// To install from the Library Manager:
// "LiquidCrystal I2C"
#include <Wire.h>
#include <LiquidCrystal_I2C.h>

// The analog pin the voltage comes in on
const int INPUT_PIN = A0;
// The Arduino's 5V: with a multimeter,
// write what its 5V pin really measures
const float VREF = 5.00;
// How many readings are averaged
const int SAMPLES = 16;
// Time between measurements (ms)
const unsigned long READ_EVERY = 300;

// I2C address: mostly 0x27, some 0x3F
LiquidCrystal_I2C lcd(0x27, 16, 2);

// The bar's five pieces: boxes with 1-5
// columns filled, as custom LCD chars
void makeBars() {
  for (int level = 0; level < 5; level++) {
    // A row, level+1 columns from left
    byte row = 0;
    for (int col = 0; col <= level; col++) {
      row |= 0x10 >> col;
    }
    byte shape[8];
    for (int y = 0; y < 8; y++) shape[y] = row;
    lcd.createChar(level, shape);
  }
}

// The average reading: steadier numbers
int readAverage() {
  long sum = 0;
  for (int i = 0; i < SAMPLES; i++) {
    sum += analogRead(INPUT_PIN);
  }
  return sum / SAMPLES;
}

// Bottom row: 0-5 V as a 16-box bar;
// 5 steps a box, 80 steps in all
void showBar(int raw) {
  int steps = (long)raw * 80 / 1023;
  lcd.setCursor(0, 1);
  for (int cell = 0; cell < 16; cell++) {
    int fill = steps - cell * 5;
    if (fill >= 5) {
      lcd.write(byte(4));
    } else if (fill > 0) {
      lcd.write(byte(fill - 1));
    } else {
      lcd.print(' ');
    }
  }
}

void setup() {
  lcd.init();
  lcd.backlight();
  makeBars();
  Serial.begin(9600);
}

void loop() {
  int raw = readAverage();
  // A 0-1023 reading, turned into volts
  float volts = raw * VREF / 1023.0;

  lcd.setCursor(0, 0);
  lcd.print("Voltage: ");
  lcd.print(volts, 2);
  // The trailing space clears old text
  lcd.print(" V ");
  showBar(raw);

  Serial.print("Voltage: ");
  Serial.print(volts, 2);
  Serial.print(" V  (A0 = ");
  Serial.print(raw);
  Serial.println(")");
  delay(READ_EVERY);
}