Plant monitor on an Arduino Uno

// Plant monitor: the soil, the rain and the water butt, once a second.
//
//   A0  the soil sensor's AOUT     A1  the rain sensor's AO
//   A2  the water level's S        pin 7  the rain sensor's DO
//   pin 8  the "water me" lamp, through 220 ohm to GND
//   every sensor's VCC -> 5 V and GND -> GND
//
// Three sensors that all look like "an analog pin that means water", and they
// disagree about which way is up. That is the thing to take away.
//
// # The soil sensor reads lower when it is wetter
//
// The capacitive stake gives about 520 counts held in the air and about 260
// with its tip in water, and the soil is somewhere between. So the percentage
// is map(reading, 520, 260, 0, 100): the two ends swapped. Those two numbers
// are this sensor's; a real one is calibrated in dry and soaked soil first.
//
// # The rain sensor reads lower too, and has a comparator
//
// Dry, the plate is open and A1 reads the full 1023. Wet, it falls. Its board
// also compares that with its trimmer and pulls DO low when it is wet enough,
// so pin 7 is a plain "is it raining" without any arithmetic.
//
// # The water level reads higher, and not in millimetres
//
// Its transistor turns on as soon as the tip is wet and then creeps up as the
// water rises: most of the reading is in the first few millimetres. It is a
// level indicator, not a gauge.
//
// # The lamp
//
// Pin 8 lights when the soil is under 30% and it is not raining: the plant
// wants water and the sky is not going to give it any.

const int SOIL = A0;
const int RAIN_AO = A1;
const int WATER = A2;
const int RAIN_DO = 7;
const int LAMP = 8;

// This sensor's two ends, in counts on a 5 V board.
const int SOIL_AIR = 520;
const int SOIL_WATER = 260;

void setup() {
  pinMode(RAIN_DO, INPUT);
  pinMode(LAMP, OUTPUT);
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: plant monitor");
}

void loop() {
  int soil = analogRead(SOIL);
  int percent = constrain(map(soil, SOIL_AIR, SOIL_WATER, 0, 100), 0, 100);
  int rain = analogRead(RAIN_AO);
  bool raining = digitalRead(RAIN_DO) == LOW;
  int water = analogRead(WATER);

  Serial.print("soil ");
  Serial.print(percent);
  Serial.print("% (");
  Serial.print(soil);
  Serial.print(")  rain ");
  Serial.print(rain);
  Serial.print(raining ? " raining" : " dry");
  Serial.print("  water ");
  Serial.println(water);

  digitalWrite(LAMP, percent < 30 && !raining ? HIGH : LOW);
  delay(1000);
}