Build a Solar-Powered Water Quality Monitoring Buoy with ESP32, pH & Turbidity Sensors

2. Overview & Engineering Objectives

Surface water bodies such as lakes, aquaculture ponds, and municipal reservoirs require continuous environmental surveillance to detect industrial effluent discharge, algal blooms, and contamination. This solar-powered floating buoy measures water chemistry and clarity autonomously and dispatches telemetry over wireless links.

Core Engineering Capabilities:

Temperature-Compensated pH Measurement: Applies the electrochemical Nernst equation to adjust raw glass-electrode pH readings against water temperature measured by a submersible DS18B20 probe.

Turbidity Scattering Analysis: Converts light scattering voltages from an optical infrared diode/phototransistor pair into standardized Nephelometric Turbidity Units (NTU).

Off-Grid Solar MPPT Harvesting: Integrates a CN3791 Maximum Power Point Tracking (MPPT) solar charger to maintain 18650 battery reserves under diffuse sunlight.

ADC1 Low-Noise Signal Pipeline: Routes analog probe signals strictly through ESP32 ADC1 channels (GPIO 34 and GPIO 35) to prevent Wi-Fi RF attenuation interference.

3. Bill of Materials (BOM)

Component

Quantity

Specification / Rating

Est. Cost (INR)

Sourcing / Affiliate Link

ESP32-WROOM-32 Dev Board

1

30-Pin Dual-Core Xtensa LX6 (Deep Sleep Capable)

₹380

Buy on Mentora Store

Analog pH Sensor Kit V2

1

Industrial probe, BNC interface, 3.3V/5V conditioning board

₹1,450

Buy on Mentora Store

Optical Turbidity Sensor Module

1

Infrared light scattering probe (0–3000 NTU)

₹680

Buy on Mentora Store

DS18B20 Waterproof Probe

1

Stainless steel encapsulated digital 1-Wire sensor

₹120

Buy on Mentora Store

CN3791 MPPT Solar Charger Board

1

6V solar panel input, 4.2V Li-ion CC/CV charge management

₹280

Buy on Mentora Store

6V 5W Solar Panel

1

Waterproof monocrystalline outdoor panel

₹450

Buy on Mentora Store

18650 Li-ion Battery (3000mAh)

1

High-capacity protected 3.7V cell

₹190

Buy on Mentora Store

IP68 Sealed Enclosure / Buoy Hull

1

Hermetic waterproof floating canister with glands

₹420

Buy on Mentora Store

Total Hardware Cost

~₹3,970

4. Analytical Chemistry & Voltage Conditioning

1. pH Measurement & The Nernst Equation

The voltage produced by a glass electrode is given by the Nernst equation: $E = E_0 - (2.303 · R · T)/(F) · (pH - 7)$

Where:

R = 8.314 J/(mol·K) (Ideal gas constant)

F = 96,485 C/mol (Faraday constant)

T = Temperature in Kelvin (T_C + 273.15)

The slope sensitivity is approximately -59.16 mV/pH at 25^circC, but varies with water temperature.

In code, calculate the dynamic slope S(T) = 0.05916 × (T_K)/(298.15) to ensure accuracy across seasonal shifts (5^circC to 40^circC).

2. Optical Turbidity Voltage-to-NTU Polynomial

The analog turbidity sensor outputs a high voltage in clear water (~4.1V at 5V, or ~2.7V at 3.3V) and drops voltage as particulate matter scatters the optical beam: $Turbidity (NTU) = -1120.4 · V^2 + 5742.3 · V - 4352.9$ (Calibrated specifically for clean reservoir waters; clamped to 0 NTU below baseline noise).

5. Real-World Troubleshooting FAQs

Q1: Why does the pH reading drift wildly or fluctuate by ± 1.5 pH?

Root Cause 1: 5V vs 3.3V Reference Mismatch: Older analog pH boards output a 0V to 5V signal centered at 2.5V (pH 7). Feeding 5V into the ESP32’s 3.3V ADC clips all acidic readings! Always select a Gravity V2 3.3V-compatible board, or power the conditioning board from a regulated 3.3V rail.

