sketch.ino

char c;
int led = 4;
int counter = 0;

void setup()
{
  Serial.begin(57600);
  pinMode(led, OUTPUT);
}

void loop()
{
    while (Serial.available() > 0) {
        c = Serial.read();
        Serial.print(c);
        if (c == 't') {
            digitalWrite(led, (counter++ % 2 == 0) ? HIGH : LOW);
        }
    }
}