How to Measure DC Voltage with Arduino and a 0-25V Sensor Module

A 0-25V voltage sensor module lets an Arduino read a higher positive DC voltage through an analogue input. The board is a simple resistor divider, not an isolated or powered sensor. Correct grounding, ADC limits and calibration matter more than the amount of code.

In this guide

1. How the module works
2. Pinout and wiring
3. Arduino example code
4. Calibrate the result
5. Understand the limits
6. Troubleshooting
7. Frequently asked questions

How the 0-25V module works

The two resistors on the module form a nominal 5:1 voltage divider. The signal pin therefore carries about one fifth of the voltage applied to the screw terminals. A 12V input produces about 2.4V at S, which a 5V Arduino analogue input can read.

The actual reading depends on resistor tolerance, the Arduino's real reference voltage and ADC behaviour. Treat 5.0 as the starting scale factor, then calibrate it against a trusted multimeter.

Pinout and Arduino Uno wiring

The NZN stock board is marked VCC<25V and has 30.1k and 7.5k resistor markings. Its three-pin header is labelled S, + and -. On this common divider layout, the centre + pin is not connected.

1. Disconnect all power. Make every connection before energising the source.
2. Join the grounds. Connect the module - pin to Arduino GND. The screw-terminal GND and Arduino ground become the same electrical point.
3. Connect the signal. Run S to Arduino A0.
4. Leave + unused. Do not connect the centre + header pin to 5V or 3.3V on this divider module.
5. Connect the measured source. Positive goes to the VCC screw terminal and negative goes to the GND screw terminal.
6. Start at a low known voltage. Use a current-limited supply or a small battery and compare the result with a multimeter before measuring anything higher.

Arduino example code

This example targets a classic Arduino Uno with its default 10-bit reading range. Set vRef to the voltage you actually measure between the Arduino 5V and GND pins.

const int sensorPin = A0;
const float vRef = 5.00;
const float dividerRatio = 5.00;

void setup() {
  Serial.begin(9600);
}

void loop() {
  int raw = analogRead(sensorPin);
  float signalVoltage = raw * (vRef / 1023.0);
  float inputVoltage = signalVoltage * dividerRatio;

  Serial.print("Raw: ");
  Serial.print(raw);
  Serial.print("  Input: ");
  Serial.print(inputVoltage, 2);
  Serial.println(" V");

  delay(500);
}

Upload the sketch, open Serial Monitor at 9600 baud and compare the displayed input voltage with your multimeter. If the reading is consistently high or low, calibrate the multiplier rather than hiding the error with rounding.

A simple averaged reading

For a steadier display, average several samples. This reduces random noise, but it does not correct the reference voltage or divider tolerance.

long total = 0;
const int samples = 20;

for (int i = 0; i < samples; i++) {
  total += analogRead(sensorPin);
  delay(2);
}

float rawAverage = total / (float)samples;
float inputVoltage = rawAverage * (vRef / 1023.0) * dividerRatio;

How to calibrate the voltage reading

1. Measure the Arduino reference. With the Arduino powered normally, measure its 5V pin relative to GND and enter that value as vRef.
2. Apply a stable test voltage. Use a source comfortably inside the range, such as 5V or 12V DC.
3. Record both readings. Note the trusted multimeter value and the Arduino result.
4. Calculate a correction factor. Divide the multimeter reading by the Arduino reading.
5. Apply the correction. Multiply the calculated input voltage by that factor, then repeat the check at a second voltage.

Example: if the multimeter shows 12.10V and the Arduino reports 11.82V, the correction factor is 12.10 / 11.82 = 1.0237. Multiply the result by 1.0237 in your sketch.

Understand the important limits

• The input must stay positive. A negative signal can damage an Arduino analogue input.
• The grounds are connected. This module is not suitable where the measured circuit must remain isolated from the Arduino or USB-connected computer.
• 25V is a nominal maximum for a 5V ADC. Component tolerance and supply variation reduce the safe margin. Do not design routine operation at the absolute edge.
• 3.3V boards need a separate calculation. A nominal 5:1 divider suggests 16.5V at a 3.3V ADC limit, but many boards have lower or configurable ADC ranges. Check the exact board documentation.
• This is not a precision instrument. ADC reference error, source impedance, resistor tolerance and electrical noise all affect the result.
• The module measures voltage in parallel. Connect it across the source. Do not place it in series like a current sensor.

Troubleshooting

• 0-25V DC voltage sensor moduleView module

Frequently asked questions

Does the + header pin power the sensor?

No. On this module the centre + pin is not connected. Leave it unused. Connect S to the analogue input and - to Arduino ground.

Can I measure a 12V battery while the Arduino uses USB power?

Only if joining the battery negative to Arduino and computer ground is safe for the complete system. The module is not isolated. If you are unsure about the grounding path, do not connect it.

Can it measure AC voltage?

No. It is a positive DC divider and has no rectifier or isolation. Use a correctly rated, isolated measurement method for AC.

Why does the reading change when USB power changes?

The default Arduino conversion depends on its analogue reference. A different USB port or supply can shift the 5V rail, which shifts the calculated result unless you measure or stabilise the reference.

Can I use the same formula on an ESP32?

Not safely by assumption. ESP32 ADC range, attenuation, resolution and calibration depend on the exact board and configuration. Check the current documentation for your board and keep the S voltage inside its specified ADC range.