// The photodiode makes 26 nA per UV index (its datasheet) and the module's
// op-amp makes 4.3 V per microamp, so OUT is 4.3 x 0.026 = 0.1118 V per UV
// index. Many sketches divide by 0.1 V instead, a round figure that reads
// about 12% high; this one divides by the datasheet's number.
//
// It sees 240 to 370 nm only, so a desk lamp reads 0 and the sun does not.

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

const float VOLTS_PER_UVI = 4.3 * 0.026;
LiquidCrystal_I2C lcd(0x27, 16, 2);

const char *band(float uvi) {
  if (uvi < 2.5) return "Low      ";
  if (uvi < 5.5) return "Moderate ";
  if (uvi < 7.5) return "High     ";
  if (uvi < 10.5) return "Very high";
  return "Extreme  ";
}

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: UV index meter");
  lcd.init();
  lcd.backlight();
}

void loop() {
  // Average 32 readings: the index moves slowly and this steadies the last digit.
  long sum = 0;
  for (int i = 0; i < 32; i++) sum += analogRead(A0);
  float volts = sum / 32.0 * 5.0 / 1024.0;
  float uvi = volts / VOLTS_PER_UVI;

  lcd.setCursor(0, 0);
  lcd.print("UV index ");
  lcd.print(uvi, 1);
  lcd.print("   ");
  lcd.setCursor(0, 1);
  lcd.print(band(uvi));

  Serial.print("mV: ");
  Serial.print(volts * 1000.0, 0);
  Serial.print("  UV index: ");
  Serial.print(uvi, 1);
  Serial.print("  ");
  Serial.println(band(uvi));
  delay(500);
}