Build a Digital Voltmeter with Arduino

Project Overview

Build a digital voltmeter that can measure DC voltages from 0V to 25V and display them on a 16×2 LCD screen. Great for your electronics workbench!

Components Required

How the Voltage Divider Works

Since the Arduino can only read up to 5V, we use a voltage divider to scale down the input:

Vout = Vin × R2 / (R1 + R2)
Vout = Vin × 7.5K / (30K + 7.5K)
Vout = Vin × 0.2

So a 25V input becomes 5V — within the safe range.

Wiring

1. Connect LCD pins (RS, EN, D4-D7) to Arduino digital pins 7, 6, 5, 4, 3, 2
2. LCD VCC to 5V, GND to GND
3. Potentiometer middle pin to LCD V0 (contrast)
4. Voltage divider output to Arduino A0
5. GND of voltage divider to Arduino GND

Arduino Code

#include <LiquidCrystal.h>

LiquidCrystal lcd(7, 6, 5, 4, 3, 2);

float vout = 0.0;
float vin = 0.0;
float R1 = 30000.0;
float R2 = 7500.0;
int value = 0;

void setup() {
  lcd.begin(16, 2);
  lcd.print("Digital Voltmeter");
  delay(1500);
  lcd.clear();
}

void loop() {
  value = analogRead(A0);
  vout = (value * 5.0) / 1024.0;
  vin = vout * (R1 + R2) / R2;
  
  lcd.setCursor(0, 0);
  lcd.print("Input Voltage:");
  lcd.setCursor(0, 1);
  lcd.print(vin, 2);
  lcd.print(" V");
  
  delay(500);
}

Calibration

1. Apply a known voltage (e.g., 5.00V from a regulated supply)
2. Measure with a reference multimeter
3. Adjust the R1 and R2 values in code if reading differs

Safety Tips

Never measure AC mains voltage with this circuit! It's designed for DC only, up to 25V maximum.