Arduino bluetooth car plus APK

Arduino Code-

int m1 = 12;
int m2 = 11;
int b1 = 10;
int b2 = 9;
int left = 8;
int right = 7;
int up = 6;
int down = 5;
int light= 13;

String readString;

void setup() {
Serial.begin(9600);
pinMode(m1, OUTPUT);
pinMode(m2, OUTPUT);
pinMode(b1, OUTPUT);
pinMode(b2, OUTPUT);
pinMode(left, OUTPUT);
pinMode(right, OUTPUT);
pinMode(up, OUTPUT);
pinMode(down, OUTPUT);
pinMode(light, OUTPUT);
}

void loop() {
while (Serial.available()) {
delay(3);
char c = Serial.read();
readString += c;
}
if (readString.length() >0) {
Serial.println(readString);
if (readString == "f")
{
digitalWrite(m1, HIGH);
digitalWrite(m2, HIGH);
}
if (readString == "b")
{
digitalWrite(b1, HIGH);
digitalWrite(b2, HIGH);
}
if (readString == "l")
{
digitalWrite(m1, HIGH);
digitalWrite(b2, HIGH);
}
if (readString == "r")
{
digitalWrite(m2, HIGH);
digitalWrite(b1, HIGH);
}
if (readString == "s")
{
digitalWrite(m1, LOW);
digitalWrite(m2, LOW);
digitalWrite(b1, LOW);
digitalWrite(b2, LOW);
digitalWrite(left, LOW);
digitalWrite(right, LOW);
digitalWrite(up, LOW);
digitalWrite(down, LOW);
}
if (readString == "ledon")
{
digitalWrite(light, HIGH);
}
if (readString == "ledoff")
{
digitalWrite(light, LOW);
}
if (readString == "left")
{
digitalWrite(left, HIGH);
}
if (readString == "right")
{
digitalWrite(right, HIGH);
}
if (readString == "up")
{
digitalWrite(up, HIGH);
}
if (readString == "down")
{
digitalWrite(down, HIGH);
}
readString="";
}
}