Documentation

// Soil Moisture Sensor - print analog reading and percent on Arduino
// Calibrate DRY_VAL by holding the probe in air, WET_VAL by submerging in water.

const int aoPin = A0;
const int doPin = 2;

const int DRY_VAL = 880;   // air / very dry
const int WET_VAL = 350;   // submerged in water

void setup() {
  Serial.begin(9600);
  pinMode(doPin, INPUT);
}

void loop() {
  int raw = analogRead(aoPin);
  int pct = map(raw, DRY_VAL, WET_VAL, 0, 100);
  pct = constrain(pct, 0, 100);

  Serial.print("Raw=");  Serial.print(raw);
  Serial.print("  Moisture="); Serial.print(pct); Serial.print(" %");
  if (digitalRead(doPin) == LOW) Serial.print("  [DRY ALARM]");
  Serial.println();

  delay(1000);
}

// ESP32 - power the soil sensor only while reading, to slow corrosion
// VCC -> GPIO 5 (switched);  AO -> GPIO 34;  DO -> GPIO 35

const int powerPin = 5;
const int aoPin    = 34;
const int doPin    = 35;

const int DRY_VAL = 3300;
const int WET_VAL = 1300;

void setup() {
  Serial.begin(115200);
  pinMode(powerPin, OUTPUT);
  pinMode(doPin, INPUT);
  analogReadResolution(12);
}

int readSoil() {
  digitalWrite(powerPin, HIGH);
  delay(200);                       // settle time
  long sum = 0;
  for (int i = 0; i < 20; i++) { sum += analogRead(aoPin); delay(5); }
  digitalWrite(powerPin, LOW);
  return sum / 20;
}

void loop() {
  int raw = readSoil();
  int pct = map(raw, DRY_VAL, WET_VAL, 0, 100);
  pct = constrain(pct, 0, 100);
  Serial.printf("Raw=%4d  Moisture=%3d %%\n", raw, pct);
  delay(60000);                     // once a minute is plenty
}

# Soil moisture on Raspberry Pi Pico (MicroPython)
# AO -> GP26 (ADC0)

from machine import ADC, Pin
import time

ao = ADC(26)

DRY = 52000   # 16-bit reading in air
WET = 22000   # 16-bit reading submerged

while True:
    raw = ao.read_u16()
    pct = (DRY - raw) * 100 // (DRY - WET)
    pct = max(0, min(100, pct))
    print("Raw={:5d}   Moisture={:3d} %".format(raw, pct))
    time.sleep(1)

#!/usr/bin/env python3
# Soil moisture + ADS1115 on Raspberry Pi
# pip install adafruit-circuitpython-ads1x15

import time
import board, busio
from adafruit_ads1x15.ads1115 import ADS1115
from adafruit_ads1x15.analog_in import AnalogIn

i2c = busio.I2C(board.SCL, board.SDA)
ads = ADS1115(i2c)
ao  = AnalogIn(ads, 0)

DRY = 3.20   # volts in air
WET = 1.30   # volts submerged

while True:
    v   = ao.voltage
    pct = max(0, min(100, int((DRY - v) * 100 / (DRY - WET))))
    print("V={:.3f}   Moisture={} %".format(v, pct))
    time.sleep(1)