#define relay_pin 2
#define humidity_sensor_vcc 3
#define humidity_sensor_pin A4

void setup() {
  // Usual setup
  pinMode(LED_BUILTIN, OUTPUT);
  pinMode(relay_pin, OUTPUT);
  pinMode(humidity_sensor_vcc, OUTPUT);
  digitalWrite(relay_pin, HIGH);
  delay(2000); // Safety delay
}

int read_humidity_sensor() {
  int value;
  digitalWrite(humidity_sensor_vcc, HIGH); // Power up sensor
  digitalWrite(LED_BUILTIN, HIGH); // Builtin led on to show activity
  delay(500); // Wait a little bit until sensor is ready

  value = analogRead(humidity_sensor_pin);

  digitalWrite(humidity_sensor_vcc, LOW); // Sensor off
  digitalWrite(LED_BUILTIN, LOW); // Activity led off
  return 1023 - value;// Capture range is 0..1023
}

void loop() {
  int soil_humidity = read_humidity_sensor();
  if(soil_humidity > 300) {
    digitalWrite(relay_pin, LOW);// Relay low = pump on
  } else {
    digitalWrite(relay_pin, HIGH);
  }
  delay(10000);// Next loop in 10 secs
}