Root Cause 2: Ground Loop Interference: Operating a pH probe and a turbidity sensor in the same liquid volume can establish ionic ground loops between their electrodes. Keep the sensors separated by at least 10cm, or briefly power down the turbidity optical LED during the pH ADC conversion.

Q2: Why does the ESP32 ADC read non-linear values near 0V and 3.3V?

Root Cause: ESP32 internal SAR ADC non-linearity below 150mV and above 3100mV.

Fix: Use two-point buffer calibration: calibrate pH neutral (pH 7.00 buffer) and pH acid (pH 4.00 buffer) directly in software ADC steps rather than relying on assumed theoretical voltages.

Q3: Why does deep sleep timer drift by 5–10% over days?

Root Cause: The internal 150kHz RC oscillator on the ESP32 drifts with ambient temperature. For exact temporal intervals, the RTC timer automatically recalibrates against the main 40MHz crystal before entering sleep.

6. Mentora Monetization & Student Support Hooks

🛒 Official Mentora Water Quality Buoy Hardware Kit: ESP32-WROOM, Gravity V2 pH probe, optical turbidity sensor, waterproof DS18B20, CN3791 MPPT board, 6V solar panel, and IP68 marine enclosure for ₹4,699.

⚡ ₹49 Mentora Token Vault: Instantly unlock the companion LoRaWAN TTN gateway bridge firmware, ThingSpeak multi-parameter dashboard code, and 3D printable catamaran buoy hull STL files.

🛠️ ₹299 1-on-1 Mentora Live Video Debugging: 30-minute session with an environmental telemetry engineer to perform two-point buffer pH calibration, eliminate optical ground loop noise, and configure MPPT battery charging.

Hardware Pinout & Wiring Connections

Hardware Pinout & Wiring Connection Guide

Project 10: Solar-Powered Water Quality Buoy

Microcontroller: ESP32-WROOM-32 Dev Module (30-pin / 36-pin)Sensors: Analog pH Sensor Kit V2 (BNC Interface), Optical Turbidity Sensor (NTU), DS18B20 Waterproof 1-Wire Temperature SensorPower Stage: 6V 5W Monocrystalline Solar Panel + CN3791 MPPT Solar Charger Board + 18650 Li-ion Battery (3.7V)

1. Complete Fritzing Pin Connection Mapping Table

Table A: Water Quality Sensors to ESP32 Signal Inputs

Component

Component Pin

ESP32 Pin

Wire Color

Signal / Bus Type

Electrical Constraints & Guidelines

pH Sensor Board (V2)

VCC

3V3 Rail

Red

3.3V DC Sensor Supply

Powered from 3.3V. Matches ESP32 ADC reference range.

pH Sensor Board (V2)

GND

GND Rail

Black

Ground Return

Common system ground reference.

pH Sensor Board (V2)

AOUT

GPIO 34

Blue

Analog Output (pH Voltage)

Connected to ADC1 Channel 6 (Input-only, no pull-ups).

Turbidity Sensor Board

VCC

3V3 Rail

Red

3.3V DC Sensor Supply

Optical LED and detector supply.

Turbidity Sensor Board

GND

GND Rail

Black

Ground Return

Common system ground.

Turbidity Sensor Board

AOUT

GPIO 35

Yellow

Analog Output (NTU Voltage)

Connected to ADC1 Channel 7. Drops voltage in turbid water.

DS18B20 Temp Probe

VDD (Red)

3V3 Rail

Red

3.3V Logic Supply

Submersible digital temperature probe.

DS18B20 Temp Probe

GND (Black)

GND Rail

Black

Ground Return

Common ground reference.

DS18B20 Temp Probe

DATA (Yellow)

GPIO 4

Yellow

1-Wire Digital Bus

REQUIRES 4.7kΩ PULL-UP RESISTOR tied to 3.3V Rail!

4.7kΩ Pull-Up Resistor

Lead 1 to Lead 2

Between 3V3 & GPIO 4

—

Bus Pull-Up

Pulls 1-Wire line HIGH during idle bus state.

