ESPHome THC-S RS485 Soil Sensor Configuration

This repository contains the ESPHome configuration for the THC-S RS485 soil sensor, a low-cost Time Domain Reflectometry (TDR) sensor for comprehensive soil monitoring.

Understanding TDR Technology

Time Domain Reflectometry (TDR) is a sophisticated measurement technique that works similar to radar. Here's how it works:

• The sensor sends an electromagnetic pulse along a probe inserted in the soil
• When this pulse hits changes in the soil (like water content), part of it reflects back
• By measuring these reflections, the sensor can determine soil properties

About the THC-S RS485

The THC-S RS485 is a Chinese-manufactured low-cost alternative to expensive TDR sensors:

• Professional TDR sensors like Teros-12 can cost $200-500
• The THC-S typically costs $30-50
• Uses RS485 protocol for communication, which is industrial standard and noise-resistant

Measurements

The sensor provides comprehensive soil data:

Soil Moisture

• Measures volumetric water content
• Uses dielectric permittivity of soil
• More accurate than capacitive sensors

Temperature

• Built-in temperature sensor
• Used for compensating other readings
• Important for plant health monitoring

EC (Electrical Conductivity)

• Measures soil's ability to conduct electricity
• Indicates nutrient availability
• Temperature compensated for accuracy

pH

• Measures soil acidity/alkalinity
• Important for nutrient availability

NPK

• Nitrogen (N): Essential for leaf growth
• Phosphorus (P): Important for root development
• Potassium (K): Key for overall plant health

Advantages Over Simpler Sensors

• More accurate than capacitive sensors
• Less affected by soil salinity
• Provides multiple measurements in one device
• More durable due to better build quality
• RS485 allows longer cable runs (up to 1000m)

Limitations

• Requires more complex setup than simple sensors
• Needs calibration for best accuracy
• More expensive than basic moisture sensors
• Requires RS485 to TTL converter for microcontroller use

Hardware Requirements

• ESP32-C3 board (or similar ESP32 board)
• THC-S RS485 soil sensor
• TTL to RS485 converter
• Power supply for the sensor (12V recommended)
• Jumper wires or proper cabling for permanent installation

Wiring

ESP32-C3	TTL-RS485 Converter
GPIO5	TX
GPIO6	RX
GPIO4	DE/RE
3.3V	VCC
GND	GND

TTL-RS485 Converter	THC-S Sensor
A+	A+
B-	B-
GND	GND
--	12V

Features

• Direct integration with Home Assistant via ESPHome
• Temperature readings in both Celsius and Fahrenheit
• Temperature-compensated EC readings
• Calibrated soil moisture readings using dielectric constant calculations
• Offset-corrected NPK readings
• Multiple WiFi network support with fallback AP
• Over-the-air updates

Installation

1. Install ESPHome if you haven't already:

pip install esphome

2. Copy soil-sensor.yaml to your ESPHome directory

3. Create and update the secrets.yaml:

wifi_ssid: "Your_SSID"
wifi_password: "Your_Password"
wifi_ssid_2: "Your_Backup_SSID"
wifi_password_2: "Your_Backup_Password"

4. Flash your ESP32:

esphome run soil-sensor.yaml

Sensor Calculations and Calibrations

The configuration includes several important calibrations derived from testing and documentation:

Soil Moisture

Uses a quadratic approximation for the apparent dielectric constant:

soil_apparent_dieletric_constant = 1.3088 + (0.1439 * raw_value) + (0.0076 * raw_value * raw_value)

EC (Electrical Conductivity)

Includes temperature compensation:

soil_ec = 1.93 * raw_ec - 270.8
soil_ec = soil_ec / (1.0 + 0.019 * (temp - 25.0))

NPK Offsets

Based on testing with known solutions:

• Nitrogen: -27 offset
• Phosphorus: -109 offset
• Potassium: -102 offset

Troubleshooting

Common issues and solutions:

1. No readings from sensor:

   • Check wiring, especially A+ and B- connections
   • Verify 12V power supply
   • Confirm RS485 converter is working (LED indicators)

2. Erratic readings:

   • Check cable length and quality
   • Verify power supply stability
   • Ensure proper grounding

3. Connection issues:

   • Verify WiFi credentials
   • Check ESP32 antenna orientation
   • Consider signal strength and distance to router

Contributing

Feel free to submit issues and pull requests if you have improvements or fixes to suggest. Please include:

• Clear description of changes
• Test results if applicable
• Any relevant documentation updates

Credits

This configuration is based on research and documentation from:

• Original Arduino code by Kromadg
• ESPHome community contributions
• Personal testing and calibration

License

MIT License - feel free to use and modify as needed. See LICENSE for more details.