Mini pump plant waterer on an Arduino Uno

// Plant waterer: a capacitive soil moisture probe, a relay and a mini pump.
//
//   A0       the probe's AOUT (VCC and GND from the breadboard rails)
//   pin 7    the relay module's IN (high pulls the relay in)
//   relay    COM to the pump's own 6 V supply, NO to the pump's V+;
//            the pump's V- to ground
//
// A capacitive probe reads higher in dry soil: about 2.55 V in air and
// 1.27 V in water on this one, so on a 5 V board about 522 and 260 counts.
// The sketch turns that into a percentage, and under 35% runs the pump for
// three seconds, then waits ten for the water to soak in before it looks
// again, so it never floods the pot chasing a probe that has not caught up.

const int PROBE = A0;
const int RELAY = 7;

const long DRY_COUNTS = 522;
const long WET_COUNTS = 260;
const int THIRSTY = 35;
const unsigned long PUMP_MS = 3000;
const unsigned long SOAK_MS = 10000;

int waterings = 0;

int moisture() {
  long counts = analogRead(PROBE);
  long percent = (DRY_COUNTS - counts) * 100 / (DRY_COUNTS - WET_COUNTS);
  if (percent < 0) percent = 0;
  if (percent > 100) percent = 100;
  return (int)percent;
}

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: plant waterer");
  pinMode(RELAY, OUTPUT);
  digitalWrite(RELAY, LOW);
}

void loop() {
  int percent = moisture();
  Serial.print("soil ");
  Serial.print(percent);
  Serial.println("%");
  if (percent < THIRSTY) {
    waterings++;
    Serial.print("dry: watering ");
    Serial.println(waterings);
    digitalWrite(RELAY, HIGH);
    delay(PUMP_MS);
    digitalWrite(RELAY, LOW);
    Serial.println("soaking");
    delay(SOAK_MS);
  } else {
    delay(1000);
  }
}