Arduino pH Meter with PH-4502C: Wiring, Calibration & Code Guide

pH measures how acidic or alkaline a solution is on a 0–14 scale, with 7 as neutral. For Arduino projects — aquarium monitors, hydroponic controllers, soil testers, wastewater loggers — the PH-4502C is the most widely available analog pH interface module, costing $10–20 and connecting directly to any Arduino analog input.

This guide covers: module specs, calibration, wiring, and three Arduino sketches of increasing sophistication.

PH-4502C Module Specifications

The PH-4502C accepts a BNC-connector glass electrode probe and outputs a 0–5V analog voltage proportional to measured pH.

PH Probe Sensor Pinout

• TO — Temperature output (analog, for software compensation)
• DO — Digital output: goes HIGH when pH crosses the threshold set by POT2
• PO — Analog pH voltage output → connect to Arduino A0
• Gnd — Ground for pH probe
• Gnd — Ground for board (connect both GND pins)
• VCC — 5V DC power
• POT 1 — Offset trimmer (nearest to BNC connector) — sets midpoint voltage for pH 7
• POT 2 — Limit trimmer — sets the pH threshold for the DO digital trigger

How pH Measurement Works

The BNC glass electrode contains a thin glass membrane separating an internal reference solution from the measured sample. Hydrogen ions (H⁺) in the sample create a potential difference across the membrane proportional to ion concentration — the Nernst equation in practice.

At pH 7 (neutral) the electrode potential is zero. At pH 4 (acidic) it goes positive; at pH 10 (alkaline) it goes negative. The PH-4502C module shifts and scales this signal so pH 7 corresponds to 2.5V at the Arduino analog input:

pH = 7 + ((2.5 − V) / 0.18)

The constant 0.18 is the expected voltage change per pH unit (electrode slope). Well-maintained glass electrodes produce 0.177–0.185 V/pH at 25°C.

Calibration

Calibration sets the offset so pH 7 reads exactly 2.5V at PO. This is a one-time setup — the module’s gain is fixed.

What you need: pH 7.0 buffer solution, multimeter, small Phillips screwdriver.

1. Disconnect the BNC probe and short-circuit the BNC connector (touch centre pin to shield with a wire).
2. Power the circuit (Arduino 5V → VCC, both GNDs connected).
3. Measure voltage at the PO pin with a multimeter.
4. Adjust POT 1 (trimmer nearest to the BNC connector) until PO reads 2.500V exactly.
5. Reconnect the probe. Rinse with distilled water, immerse in pH 7.0 buffer solution.
6. Serial Monitor should now show values between 6.8 and 7.2. Fine-tune POT 1 in small increments if needed.

Tip: Always rinse with distilled water between measurements and buffer solutions. Never wipe the glass bulb — the glass membrane scratches easily and degrades quickly.

Wiring Diagram

PH-4502C PO  → Arduino A0
PH-4502C GND → Arduino GND  (connect both GND pins)
PH-4502C VCC → Arduino 5V

Sketch 1: Basic pH Reading

#include <Arduino.h>

int pHSense = A0;
int samples = 10;
float adc_resolution = 1024.0;

void setup() {
    Serial.begin(9600);
    delay(100);
    Serial.println("CimpleO pH Sense");
}

float ph(float voltage) {
    return 7 + ((2.5 - voltage) / 0.18);
}

void loop() {
    int measurings = 0;
    for (int i = 0; i < samples; i++) {
        measurings += analogRead(pHSense);
        delay(10);
    }
    float voltage = 5.0 / adc_resolution * measurings / samples;
    Serial.print("pH= ");
    Serial.println(ph(voltage));
    delay(3000);
}

Sketch 2: Moving Average Filter

Glass electrode output carries electrical noise. Collecting 30 samples over 3 seconds and discarding the highest and lowest 5 before averaging significantly improves stability:

#include <Arduino.h>

#define SAMPLES 30
#define TRIM    5

int pHSense = A0;

float ph(float voltage) {
    return 7.0 + ((2.5 - voltage) / 0.18);
}

float readPH() {
    int buf[SAMPLES];
    for (int i = 0; i < SAMPLES; i++) {
        buf[i] = analogRead(pHSense);
        delay(30);
    }
    // sort ascending
    for (int i = 0; i < SAMPLES - 1; i++)
        for (int j = i + 1; j < SAMPLES; j++)
            if (buf[i] > buf[j]) { int t = buf[i]; buf[i] = buf[j]; buf[j] = t; }
    // average middle values
    long sum = 0;
    for (int i = TRIM; i < SAMPLES - TRIM; i++) sum += buf[i];
    float voltage = 5.0 / 1024.0 * sum / (SAMPLES - 2 * TRIM);
    return ph(voltage);
}

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

void loop() {
    Serial.print("pH= ");
    Serial.println(readPH(), 2);
    delay(1000);
}

Sketch 3: Temperature Compensation

pH readings shift approximately 0.003 units per °C away from neutral. At pH 4 in a 35°C environment, uncompensated error reaches ±0.03 pH. Connect the TO pin to Arduino A1 for temperature-corrected readings:

#include <Arduino.h>

int pHSense  = A0;
int tempSense = A1;

float ph(float voltage) {
    return 7.0 + ((2.5 - voltage) / 0.18);
}

float readTemperature() {
    int raw = analogRead(tempSense);
    return raw * 5.0 / 1024.0 * 100.0; // 10mV/°C linear sensor
}

void loop() {
    int raw = 0;
    for (int i = 0; i < 10; i++) { raw += analogRead(pHSense); delay(10); }
    float voltage = 5.0 / 1024.0 * raw / 10.0;
    float rawPH   = ph(voltage);
    float temp    = readTemperature();
    float compPH  = rawPH + (temp - 25.0) * 0.03;

    Serial.print("Temp: "); Serial.print(temp, 1);
    Serial.print("°C  pH= ");  Serial.println(compPH, 2);
    delay(3000);
}

PH-4502C vs Alternative pH Modules

For Arduino prototypes, the PH-4502C provides sufficient accuracy at minimal cost. When moving to a production device — especially where readings drive automated dosing or safety alerts — the Atlas Scientific EZO-pH’s precision and I2C interface justify the higher price.

If you’re working with IoT hardware, you might also find our JRD-100 UHF RFID module guide useful, or learn how to stream sensor data in real time with Kafka. For cellular connectivity in field-deployed sensors, see our A9G GSM/GPS module guide.

Building a connected device that needs to move beyond a prototype? CimpleO’s IoT engineering team covers the full stack — from firmware and PCB design to cloud backend and mobile dashboard. Get in touch to discuss your project.