unsigned long sentAt = 0;
unsigned long count = 0;
bool ledOn = false;

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Send 1 for on, 0 for off");
}

void loop() {
  while (Serial.available() > 0) {
    const char command = Serial.read();
    if (command == '1' || command == '0') {
      ledOn = command == '1';
      digitalWrite(LED_BUILTIN, ledOn ? HIGH : LOW);
      Serial.println(ledOn ? "ACK on" : "ACK off");
    }
  }
  const unsigned long now = millis();
  if (now - sentAt >= 1000) {
    sentAt = now;
    Serial.print("count="); Serial.print(count++);
    Serial.print(",led="); Serial.println(ledOn ? 1 : 0);
  }
}