Table B: Solar Power, Battery & Voltage Divider

Sub-Circuit / Component

Terminal

Connected Node

Wire Color

Purpose / Operation

6V Solar Panel

Positive (+)

CN3791 Solar (+) Input

Red

Solar photovoltaic energy harvesting.

6V Solar Panel

Negative (-)

CN3791 Solar (-) Input

Black

Solar panel ground return.

18650 Li-ion Cell

Positive (+)

CN3791 BAT (+) Terminal

Red

3.7V–4.2V rechargeable battery storage.

18650 Li-ion Cell

Negative (-)

CN3791 BAT (-) Terminal

Black

Battery ground return.

CN3791 Regulated Output

VOUT (+)

ESP32 3V3 Pin (or LDO IN)

Red

Supplies clean DC power to system.

CN3791 Regulated Output

GND (-)

Common GND Rail

Black

Common system ground.

Battery Divider (Top 100kΩ)

Lead 1 to Lead 2

Battery (+) to GPIO 32

Yellow

Halves battery voltage for safe ADC sampling (0–2.1V).

Battery Divider (Bottom 100kΩ)

Lead 1 to Lead 2

GPIO 32 to GND Rail

Black

Bottom leg of 2:1 resistive divider.

2. Circuit Connection Schematic Diagram (ASCII Art)

+-------------------------------------------------------------+

| ESP32-WROOM-32 |

| |

| [ 3V3 ] [ GND ] [GPIO 34] [GPIO 35] [GPIO 4] [GPIO 32]

+----+--------+------------+----------+---------+---------+---+

|        |            |          |         |         |

+3.3V | GND | pH |Turbidity | 1-Wire | Battery |

Rail | Rail | Signal | Signal | Data | Divider |

=======+========+============+==========+=========+=========+====

|        |            |          |         |         |

|        |            |          |     [4.7k]        |

|        |            |          |     (Pullup)      |

|        |            |          |    +----+---+     |

|        |            |          |    |    |         |

+--+--------+----+ | | | +--+---------+----+

| VCC  GND  AOUT |       |          |    | | VDD GND  DATA   |

|  ANALOG pH     |<------+          |    | | DS18B20 SENSOR  |

|  SENSOR (V2)   |                  |    | +-----------------+

+----------------+ | |

|        |                       |    |

+--+--------+----+ | |

| VCC  GND  AOUT |                  |    |

|    TURBIDITY   |<-----------------+    |

|     SENSOR     |                       |

+----------------+ |

|

BATTERY MONITOR DIVIDER |

Battery (+) ---> [100k] ---+---> [GPIO 32] (ADC1)

|

GND <--- [100k]-+

3. Golden Engineering Rules for Water Quality Probes

ADC1 Exclusivity:

Both pH (GPIO 34), Turbidity (GPIO 35), and Battery (GPIO 32) are wired to ADC1. This prevents the Wi-Fi modem from blocking analog measurements during cloud telemetry bursts.

Waterproof Isolation & Common Ground:

Water is an electrical conductor. Ensure probe ground shields are tied directly to the common ground plane of the buoy to eliminate galvanic voltage potentials between the pH glass electrode reference and the turbidity probe.

Storage Solution Precautions:

The glass bulb of the pH probe must never dry out. For transport or storage, keep the protective cap filled with 3M KCl (Potassium Chloride) storage solution.

Complete Tested Arduino C++ Firmware

``cpp

/*

• ==============================================================================
• Project: Solar-Powered Water Quality Buoy with Turbidity & pH Sensor
• Microcontroller: ESP32-WROOM-32 (Xtensa Dual-Core)
• Author: Mentora Maker Lab (Spark Autonomous Content Intelligence)
• License: MIT

*

• Hardware Peripherals:
• - Analog pH Sensor (Gravity V2): Signal -> GPIO 34 (ADC1 Channel 6)
• - Optical Turbidity Sensor : Signal -> GPIO 35 (ADC1 Channel 7)
• - DS18B20 Waterproof Temp : Data -> GPIO 4 (1-Wire Bus with 4.7k pullup)
• - Battery Divider (100k + 100k): Signal -> GPIO 32 (ADC1 Channel 4)
• - Power Stage: CN3791 MPPT Solar Charger + 18650 Li-ion + Deep Sleep Timer
• ==============================================================================

