Wireless Soil Sensor Gateway

Battery-powered ESP32-S3 gateway for SDI-12 soil sensors with deep sleep

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Overview

A battery-powered field gateway that reads 4 soil sensors over the SDI-12 protocol, logs the data, and goes into deep sleep for 1 hour to conserve power. Designed for precision agriculture, environmental monitoring, and research stations.

Each sensor connects on its own dedicated SDI-12 data line with individual level shifting, allowing independent communication without bus collisions.

What it does:

• Reads 4 METER GS3 (or compatible) soil sensors via SDI-12
• Measures soil moisture (VWC), temperature, and electrical conductivity
• Deep sleep between cycles — 1 hour interval, ~10 μA sleep current
• MOSFET-switched sensor power — sensors are powered off during sleep
• State machine architecture — non-blocking, timeout-protected
• 9V battery powered with voltage regulation
• Individual BSS138 level shifters per sensor line

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Schematic

Download full schematic (PDF)

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Architecture

┌─────────┐     ┌────────────────────────────────────────────┐
│  9V      │     │              ESP32-S3                      │
│  Battery │────►│                                            │
└─────────┘     │  GPIO 8  ──► BSS84 P-FET ──► Sensor Power  │
                │                                            │
                │  GPIO 15 ◄──► BSS138 ◄──► SDI-12 Sensor 1  │
                │  GPIO 16 ◄──► BSS138 ◄──► SDI-12 Sensor 2  │
                │  GPIO 17 ◄──► BSS138 ◄──► SDI-12 Sensor 3  │
                │  GPIO 18 ◄──► BSS138 ◄──► SDI-12 Sensor 4  │
                │                                            │
                │  Deep Sleep (1 hour) ──► Wake ──► Read ──►  │
                └────────────────────────────────────────────┘

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Measurement Cycle

Boot → Power ON sensors → Wait 2s → Send M! → Wait → Read D0! → Next sensor → Power OFF → Deep Sleep (1h)

1. ESP32 wakes from deep sleep (or boots fresh)
2. MOSFET switches ON sensor power (GPIO 8 → LOW)
3. Wait 2 seconds for sensors to stabilize
4. For each sensor (1–4):
   • Send aM! command (start measurement)
   • Wait for sensor-requested delay (typically 1–3 sec)
   • Send aD0! command (read data)
   • Parse response: VWC, temperature, EC
5. Power OFF sensors (GPIO 8 → HIGH)
6. Enter deep sleep for 3600 seconds (1 hour)

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Hardware

Component	Qty	Purpose
ESP32-S3 DevKit	1	Main controller
METER GS3 (or compatible)	4	Soil moisture / temp / EC sensors
BSS138 N-MOSFET	4	3.3V ↔ sensor voltage level shifting (data lines)
BSS84 P-MOSFET	1	Sensor power switching
BC547 NPN	1	Gate driver for BSS84
TVS diode	1	ESD protection on SDI-12 bus
9V battery	1	Power source
Resistors	—	4.7kΩ pull-ups, 10kΩ pull-downs, 150Ω series, 100kΩ gate

Pin Map

ESP32-S3 Pin Assignment
───────────────────────────────────────
GPIO 8      → Sensor power control (ENERGY_SAVING)
GPIO 15     → SDI-12 Data Line — Sensor 1 (address '1')
GPIO 16     → SDI-12 Data Line — Sensor 2 (address '2')
GPIO 17     → SDI-12 Data Line — Sensor 3 (address '3')
GPIO 18     → SDI-12 Data Line — Sensor 4 (address '4')

Power Circuit

9V Battery ──► BC547 (base from GPIO 8)
                  │
                  ▼
              BSS84 P-FET ──► Sensor V+ rail
                  │
              100kΩ pull-up

When GPIO 8 is LOW → BC547 ON → BSS84 gate pulled LOW → sensors powered. When GPIO 8 is HIGH → BC547 OFF → BSS84 gate HIGH → sensors off (~10 μA total).

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Software

State Machine

State	Description
IDLE	Wait for cycle trigger (deep sleep wake or interval)
WAKE_SENSORS	Switch on sensor power via MOSFET
WAIT_WAKE	2-second stabilization delay
SEND_MEASUREMENT	Send aM! to current sensor
WAIT_MEASUREMENT	Wait sensor-requested time (1–3 sec)
READ_DATA	Send aD0!, parse response
FINISH_CYCLE	Power off sensors, enter deep sleep

Configuration

const bool USE_DEEP_SLEEP = true;           // true = deep sleep, false = polling
const unsigned long DEEP_SLEEP_TIME_SEC = 3600;  // 1 hour
const unsigned long WAKE_DELAY_MS = 2000;        // sensor warm-up
const unsigned long RESPONSE_TIMEOUT_MS = 5000;  // M! timeout
const unsigned long READ_TIMEOUT_MS = 5000;      // D0! timeout

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Project Structure

Wireless-Soil-Sensor-Gateway/
├── Wireless_Soil_Sensor_Gateway.ino   # Main firmware
├── schematic.pdf                       # Fritzing circuit diagram
├── screenshots/
│   └── schematic.png                   # Circuit preview
├── README.md
└── LICENSE

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Quick Start

1. Wire up

Connect sensors and MOSFETs according to the schematic. Each sensor gets its own SDI-12 data line through a BSS138 level shifter.

2. Flash

1. Open Wireless_Soil_Sensor_Gateway.ino in Arduino IDE
2. Select board: ESP32-S3 Dev Module
3. Install library: SDI-12 (by EnviroDIY)
4. Upload
5. Open Serial Monitor (115200 baud)

3. Deploy

Connect 9V battery, place sensors in soil at desired depths. The device will read all 4 sensors and sleep for 1 hour between cycles.

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SDI-12 Protocol

The SDI-12 (Serial Data Interface at 1200 baud) protocol is an industry standard for environmental sensors. Each sensor has a unique address ('1'–'4') and responds to ASCII commands:

Command	Description	Example Response
aM!	Start measurement	a0013 (wait 1 sec, 3 values)
aD0!	Read data	a+0.234+22.5+1.45 (VWC, temp, EC)
aI!	Sensor info	Manufacturer, model, version

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Key Design Decisions

Individual data lines — Each sensor has its own GPIO + level shifter instead of a shared bus. This avoids address conflicts and allows parallel communication.

MOSFET power switching — A BSS84 P-channel MOSFET cuts power to all sensors during sleep, reducing current from ~50 mA to ~10 μA.

Non-blocking state machine — The firmware uses a state machine with timeouts instead of blocking delays, ensuring robust operation even if a sensor fails to respond.

Deep sleep — The ESP32-S3 deep sleep mode draws ~10 μA. With a 9V battery (500 mAh), the device can run for months in the field.

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Author

Temur Eshmurodov — @myseringan

License

MIT License — free to use and modify.