Source: https://mokxi.com/learn/arduino-voltage-sensor-module
Updated: 2026-11-02

Learn

# Measure a battery or supply with the Arduino voltage sensor module

Beginner

Written by the Mokxi team, updated November 2, 2026

Start building free Open the editor

Or see every lesson.

- 465 parts on the bench
- 43 boards running now
- 1.00x real time, on every board

Click to open it in the editor

A 0 to 25 V voltage sensor module across a bench supply, with S on A0 and an I2C LCD, on a simulated ATmega328P. Step the supply’s voltage while it runs.

The voltage sensor module is the small red board with a green screw terminal in most Arduino sensor kits. It lets an Arduino measure a voltage higher than its own 5 V, like a 12 V battery or a bench supply, so a project can warn you when a battery is getting flat.

The circuit above is an Uno with the module across a bench supply set to 12 V, and a 16x2 I2C LCD. Press Run and the screen shows the voltage. Press the supply’s VOLTAGE buttons and the reading follows. It runs in your browser on the sketch below.

## What you need

- An Arduino Uno
- A voltage sensor module (0 to 25 V)
- A battery or a bench supply to measure
- A 16x2 LCD with an I2C backpack
- A half-size breadboard and jumper wires

## Wiring

Part and pin | Goes to | Note

Module VCC (screw terminal) | Plus of the voltage to measure | Up to 25 V on a 5 V Arduino

Module GND (screw terminal) | Minus of the voltage to measure |

Module S | A0 | A fifth of the input

Module - | GND | The same trace as the terminal GND

Module + | Nothing | Not connected on the board

LCD SDA and SCL | A4 and A5 | Backpack address 0x27

## It is just two resistors

There is no chip on the board. A 30 kohm resistor runs from the terminal’s VCC to S, and a 7.5 kohm resistor from S to GND. That is a voltage divider, and it makes S one fifth of the input: 7.5 / (30 + 7.5) = 0.2. So the sketch reads S and multiplies by 5.

In the circuit above, 12 V on the terminal is 2.4 V on S, and A0 reads 491. The sketch turns that into 491 x 5 / 1023 x 5 and the screen shows 12.00 V.

The board loads whatever it measures with its 37.5 kohm, about a third of a milliamp at 12 V. That is nothing for a battery, and Mokxi puts the two real resistors in the circuit, so it is there in the simulation too.

## Where 25 V comes from, and why it is 16.5 V on a 3.3 V board

The limit is the Arduino, not the board. An Uno can read up to 5 V, and 5 V on S is 25 V on the terminal: step the supply to 25 V and A0 reads 1023, which the sketch shows as 25.00 V. Anything higher reads 1023 too, and pushes S above the Uno’s supply.

On a 3.3 V board like an ESP32, a Pico or an Arduino Due, the top of the ADC is 3.3 V, so the same module reads at most 3.3 x 5 = 16.5 V. Change the 5.0 in the sketch to 3.3 as well.

One step of the Uno’s ADC is 4.9 mV at S, so 24.4 mV on the terminal. Set the supply to 9 V and A0 reads 368, which shows as 8.99 V. That last digit is the converter’s step, not a fault.

The loop, from the DC voltmeter

## The shared ground

The terminal’s GND and the header’s minus are the same trace on the board. So when you wire minus to the Arduino’s GND, the minus of whatever you are measuring is now on the Arduino’s ground. That is how it measures at all, and it is why this module must never go near mains or anything that cannot share a ground with your Arduino.

## Try it in the editor

Add an LED that lights when the voltage drops under 11.8 V, the point where a 12 V lead-acid battery is nearly flat. Or swap the bench supply for a 9 V battery part and watch its voltage sag when you add a load.

## Common mistakes

Leaving minus unconnected. Without a shared ground the Arduino has nothing to measure S against, and the reading wanders.

Putting the voltage on the header’s plus pin. It goes nowhere on the board. The voltage goes into the screw terminal.

Measuring more than 16.5 V with a 3.3 V board. The 25 V on the listing assumes a 5 V Arduino.

Forgetting the factor of 5. Without it, a 12 V battery reads 2.4 V.

## Questions

How do I connect a voltage sensor module to an Arduino?

The voltage to measure goes into the screw terminal, plus to VCC and minus to GND. On the header, S goes to an analog pin such as A0 and minus goes to the Arduino’s GND. The plus pin is not connected.

What is the formula for the Arduino voltage sensor?

The board is a 30 k and 7.5 k divider, so the input is five times the voltage on S: volts = analogRead(A0) x 5.0 / 1023 x 5 on a 5 V Arduino.

Can the voltage sensor module measure 25 V on an ESP32?

No. On a 3.3 V board the most it can read is 3.3 x 5 = 16.5 V. The 25 V figure is for a 5 V Arduino.

What does Mokxi not model on the voltage sensor?

The resistors are exact here, so the ratio is exactly 5; a real board’s 1% or 5% resistors can need a small correction. What happens to the Arduino’s input above 25 V is not modeled either.

Related

## Keep going

Voltage Dividers and Why They Are Bad Power Supplies Analog vs Digital Pins, and ADC Resolution Arduino Battery Level Indicator: 18650 on A0 Arduino LCD I2C 16x2: Wiring, Code, Scrolling Text Arduino ACS712 Current Sensor: Wiring and Code How the voltage sensor is modeled The Arduino Uno simulator The project page, with the full sketch Beginner

## Build this for real

Open the editor, change a value and watch the number move with it. Nothing to install, and no account needed.

Start building free Open the editor