hc05

//master
#include <SoftwareSerial.h>

SoftwareSerial BTserial(6, 7); // RX | TX

#define __BAUD 38400

void setup(){
  BTserial.begin(__BAUD);
  delay(1000);
  Serial.println("ready...");
}

void loop() {
  BTserial.write('a');
  delay(1000);
  BTserial.write('s');
  delay(1000);

}

//slave
#include <SoftwareSerial.h>

SoftwareSerial BTserial(6, 7); // RX | TX

#define __BAUD 38400

void setup(){
  Serial.begin(__BAUD);
  BTserial.begin(__BAUD);
  delay(1000);
  Serial.println("ready...");
  pinMode(10, OUTPUT);
}

void loop() {
  if (BTserial.available()) {
    Serial.write("comunicazione");

    char c = BTserial.read();
    Serial.write(c);
    if (c == 'a') {
      digitalWrite(10, HIGH);
    }
    if (c == 's') {
      digitalWrite(10, LOW);
    }
  }
}