Plant Moisture Monitor

#include <Arduino.h>
#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>


// Forward declarations
int readSoilRaw();
int moisturePercentFromRaw(int raw);
void drawStatus(int raw, int percent, bool needsWater);
void sampleAndUpdate(bool forceRedraw);

constexpr int SOIL_PIN = 34;
constexpr int I2C_SDA_PIN = 21;
constexpr int I2C_SCL_PIN = 22;
constexpr uint8_t SCREEN_WIDTH = 128;
constexpr uint8_t SCREEN_HEIGHT = 64;
constexpr uint8_t OLED_ADDRESS = 0x3C;
constexpr unsigned long SAMPLE_INTERVAL_MS = 15000UL;

// Calibrate these per probe: record raw readings in dry air and saturated potting soil.
constexpr int DRY_RAW = 3000;
constexpr int WET_RAW = 1400;
constexpr int WATER_THRESHOLD_PERCENT = 35;

Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, -1);
int lastPercent = -1;
int lastRaw = -1;
bool lastNeedsWater = false;
unsigned long lastSampleMs = 0;

int readSoilRaw() {
  constexpr int sampleCount = 16;
  uint32_t total = 0;
  for (int i = 0; i < sampleCount; ++i) {
    total += analogRead(SOIL_PIN);
    delay(3);
  }
  return static_cast<int>(total / sampleCount);
}

int moisturePercentFromRaw(int raw) {
  long percent = map(raw, DRY_RAW, WET_RAW, 0, 100);
  return constrain(percent, 0, 100);
}

void drawStatus(int raw, int percent, bool needsWater) {
  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.println(F("PLANT WATER MONITOR"));
  display.drawLine(0, 10, 127, 10, SSD1306_WHITE);

  display.setTextSize(2);
  display.setCursor(0, 17);
  display.print(percent);
  display.println(F("%"));

  display.setTextSize(1);
  display.setCursor(0, 40);
  if (needsWater) {
    display.println(F("Status: WATER NOW"));
  } else {
    display.println(F("Status: moisture OK"));
  }
  display.setCursor(0, 54);
  display.print(F("Raw ADC: "));
  display.print(raw);
  display.display();
}

void sampleAndUpdate(bool forceRedraw) {
  const int raw = readSoilRaw();
  const int percent = moisturePercentFromRaw(raw);
  const bool needsWater = percent < WATER_THRESHOLD_PERCENT;

  Serial.print(F("Soil raw="));
  Serial.print(raw);
  Serial.print(F(" moisture="));
  Serial.print(percent);
  Serial.println(F("%"));

  if (forceRedraw || percent != lastPercent || needsWater != lastNeedsWater) {
    drawStatus(raw, percent, needsWater);
    lastPercent = percent;
    lastRaw = raw;
    lastNeedsWater = needsWater;
  }
}

void setup() {
  Serial.begin(115200);
  analogReadResolution(12);
  analogSetPinAttenuation(SOIL_PIN, ADC_11db);

  Wire.begin(I2C_SDA_PIN, I2C_SCL_PIN);
  if (!display.begin(SSD1306_SWITCHCAPVCC, OLED_ADDRESS)) {
    Serial.println(F("SSD1306 not found at 0x3C."));
    while (true) {
      delay(1000);
    }
  }

  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 20);
  display.println(F("Starting monitor..."));
  display.display();
  sampleAndUpdate(true);
  lastSampleMs = millis();
}

void loop() {
  const unsigned long now = millis();
  if (now - lastSampleMs >= SAMPLE_INTERVAL_MS) {
    lastSampleMs = now;
    sampleAndUpdate(false);
  }
}