*/

#include <OneWire.h>

#include <DallasTemperature.h>

#include <WiFi.h>

#include <HTTPClient.h>

// -----------------------------------------------------------------------------

// USER CONFIGURATION (WIFI & TELEMETRY TARGET)

// -----------------------------------------------------------------------------

const char* WIFISSID = "YOURWIFI_SSID";

const char* WIFIPASSWORD = "YOURWIFI_PASSWORD";

const char* SERVER_ENDPOINT = "http://api.mentora.in/telemetry/buoy";

// Hardware Pin Definitions

#define PHADCPIN 34

#define TURBIDITYADCPIN 35

#define BATTERYADCPIN 32

#define ONEWIREBUS_PIN 4

#define STATUSLEDPIN 2

// Deep Sleep Interval

#define SLEEPDURATIONSEC 600ULL // Wake up every 10 minutes (600 seconds)

// Calibration Constants for pH (Derived from standard buffer tests at 25C)

const float PHNEUTRALVOLTAGE = 1.65; // Voltage at pH 7.00

const float PHACIDVOLTAGE = 2.05; // Voltage at pH 4.00

// Setup 1-Wire DS18B20 Temperature Bus

OneWire oneWire(ONEWIREBUS_PIN);

DallasTemperature tempSensors(&oneWire);

// RTC Persistent Memory across sleep cycles

RTCDATAATTR int wakeCycleCount = 0;

// -----------------------------------------------------------------------------

// MULTI-SAMPLE FILTERED ADC SAMPLING

// -----------------------------------------------------------------------------

float readAverageADCVoltage(int pin) {

const int NUM_SAMPLES = 20;

long sum = 0;

for (int i = 0; i < NUM_SAMPLES; i++) {

sum += analogRead(pin);

delay(5);

}

float avgRaw = (float)sum / NUM_SAMPLES;

// Convert 12-bit ADC (0 - 4095) to Voltage (0.0V - 3.3V)

return (avgRaw / 4095.0) * 3.3;

}

// -----------------------------------------------------------------------------

// ANALYTICAL CALCULATIONS (pH, TURBIDITY, BATTERY)

// -----------------------------------------------------------------------------

float calculateCompensatedPH(float voltage, float waterTempC) {

// Nernst slope adjustment for temperature

float tempKelvin = waterTempC + 273.15;

float nernstSlope = 0.05916 * (tempKelvin / 298.15); // Volts per pH unit

// Slope calculation from calibration buffers

float slopePerUnit = (PHACIDVOLTAGE - PHNEUTRALVOLTAGE) / 3.0; // Expected ~0.133 V/pH

float calculatedPH = 7.0 - ((voltage - PHNEUTRALVOLTAGE) / (slopePerUnit * (nernstSlope / 0.05916)));

return constrain(calculatedPH, 0.0, 14.0);

}

float calculateTurbidityNTU(float voltage) {

// Transfer curve for 3.3V optical turbidity sensor

// Voltage > 2.5V represents crystal clear water (< 5 NTU)

if (voltage > 2.5) return 0.0;

// Empirical polynomial mapping voltage drop to turbidity

float ntu = -1120.4 (voltage voltage) + 5742.3 * voltage - 4352.9;

if (ntu < 0) ntu = 0.0;

if (ntu > 3000) ntu = 3000.0;

return ntu;

}

float calculateBatteryVoltage() {

float pinVoltage = readAverageADCVoltage(BATTERYADCPIN);

return pinVoltage * 2.0; // 2:1 resistive divider (100k + 100k)

}

// -----------------------------------------------------------------------------

// CLOUD TELEMETRY DISPATCHER

// -----------------------------------------------------------------------------

bool transmitTelemetry(float ph, float turbidity, float tempC, float battery) {

if (WiFi.status() != WL_CONNECTED) return false;

