int photo = A0;    
int but = 3;       
int led = 2;       

bool svet = false;   
bool alarm = false;          

void setup() {
  pinMode(photo, INPUT);
  pinMode(but, INPUT_PULLUP);
  pinMode(led, OUTPUT);
  digitalWrite(led, LOW);
}

void loop() {
  int level = analogRead(photo);
  if (level < 500 && digitalRead(but) == LOW) {
    digitalWrite(led, HIGH);
  }
  else {
    digitalWrite(led, LOW);
  }
  
}