🌱 ESP32 Soil Moisture Monitoring System

A production-style embedded IoT system built on the ESP32-WROOM-32 using ESP-IDF, featuring real-time sensing, persistent configuration, and a lightweight HTTP interface.

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🚀 Features

• 📡 Wi-Fi connectivity (STA mode) with event-driven handling

• 🌐 Embedded HTTP server

  • /data endpoint returns live soil moisture in JSON

• 📟 SSD1306 OLED display (I2C)

  • Custom framebuffer + bitmap font rendering

• 🌡️ ADC-based soil moisture sensing

  • Averaged sampling for stability

• 💾 Non-Volatile Storage (NVS)

  • Stores Wi-Fi credentials persistently

• ⚙️ FreeRTOS-based execution

  • Deterministic periodic sampling loop

• 🧠 Clean modular architecture

  • Separation of drivers, networking, storage, and application logic

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🏗️ System Architecture

                +----------------------+
                |     Soil Sensor      |
                |   (Analog Output)    |
                +----------+-----------+
                           |
                           v
                +----------------------+
                |   ADC (Oneshot)      |
                |  Averaging Filter    |
                +----------+-----------+
                           |
                           v
                +----------------------+
                |   Application Logic  |
                | (main.c loop)        |
                +----+-----------+-----+
                     |           |
                     v           v
        +----------------+   +------------------+
        | OLED Display   |   | HTTP Server      |
        | (SSD1306 I2C)  |   | /data endpoint   |
        +----------------+   +------------------+
                     |
                     v
             +------------------+
             | Wi-Fi Stack      |
             | (Event Driven)   |
             +------------------+
                     |
                     v
             +------------------+
             | NVS Storage      |
             | (SSID/PASSWORD)  |
             +------------------+

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📡 API

GET /data

Returns current soil moisture:

{
  "status": "ok",
  "sensor_value": 0
}

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⚙️ Hardware

• ESP32-WROOM-32
• Capacitive Soil Moisture Sensor
• SSD1306 OLED Display (128x64, I2C)
• Breadboard + jumper wires
• Pull up resistors

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🔌 Pin Configuration

Component	ESP32 Pin
Soil Sensor	GPIO34 (ADC1_CH6)
OLED SDA	GPIO21
OLED SCL	GPIO22

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🧠 Key Technical Decisions

1. ADC Sampling Strategy

• Uses oneshot mode instead of continuous
• 32 samples averaged to reduce noise
• Controlled delay ensures stable readings

2. Custom Display Driver

• Full framebuffer implementation
• Manual pixel control
• Scaled bitmap font rendering (no external libs)

3. Event-Driven Wi-Fi

• Uses ESP-IDF event loop

• Handles:

  • Connection
  • Disconnection
  • Auth failures

• Tracks state via FreeRTOS Event Groups

4. Persistent Storage (NVS)

• SSID & password stored in flash
• Abstracted via storage.c
• Safe open/read/write pattern with fallback init

5. Lightweight HTTP Server

• Based on esp_http_server
• Minimal memory footprint
• Shared state via controlled setter (set_moisture())

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🧵 Runtime Flow

Boot
 ├── Initialize NVS
 ├── Store Wi-Fi credentials
 ├── Connect to Wi-Fi
 ├── Initialize ADC
 ├── Initialize OLED
 ├── Start HTTP server
 └── Loop:
      ├── Sample ADC (32 samples)
      ├── Compute moisture %
      ├── Update display
      ├── Update HTTP state
      ├── Maintain Wi-Fi connection
      └── Delay (~3.3s)

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💾 Memory Architecture (NVS Focus)

NVS Namespace: "storage"

Key	Type	Description
ssid	string	Wi-Fi SSID
password	string	Wi-Fi password

Flow

storage_write_string()  → nvs_set_str() → nvs_commit()
storage_read_string()   → nvs_get_str()

• Lazy initialization handled internally
• Default fallback values supported
• Clean handle lifecycle (open → use → close)

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🔍

• ✅ Direct ESP-IDF usage
• ✅ Understanding of embedded memory (NVS, buffers)
• ✅ RTOS-aware design patterns
• ✅ Driver-level hardware control
• ✅ Networking stack integration

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⚠️ Limitations / Future Improvements

• No HTTPS (currently HTTP only)
• No OTA updates
• No sensor calibration curve (linear approximation used)
• Web UI could be added (currently API only)
• ADC calibration using eFuse could improve accuracy

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🧪 Build & Flash

Using PlatformIO:

pio run
pio run --target upload
pio device monitor

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📸 Demo

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