#include <SoftwareSerial.h>;
SoftwareSerial mySerial(PB3, PB4);
void setup() {
  // put your setup code here, to run once:
  mySerial.begin(9600);
  pinMode (PB0, OUTPUT);
  digitalWrite(PB0, 1);
}

void loop() {
  // put your main code here, to run repeatedly:
  if ( mySerial.available() > 0) {
    char d;
    char a = 'b';
    d = mySerial.read();
    if (d == a) {
      mySerial.println('c');
      digitalWrite(PB0, 0);
      delay(1);
      digitalWrite(PB0, 1);
    }
  }
}