const int LDR_PIN      = A0;
const int RELAY_PIN    = 7;
const int LED_GREEN    = 8;
const int LED_RED      = 9;
const int LED_YELLOW   = 10;
const int SWITCH_PIN   = 2;
const int LIGHT_THRESHOLD = 500;

void setup() {
  pinMode(RELAY_PIN,  OUTPUT);
  pinMode(LED_GREEN,  OUTPUT);
  pinMode(LED_RED,    OUTPUT);
  pinMode(LED_YELLOW, OUTPUT);
  pinMode(SWITCH_PIN, INPUT_PULLUP);
  Serial.begin(9600);
}

void loop() {
  int switchState = digitalRead(SWITCH_PIN);
  int lightLevel  = analogRead(LDR_PIN);

  if (switchState == LOW) {
    digitalWrite(LED_YELLOW, LOW);
    if (lightLevel > LIGHT_THRESHOLD) { switchToSolar(); }
    else { switchToGrid(); }
  } else {
    digitalWrite(LED_YELLOW, HIGH);
    if (lightLevel > LIGHT_THRESHOLD) { switchToSolar(); }
    else { switchToGrid(); }
  }
  delay(1000);
}

void switchToSolar() {
  digitalWrite(RELAY_PIN, HIGH);
  digitalWrite(LED_GREEN, HIGH);
  digitalWrite(LED_RED,   LOW);
}

void switchToGrid() {
  digitalWrite(RELAY_PIN, LOW);
  digitalWrite(LED_GREEN, LOW);
  digitalWrite(LED_RED,   HIGH);
}