🌱 ESPHome Garden Soil Moisture Node

Battery-powered ESP32 soil moisture node using ESPHome, designed for long-term, low-maintenance garden deployment.
It measures:

• Soil moisture (capacitive VH400 probe)
• Garden air temperature & humidity
• Enclosure temperature & humidity
• Battery voltage and ESP 5 V rail voltage (prevents Sleep)

All sensors receive battery positive (V+), and their ground return path is switched through a MOSFET, ensuring near-zero sensor current during deep sleep.

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Design Summary

• ESP32 running ESPHome (ESP-IDF)
• Dual SHT31 sensors for ambient and enclosure temperature/humidity
• VH400 capacitive soil moisture probe powered from a 5 V boost converter
• Hard power removal from sensors using a low-side MOSFET
• Custom ESPHome I²C bus component delays I²C initialization until sensors are powered

This project targets users already familiar with ESPHome and Home Assistant. Electronics knowledge is helpful but not required.

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Hardware Overview

Major Components

• MCU: ESP32 dev board
• Sensors:
  • SHT31 @ I²C address 0x44 (garden)
  • SHT31 @ I²C address 0x45 (enclosure)
  • Vegetronix VH400 moisture probe (analog output)
• Power:
  • Single-cell LiPo (4.2 V max)
  • 5 V boost converter for VH400
  • Sensors connected directly to battery V+
  • Sensor ground switched via IRLZ44N MOSFET
• Switching & Measurement:
  • MOSFET gate controlled by ESP32 GPIO17 (via 200 Ω, 100 kΩ pull-down)
  • Boost rail sense (30 kΩ / 20 kΩ divider → GPIO36)
  • Battery sense (36 kΩ / 100 kΩ divider → GPIO32)
• GPIO Use:
  • GPIO21 – I²C SDA
  • GPIO22 – I²C SCL
  • GPIO34 – VH400 analog input
  • GPIO32 – battery voltage ADC
  • GPIO36 – 5 V rail ADC
  • GPIO17 – MOSFET gate
  • GPIO39 – external wake

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Power & Sensor Architecture

The system uses battery V+ to power all sensors, but prevents any actual current flow until sensor ground is connected.

Key Points

• Sensor V+ is always present.
• Sensor ground is routed through an IRLZ44N MOSFET.
• When the MOSFET is off, sensors are effectively unpowered.
• This prevents:
  • I²C bus lockups on boot
  • Sensor leakage current during deep sleep
  • Boost converter idle draw

GPIO17 HIGH enables the MOSFET and powers all sensors; LOW disables them.

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Wiring Diagram

Battery V+ (to sensors)                     ESP32 (center)                   Sensors 
─────────────────────────────        ─────────────────────────────      ────────────────
5V                                   ┌──────────────────────────┐        
  │                                  |                          |
[30 kΩ]                              │            ESP32         │
● Node (5V sense tap)               ◄┤ GPIO36  adc_5v_raw       |   
  |                                 ◄| GPIO39  WAKE_IN          |
[20 kΩ]                              │                          │
  │                                  │                          │
 GND                                 │                          │
                                     │                          │
LiPo 4.2 V max                      ◄│  ----------------------- │ ------► SHT31 (4.2V)  
  │                                  |                          |         VH400 (1.5–4.5V → 5 V) 
[36 kΩ]                              │                          │         
  │                                  │                          │
● Node (LiPo sense tap)             ◄│ GPIO32 adc_batt_raw      │
  │                                  │                          │
[100 kΩ]                             │                          │
  │                                  │    ─┤ GPIO34   ADC_SOIL  ──► VH400 analog out
 GND                                 │    ─┤ GPIO21   I2C_SDA   ──► SDA of SHT31 @0x44, @0x45
                                     |    ─┤ GPIO22   i2C-SCL   ──► SCL of SHT31 @0x44, @0x45
                                     |
                                     │    ─┤ GPIO17   VDD_SW    ──200 Ω──► IRLZ44N GATE
                                     │                            [100 kΩ] gate pull-down → GND
                                     │                          |
                                     └──────────────────────────┘

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ESPHome Behavior

The ESPHome configuration:

• Uses ESP-IDF for low-level control.
• Manages dual SHT31 sensors on I²C.
• Reads VH400 analog moisture values.
• Measures battery and boost voltages via ADC.
• Controls the MOSFET gate (GPIO17) to power-cycle sensors each wake cycle.
• Performs deep sleep for long life, with adaptive intervals.

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Custom I²C Bus Component

Delayed I²C Initialization

A custom I²C component ensures I²C doesn't initialize until sensors are truly powered.

File location:

copy: /homeassistant/esphome/components/i2c/i2c_bus_esp_idf.h 
to:   /homeassistant/esphome/my_components/i2c/i2c_bus_esp_idf.h

Modification

// Original:
// float get_setup_priority() const override { return setup_priority::BUS; }

// Modified:
float get_setup_priority() const override { return setup_priority::DATA; }

Reason

• ESPHome orders initialization based on setup priority.
• By switching to DATA, the I²C bus initializes later, after:
  • GPIO17 enables the sensor ground MOSFET
  • Sensors and the boost converter stabilize

This prevents intermittent failures such as:

• I²C scan failures
• Missing devices on wake
• First-read failures after deep sleep

ESPHOME Components Access Instructions

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Wake Cycle Behavior

1. ESP32 wakes.
2. GPIO17 → HIGH to enable the MOSFET and power sensors.
3. Short stabilization delay.
4. Modified I²C bus initializes.
5. Sensors read:
   • SHT31 ×2
   • VH400 analog moisture
   • Battery & 5 V rail (Prevents Sleep)
6. Values published to Home Assistant.
7. Sleep interval chosen (normal, low battery, or night).
8. GPIO17 → LOW cuts all sensor power.
9. ESP32 enters deep sleep.

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Home Assistant Entities

Typical entities exposed:

• Ambient temperature / humidity
• Enclosure temperature / humidity
• Soil moisture (processed)
• Raw VH400 analog voltage
• Battery voltage
• Boost converter voltage
• Battery percentage
• Wi-Fi RSSI
• Sleep/wake event entity

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License

GNU General Public License v3.0 (GPL-3.0)
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