Plant Moisture Monitor

• Why

  Capacitive, not resistive

  Resistive probes corrode in wet soil within weeks. A capacitive probe has no exposed metal and lasts for years.

• Why

  Hourly readings

  Soil dries over days, so one reading an hour is plenty and keeps the battery going for months.

• Why

  Probe power is switched

  The probe only gets power for 50 ms per reading through GPIO1, which saves about 5 mA the rest of the time.

• Why

  SHT40 for air readings

  It's tiny, accurate to 0.2 °C and draws almost nothing between readings.

// Plant Moisture Monitor — wakes every hour, measures soil moisture,
// temperature and humidity, posts them over Wi-Fi, then sleeps again.

#include <Arduino.h>
#include <WiFi.h>
#include <HTTPClient.h>
#include <Wire.h>
#include <Adafruit_SHT4x.h>
#include "pins.h"
#include "secrets.h"   // WIFI_SSID, WIFI_PASS, WEBHOOK_URL

constexpr uint64_t SLEEP_US = 60ULL * 60ULL * 1000000ULL;  // 1 hour
constexpr uint16_t SOIL_DRY = 2900;   // Raw ADC in dry air (calibrate!)
constexpr uint16_t SOIL_WET = 1250;   // Raw ADC in a glass of water

Adafruit_SHT4x sht;

uint8_t readSoilPercent() {
  digitalWrite(PIN_SOIL_PWR, HIGH);     // Power the probe only while reading
  delay(50);
  uint32_t sum = 0;
  for (int i = 0; i < 16; i++) sum += analogRead(PIN_SOIL_ADC);
  digitalWrite(PIN_SOIL_PWR, LOW);
  const int raw = sum / 16;
  return constrain(map(raw, SOIL_DRY, SOIL_WET, 0, 100), 0, 100);
}

bool connectWifi(uint32_t timeoutMs = 8000) {

3 files: src/main.cpp, include/pins.h, platformio.ini

• Why

  Deep sleep between readings

  The chip is fully asleep for 59 minutes and 58 seconds of every hour, drawing a few microamps.

• Why

  Calibrate once

  SOIL_DRY and SOIL_WET are set by holding the probe in air and in water. The app will walk you through it.

Outer size

58 × 38 × 22 mm

Material

PETG, 0.2 mm layers

Print time

1 h 40 min, no supports

• Why

  Drip edge on the lid

  The lid overlaps the base by 2 mm so water from watering runs off instead of in.

1. 1Plug in the battery and probe.
2. 2Screw the board to the base.
3. 3Snap on the lid and push the probe into the soil.

• Why

  Fit check passed

  1.5 mm clearance above the tallest part.