Automated pH System

An Arduino system for measuring and calibrating the pH of a liquid, with automatic heating of the sample to a reference temperature (PID control) and two pumps for dosing calibration solutions (base / acid). Controlled via a 4x3 matrix keypad, with status shown on a 16x2 I2C LCD.

Table of Contents

• How It Works
• Hardware
• Pinout
• Libraries
• Installation
• Usage
• Code Structure
• PID Parameters
• Known Limitations / TODO

How It Works

The system has three main modes, selected via the keypad while a menu scrolls across the LCD:

Key	Mode	Description
1	Calibration	Enter two known pH values (e.g. 8.3 and 2.7); the system measures the sensor voltage in both solutions and computes the line coefficients m and b (linear calibration: pH = m*V + b)
2	Measurement	Measures the pH sensor's voltage and converts it to a pH value using the last calibration
3	Pump 1 (base)	Runs the pump that doses the base solution for ~1.5 s
4	Pump 2 (acid)	Runs the pump that doses the acid solution for ~1.5 s

Before every measurement or calibration, the system waits (pertilijerWHILE) for the sample temperature (read from the DS18B20 sensor) to reach at least 24.5 °C, since pH readings are temperature-sensitive. After that, throughout the whole measurement, the temperature is further held at 25 °C via PID control of the heater relay (pertilijerPID).

Hardware

• Arduino (Uno/Nano or similar, ATmega328-class)
• 16x2 LCD with I2C backpack (address 0x27)
• 4x3 matrix keypad
• DS18B20 digital temperature sensor (OneWire)
• pH sensor (analog output)
• Relay for the heater (Peltier/heating element)
• 2x DC pump (driven via H-bridge/relay module, 2 pins each for direction/enable)
• Power supply for the pumps (separate ENABLE_PUMPA pin)

Pinout

Pin	Function
A0	DS18B20 (OneWire bus, temperature)
A1	pH sensor (analog input)
2	Pump enable
3	Pump 2 (acid) — channel A
1	Pump 2 (acid) — channel B
6	Pump 1 (base) — channel A
5	Pump 1 (base) — channel B
4	Heater relay (Peltier)
7,8,9,10	Keypad rows
11,12,13	Keypad columns
SDA/SCL	LCD (I2C)

⚠️ Pin 1 is the hardware RX pin for serial communication. If USB serial is used for debugging (Serial.begin), it will conflict with this pin — Serial is currently disabled in the code (commented out).

Libraries

Install the following libraries (via the Arduino Library Manager or manually):

• Keypad (Mark Stanley / Alexander Brevig)
• Wire (built-in)
• LiquidCrystal_I2C
• OneWire
• DallasTemperature

Installation

1. Install the Arduino IDE (or use PlatformIO).
2. Install the libraries listed above via Sketch → Include Library → Manage Libraries.
3. Wire up the hardware according to the pinout.
4. Open the .ino file, select the correct board and port.
5. Upload the code to the Arduino.

Usage

1. On power-up, the LCD shows a scrolling menu: Izaberi: 1-Kalibracija 2-Merenje 3-BAZA 4-KISELOST.
2. For first use, run calibration (1):
   • Enter the first known pH value (integer + decimal part, # to confirm, * to delete a digit).
   • Enter the second known pH value.
   • The system waits for the temperature to reach 24.5 °C, then measures the voltage in the first solution (~120 s).
   • Move the sensor to the second solution when prompted (10 s pause for repositioning).
   • The system measures the second voltage and computes the calibration coefficients.
3. For measurement (2): the system waits for temperature, measures voltage (~120 s), and displays the computed pH value.
4. For dosing (3/4): activates the corresponding pump for ~1.5 seconds.

Note: measurement durations in the code are set to 120 cycles x 1 s (~2 min) — the comments indicate it should be 300 s, but it was shortened for testing. Adjust the br<120 conditions in VphMerenje() and VphKalibracija() as needed.

Code Structure

├── setup()              → inicijalizacija LCD-a, pinova, senzora
├── loop()                → poziva odabirModa() u beskonačnoj petlji
├── odabirModa()          → glavna state-mašina, čita meni i poziva odgovarajuću funkciju
├── modovi()              → prikazuje skrolujući meni i čeka izbor korisnika
├── vodenaPumpa()         → aktivira pumpu 1 ili 2
├── pertilijerWHILE()     → čeka da temperatura dostigne minimalnu vrednost pre merenja
├── pertilijerPID()       → PID regulacija temperature tokom merenja
├── racunajPID()          → implementacija PID formule (P, I, D)
├── tempNtc()             → čita temperaturu sa DS18B20
├── VphMerenje() / VphKalibracija() → čitanje napona pH senzora tokom vremena
├── pocetnaNapon()        → inicijalno očitavanje napona
├── racunanjeM() / racunanjeB() → računanje koeficijenata kalibracione prave
├── merenjePh()           → primena kalibracije na izmereni napon → pH vrednost
├── unosCelobrojneVrednosti() → unos decimalnog broja preko tastature
└── ispisTeksta()         → skrolujući ispis teksta na LCD-u

PID Parameters

Parameter	Value	Description
SETPOINT	25.0 °C	Target sample temperature
MRTVA_ZONA	0.5	Dead-zone threshold below which the relay stays off
Kp	2.0	Proportional coefficient
Ki	0.05	Integral coefficient
Kd	1.0	Derivative coefficient
PID_PERIOD	1000 ms	PID computation period

The relay is used as an on/off output (no PWM control of the heater), so the PID output only acts as a threshold: above MRTVA_ZONA the relay turns on, otherwise it stays off.

Known Limitations / TODO

• No check for DEVICE_DISCONNECTED_C (disconnected temperature sensor) — the check exists in the code but is commented out.
• Serial debugging is disabled; all output goes to the LCD only.
• Audio feedback (zvucniSignal()) is implemented but commented out throughout the code.
• Measurement duration is set to ~2 min instead of the intended 5 min (a code comment indicates this is temporary, for testing).
• The file contains a fair amount of commented-out legacy code (an older loop() without the odabirModa() function) that could be removed for readability.