int led=5;
void setup() {
  // put your setup code here, to run once:
pinMode(led,OUTPUT);
Serial.begin(9600);
}

void loop() {
  // put your main code here, to run repeatedly:
if(Serial.available()>0)
{
  char s=Serial.read();
  if(s=='A')
  {
    digitalWrite(led,HIGH);
  }
else if(s=='a')
{
  digitalWrite(led,HIGH);
}
else
{
digitalWrite(led, LOW);
}
}
}