const int phototransistor = A2 ; // connected to PA2 , A2 in arduino
const int mosfet = 7 ; // connected to PA7 , 7 in arduino
int photovalue;
int treshold = 300; // threshold for the phototransistor
bool night = 0; // one time during night

int moisture_duration = 15000; // 15 seconde of moisture

unsigned long previous_time;
unsigned long actual_time;
unsigned long interval = 1000 * 3600 * 4; // every 4 hours

void setup() {
  pinMode(phototransistor, INPUT);
  pinMode(mosfet, OUTPUT);
  previous_time = millis();

}

void loop() {

  actual_time = millis();
  photovalue = analogRead(phototransistor);

// after 50 days millis overflow , time should be reset
  if(actual_time < previous_time){
    previous_time = millis();
  }  

// the sun shine
  if(photovalue < treshold && actual_time > (previous_time + interval)){
    digitalWrite(mosfet, HIGH);
    delay(moisture_duration);
    digitalWrite(mosfet, LOW);
    previous_time = millis();
    night = 0;    
  }

// one time during the night
  if(photovalue > treshold && actual_time > (previous_time + interval) && night == 0 ){
    digitalWrite(mosfet, HIGH);
    delay(moisture_duration);
    digitalWrite(mosfet, LOW);
    previous_time = millis();
    night = 1;    
  }

}