Automatic plant waterer on an Arduino Uno

// Automatic plant waterer: soil moisture, a relay, a pump and a tank sensor.
//
// Wiring:
//   soil moisture stake   AOUT -> A0   VCC -> 5 V   GND -> GND
//   tank water level      S -> A1      VCC -> 5 V   GND -> GND
//   relay module          IN -> 7      VCC -> 5 V   GND -> GND
//   pump circuit          battery + -> relay COM, relay NO -> pump -> GND
//                         (the battery minus joins GND, so there is one ground)
//   water now button      pin 2 -> pushbutton -> GND
//   16x2 LCD, I2C 0x27    SDA -> A4    SCL -> A5    VCC -> 5 V   GND -> GND
//
// When the soil reads drier than 35%, the relay runs the pump for three
// seconds, then waits for the water to soak down to the stake before it looks
// again, and repeats until the soil reads 60%. Watering in short bursts is
// what keeps a pot from flooding: the stake only sees the water once it has
// had time to spread.
//
// The tank sensor guards the pump. A small pump running dry burns out, so with
// the tank low the screen asks for a refill and the pump stays off.
//
// In the simulator nothing drinks the water: the tank starts empty, so fill it
// with its slider, then drag the soil's moisture down and watch it water.

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

const uint8_t SOIL_PIN = A0;
const uint8_t TANK_PIN = A1;
const uint8_t RELAY_PIN = 7;
const uint8_t BUTTON_PIN = 2;

// The stake reads higher when it is drier. These are the readings in air and
// in a glass of water; measure your own and put them here.
const int SOIL_DRY = 522;
const int SOIL_WET = 260;
const uint8_t START_BELOW = 35;  // percent
const uint8_t STOP_AT = 60;      // percent
const int TANK_LOW = 120;        // below this the tank is nearly empty

const uint32_t PUMP_MS = 3000;
const uint32_t SOAK_MS = 5000;

LiquidCrystal_I2C lcd(0x27, 16, 2);

enum State : uint8_t { IDLE, PUMPING, SOAKING };
State state = IDLE;
uint32_t stateAt = 0;
uint16_t waterings = 0;
bool buttonWas = false;
bool manual = false;  // this watering was asked for with the button

static uint8_t soilPercent()
{
  long raw = analogRead(SOIL_PIN);
  long p = (long)(SOIL_DRY - raw) * 100 / (SOIL_DRY - SOIL_WET);
  return (uint8_t)(p < 0 ? 0 : (p > 100 ? 100 : p));
}

static bool tankLow()
{
  return analogRead(TANK_PIN) < TANK_LOW;
}

static void pump(bool on)
{
  digitalWrite(RELAY_PIN, on ? HIGH : LOW);
}

/** The top line is the soil; the bottom line is what it is doing. */
static void screen(uint8_t moisture, const char *status, int count = -1)
{
  lcd.setCursor(0, 0);
  lcd.print("Soil ");
  if (moisture < 100) lcd.print(' ');
  if (moisture < 10) lcd.print(' ');
  lcd.print(moisture);
  lcd.print("%  ");
  lcd.print(moisture < START_BELOW ? "DRY " : (moisture >= STOP_AT ? "WET " : "OK  "));
  lcd.setCursor(0, 1);
  uint8_t used = (uint8_t)lcd.print(status);
  if (count >= 0) {
    used = (uint8_t)(used + lcd.print(count));
    used = (uint8_t)(used + lcd.print(" times"));
  }
  for (uint8_t i = used; i < 16; i++) {
    lcd.print(' ');
  }
}

static void startPump(const char *why)
{
  state = PUMPING;
  stateAt = millis();
  waterings++;
  pump(true);
  Serial.print("Pump on: ");
  Serial.println(why);
}

void setup()
{
  Serial.begin(9600);
  pinMode(RELAY_PIN, OUTPUT);
  pump(false);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  lcd.begin();
  lcd.backlight();
  lcd.print("Plant waterer");
  delay(800);
  lcd.clear();
  Serial.println("Plant waterer: starts below 35%, stops at 60%");
}

void loop()
{
  uint32_t now = millis();
  uint8_t moisture = soilPercent();
  bool low = tankLow();
  bool button = digitalRead(BUTTON_PIN) == LOW;
  bool pressed = button && !buttonWas;
  buttonWas = button;

  // The tank comes first: never run the pump dry.
  if (low && state == PUMPING) {
    pump(false);
    state = IDLE;
    Serial.println("Pump off: the tank is empty");
  }

  switch (state) {
    case IDLE:
      if (low) {
        screen(moisture, "Refill the tank");
      } else if (pressed) {
        manual = true;
        startPump("button");
      } else if (moisture < START_BELOW) {
        manual = false;
        startPump("soil is dry");
      } else {
        screen(moisture, "Watered ", waterings);
      }
      break;
    case PUMPING:
      screen(moisture, "Watering...");
      if (now - stateAt >= PUMP_MS) {
        pump(false);
        state = SOAKING;
        stateAt = now;
        Serial.println("Pump off, letting it soak");
      }
      break;
    case SOAKING:
      screen(moisture, "Soaking in...");
      if (now - stateAt >= SOAK_MS) {
        state = IDLE;
        if (!manual && moisture < STOP_AT && !low) {
          startPump("still dry");
        } else if (moisture >= STOP_AT) {
          Serial.print("Done: soil at ");
          Serial.print(moisture);
          Serial.println("%");
        }
      }
      break;
  }
  delay(100);
}