void setup() {
  for(int i=8; i <= 13; i++)
    pinMode(i,OUTPUT);
}

long long int start = millis();
float tensiune[6]={0,0,0,0,0,0};
float fadeAmount[6]={0.1,0.15,0.25,0.25,0.15,0.1};

void loop() {

  while(millis()-start < 20) {
    for(int i=8; i <= 13; i++)
      if(millis()-start < 20 * tensiune[i-8] / 5)
        digitalWrite(i, HIGH);
      else digitalWrite(i, LOW);
  }

  for(int i=0; i<= 5; i++) {
    if(tensiune[i] < 0 || tensiune[i] > 5)
      fadeAmount[i] = - fadeAmount[i];
    tensiune[i] += fadeAmount[i];
  }

  start = millis();

}