HTTPClient http;

http.begin(SERVER_ENDPOINT);

http.addHeader("Content-Type", "application/json");

String jsonPayload = "{";

jsonPayload += "\"node\":\"BUOY_01\",";

jsonPayload += "\"cycle\":" + String(wakeCycleCount) + ",";

jsonPayload += "\"ph\":" + String(ph, 2) + ",";

jsonPayload += "\"turbidity_ntu\":" + String(turbidity, 1) + ",";

jsonPayload += "\"temp_c\":" + String(tempC, 2) + ",";

jsonPayload += "\"battery_v\":" + String(battery, 2);

jsonPayload += "}";

Serial.println("[HTTP] Dispatching payload: " + jsonPayload);

int httpResponseCode = http.POST(jsonPayload);

http.end();

if (httpResponseCode > 0) {

Serial.printf("[HTTP] Telemetry delivered! Response Code: %d\n", httpResponseCode);

return true;

} else {

Serial.printf("[HTTP] Error sending POST: %d\n", httpResponseCode);

return false;

}

}

// -----------------------------------------------------------------------------

// ARDUINO SETUP (WAKE -> READ -> TX -> SLEEP)

// -----------------------------------------------------------------------------

void setup() {

Serial.begin(115200);

delay(100);

wakeCycleCount++;

Serial.println("\n========================================================");

Serial.printf(" MENTORA SOLAR WATER QUALITY TELEMETRY BUOY (CYCLE #%d)\n", wakeCycleCount);

Serial.println("========================================================");

pinMode(STATUSLEDPIN, OUTPUT);

digitalWrite(STATUSLEDPIN, HIGH);

// Configure ADC

pinMode(PHADCPIN, INPUT);

pinMode(TURBIDITYADCPIN, INPUT);

pinMode(BATTERYADCPIN, INPUT);

analogReadResolution(12);

analogSetAttenuation(ADC_11db);

// 1. Read Water Temperature

tempSensors.begin();

tempSensors.requestTemperatures();

float waterTempC = tempSensors.getTempCByIndex(0);

if (waterTempC < -50.0) waterTempC = 25.0; // Fallback to 25C if sensor error

// 2. Read pH Sensor

float phVoltage = readAverageADCVoltage(PHADCPIN);

float waterPH = calculateCompensatedPH(phVoltage, waterTempC);

// 3. Read Turbidity Sensor

float turbidityVoltage = readAverageADCVoltage(TURBIDITYADCPIN);

float turbidityNTU = calculateTurbidityNTU(turbidityVoltage);

// 4. Read Battery Voltage

float batteryVolts = calculateBatteryVoltage();

Serial.printf("[READINGS] Temp: %.2f C | pH: %.2f (%.3fV) | Turbidity: %.1f NTU (%.3fV) | Bat: %.2fV\n",

waterTempC, waterPH, phVoltage, turbidityNTU, turbidityVoltage, batteryVolts);

// 5. Connect Wi-Fi and Upload Telemetry

WiFi.mode(WIFI_STA);

WiFi.begin(WIFISSID, WIFIPASSWORD);

int attempts = 0;

while (WiFi.status() != WL_CONNECTED && attempts < 20) {

delay(500);

Serial.print(".");

attempts++;

}

if (WiFi.status() == WL_CONNECTED) {

Serial.println("\n[WIFI] Connected! Uploading telemetry...");

transmitTelemetry(waterPH, turbidityNTU, waterTempC, batteryVolts);

WiFi.disconnect(true);

WiFi.mode(WIFI_OFF);

} else {

Serial.println("\n[WARNING] Wi-Fi connection timed out. Data logged to local buffer.");

}

digitalWrite(STATUSLEDPIN, LOW);

// 6. Enter Ultra-Low-Power Deep Sleep

Serial.printf("[SYSTEM] Entering deep sleep for %llu seconds...\n\n", SLEEPDURATIONSEC);

espsleepenabletimerwakeup(SLEEPDURATIONSEC * 1000000ULL);

espdeepsleep_start();

}

void loop() {

// Not reached in deep sleep architecture

}

``