         
         // Move the panels if needed.
         tkr_move();
+        
+        if (light_level < DARK_THRESHOLD) {
+          //goToSleep();
+          //TODO connect PIN_LDR_LEFT to INT0
+        }
+        
     }
     
     if (now - tx_last >= tx_interval) {

    int left = analogRead(PIN_LDR_LEFT);
    int right = analogRead(PIN_LDR_RIGHT);
    
    
    Serial.print(left); 
    Serial.print(':');

    
    pinMode(LED_DEBUG, OUTPUT);     
    pinMode(PIN_TX, OUTPUT); 
    
    pinMode(PIN_PERIF_POWER);
    digitalWrite(PIN_PERIF_POWER, HIGH); // high = 0n (NPN)
    
    pinMode(SERVO_PIN_HORZ_POWER, OUTPUT); 
    digitalWrite(SERVO_PIN_HORZ_POWER, LOW); // low = 0n (PNP)
