/*
   Simple Arduino DC Voltmeter with LCD (I2C)

   ┌──────────────┐
   │   Arduino    │
   │              │
   │   A0 ◄─────●───────┐
   │              │      │
   │             [R1]   VIN (+)
   │           100kΩ     │
   │              │      │
   │              ●─────[R2]───► GND
   │                     10kΩ
   │
   │ SDA (A4) ─────────► LCD SDA
   │ SCL (A5) ─────────► LCD SCL
   │ 5V       ─────────► LCD VCC
   │ GND      ─────────► LCD GND
   └──────────────┘

   - R1 = 100kΩ (top resistor, VIN to A0)
   - R2 = 10kΩ  (bottom resistor, A0 to GND)
   - Voltage divider ratio = 11
   - Max input voltage ≈ 55V
*/

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

const int analogPin = A0;
const float Vref = 5.0;
const float R1 = 100000.0;
const float R2 = 10000.0;

LiquidCrystal_I2C lcd(0x27, 16, 2);

void setup() {
  lcd.init();
  lcd.backlight();
}

void loop() {
  int raw = analogRead(analogPin);
  float Vout = (raw * Vref) / 1023.0;
  float Vin = Vout * ((R1 + R2) / R2);

  if (Vin < 0.09) Vin = 0.0;

  lcd.setCursor(0, 0);
  lcd.print("Voltage: ");
  lcd.print(Vin, 2);
  lcd.print(" V");

  delay(500);
}