HC-05 Bluetooth Module with an Arduino Uno

// HC-05 Bluetooth Module on Arduino Uno: wiring and a free simulation at https://www.tinkered.ai/components/hc-05/arduino-uno
#include <SoftwareSerial.h>

SoftwareSerial bt(10, 11);   // RX, TX: the module's TXD, RXD (through the divider)
const int STATE_PIN = 7;

void setup() {
  Serial.begin(9600);
  bt.begin(9600);            // the HC-05's data-mode speed out of the box
  pinMode(LED_BUILTIN, OUTPUT);
  pinMode(STATE_PIN, INPUT);
  Serial.println("Pair with HC-05 (PIN 1234) and send ON or OFF");
}

String line;
uint32_t last = 0;

void loop() {
  while (bt.available()) {
    char c = bt.read();
    if (c == '\n' || c == '\r') {
      if (line == "ON")  { digitalWrite(LED_BUILTIN, HIGH); bt.println("LED on"); }
      if (line == "OFF") { digitalWrite(LED_BUILTIN, LOW);  bt.println("LED off"); }
      if (line.length()) Serial.println("phone: " + line);
      line = "";
    } else {
      line += c;
    }
  }
  if (millis() - last > 2000 && digitalRead(STATE_PIN)) {
    last = millis();
    bt.print("uptime ");
    bt.println(millis() / 1000);
  }
}