DC voltmeter on an Arduino Uno

// DC voltmeter: a 0 to 25 V voltage sensor module across a bench supply,
// and the voltage on a 16x2 I2C LCD and the serial monitor.
//
//   A0      the sensor's S
//   -       the sensor's header minus, to GND
//   VCC GND the sensor's screw terminal, across the supply's V+ and V-
//   LCD     SDA -> A4    SCL -> A5    VCC -> 5 V    GND -> GND
//
// The module is two resistors and nothing else: 30 k from the terminal's VCC
// to S and 7.5 k from S to its GND. So S is a fifth of the voltage on the
// terminal, and the sketch multiplies by 5 to get it back.
//
// The terminal's GND and the header's minus are one trace on the board, so
// wiring minus to the Arduino ties the supply's minus to the Arduino's ground.
// That is what lets it measure at all, and why it cannot measure anything
// that must not share a ground with the Arduino.
//
// On a 5 V Uno, S can go up to 5 V, which is 25 V on the terminal: that is
// the "0 to 25 V" on the listing. One converter step is 5 / 1023 V at S, so
// 24.4 mV on the terminal. (On a 3.3 V board the top is 16.5 V.)
//
// Press the supply's VOLTAGE buttons while it runs and watch the reading
// follow.

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

const int SENSE = A0;
LiquidCrystal_I2C lcd(0x27, 16, 2);

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: DC voltmeter");
  lcd.init();
  lcd.backlight();
  lcd.print("DC voltmeter");
}

void loop() {
  // Average sixteen readings: the sum fits a long easily, and the average
  // smooths a step of noise on a real supply.
  long sum = 0;
  for (int i = 0; i < 16; i++) sum += analogRead(SENSE);
  int raw = (sum + 8) / 16;

  float atS = raw * 5.0 / 1023.0;  // the voltage at S
  float volts = atS * 5.0;         // the divider is 5 to 1

  lcd.setCursor(0, 1);
  lcd.print(volts, 2);
  lcd.print(" V  ");
  lcd.print(raw);
  lcd.print("    ");

  Serial.print("ADC: ");
  Serial.print(raw);
  Serial.print("  volts: ");
  Serial.println(volts, 2);
  delay(250);
}