#include <Arduino.h>
#include <BluetoothSerial.h>

#define TRIGGER_PIN 3
#define ECHO_PIN 4
#define SPEED_SOUND 34300 //speed in cm/s
#define timediff 100 //ms

BluetoothSerial SerialBT;

void setup() {
  Serial.begin(115200);
  Serial.println("Hello, ESP32!");
  SerialBT.begin("Hello ESP32_BT");
  pinMode(TRIGGER_PIN,OUTPUT);
  pinMode(ECHO_PIN,INPUT);
  digitalWrite(TRIGGER_PIN,LOW);
}

void loop() {
  char op;
  SerialBT.print("Enter M to measure: ");
  if (SerialBT.available()) {
    op = SerialBT.read();
  }
  SerialBT.println(op);
  if(op != 'M'){
    SerialBT.println("Not measuring!");
    delay(100);
  } else{
    digitalWrite(TRIGGER_PIN,HIGH);
    delayMicroseconds(10);
    digitalWrite(TRIGGER_PIN,LOW);
    long time1=pulseIn(ECHO_PIN,HIGH);
    float distance1=((time1/1e6)*SPEED_SOUND)/2;
    delay(timediff);
    digitalWrite(TRIGGER_PIN,HIGH);
    delayMicroseconds(10);
    digitalWrite(TRIGGER_PIN,LOW);
    long time2=pulseIn(ECHO_PIN,HIGH);
    float distance2=((time2/1e6)*SPEED_SOUND)/2;

    float ans=(distance2-distance1)/(timediff/1000);
    Serial.print("Speed Reading in cm/s: ");
    Serial.println(ans);
    SerialBT.print("Speed Reading in cm/s: ");
    SerialBT.println(ans);
    delay(2000);
  }
}