#include<SoftwareSerial.h>

int rxPin = 3;
int txPin = 4;
char chr;
SoftwareSerial myserial(rxPin, txPin);
void setup()
{
  pinMode(0, OUTPUT);
  myserial.begin(9600);
}

void loop()
{
  char chr = myserial.read();
  if (chr == 'd') {
    for (int i = 0; i < 50; ++i) {
      digitalWrite(0, HIGH);
      delayMicroseconds(2000);
      digitalWrite(0, LOW);
      delayMicroseconds(18000);
    }
  }
  else if(chr == 'D') {
    for (int i = 0; i < 50; ++i) {
      digitalWrite(0, HIGH);
      delayMicroseconds(1000);
      digitalWrite(0, LOW);
      delayMicroseconds(19000);
    }
  }
}