/* Ultrasonic distance sensor example - Better Version

   Paul Carpenter, PC Services
   26-Jan-2017

   Update 4-Feb-2017 PC make variables more obvious
   Update 13-May-2017 PC add half bit rounding to distance calculation

   Uses HC-SR04 four pin ultrasonic sensor to continuously output distances
   found when in range (every second).

   Results WHEN valid output onto serial port at 115,200 baud

   Both LEDs ON when program starts and any other time for no start of echo
   received (probably bad wiring or faulty unit) on start up also serial port
   error string
    
   Lights RED Led when No Reading received or out of range
   Lights Yellow LED when too close (less than 5 cm)
   LEDs are connected via 330R or similar resistor other end of LED to GND

   Works on principle sound travels at 343.2 m/s in dry STILL air at 20 deg C

   So time per cm is 29.137529 micro seconds /cm

   For round trip (there and back)  58.275058 micro seconds /cm

   In order to reduce code size and execution time values are kept as integers
   and factor of 58 to calculate distances
*/

/* Pin definitions */
#define echopin     21
#define trigpin     22
//#define redled      6
//#define yellowled   7

/* time between readings in ms
   On Arduino Mega readings belown 20ms interval could 
   have residual echo issues */
#define INTERVAL    30

/* Scale factor round trip micro seconds per cm */
#define SCALE_CM    58
#define SCALE_CM_ROUND  (SCALE_CM/2)

/* Timeout for distance sensing rather than 1 second in us */
#define MAX_ECHO    300000
#define MIN_ECHO    (3 * SCALE_CM)

/* Timeout for start of ECHO pulse being received in us */
#define MAX_START_ECHO  1000

/* Limits for application ranges in cm */
#define MIN_RANGE   4 //era 4
#define MAX_RANGE   500 // era 500

#define MAX_ERROR 10

/* calculated distance in centimetres */
unsigned long distance;
unsigned int echotime;
unsigned long next_time, new_time;


/* Distance sensor function to get echo time
   Note most ultrasonic distance sensors are only usable beyond 3 cm
   So usable range is 3 * SCALE_CM to MAX_ECHO, therefore anything below
   3 * SCALE_CM should be treated as error

   Returns echo time in microseconds
        Maximum MAX_ECHO
        Minimum 3 * SCALE_CM    (minimum usable)
   error codes
        Error   2   Echo HIGH before start
        Error   1   Echo did not start
        Error   0   NO ECHO (Timeout)

   Timeout for measurements set by MAX_ECHO
*/
unsigned long GetEchoTime( )
{
unsigned long start_time;
unsigned long end_time;

/* check Echo if high return error */
if( digitalRead( echopin ) )
  return( 2 );
  
/* note start time */
start_time = micros( );

/* send the trigger pulse */
digitalWrite( trigpin, HIGH );
delayMicroseconds( 10 );
digitalWrite( trigpin, LOW );

/* Set timeout for start of echo pulse */
end_time = start_time + MAX_START_ECHO;

/* check ECHO pin goes high within MAX_START_ECHO
   if not return error of 1  */
while( !digitalRead( echopin ) )
   if( micros( ) > end_time )
     return( 1 );

/* Check for Length of echo occurred or timeout */
start_time = micros( );
end_time = start_time + MAX_ECHO;
while( digitalRead( echopin ) )
   if( micros( ) > end_time )
     break;
end_time = micros( );
     
/* Return time or timeout   */
return( ( start_time < end_time ) ? end_time - start_time: 0 );
}


void setup( )
{
/* set time from reset */
next_time = INTERVAL;

Serial.begin( 115200 );

/* Configure pins and ensure trigger is OFF */
pinMode( trigpin, OUTPUT );
digitalWrite( trigpin, LOW );
pinMode( echopin, INPUT );

/* Configure LED drive and both LEDs On at start up */
//pinMode( redled, OUTPUT );
//pinMode( yellowled, OUTPUT );
//digitalWrite( redled, HIGH );
//digitalWrite( yellowled, HIGH );

/* Send signon message */
Serial.println( F( "PC Services - Better Range test" ) );

/* Do test reading to check if unit connected */
distance = GetEchoTime( );
if( distance > 0 && distance <= 10 )
  {
  Serial.println( F( "No unit found - Error = " ) );
  Serial.println( distance );
  }
}


void loop( )
{
new_time = millis( );           /* check if to run this time */
if( new_time >= next_time )
  {
  /* Turn LEDs Off */
//  digitalWrite( redled, LOW );
 // digitalWrite( yellowled, LOW );

  /* Calculate distance */
  echotime = GetEchoTime( );
  /* only scale valid readings 0 is timeout or 1 is no echo
     realistically minimum accurate or physical range is 3cm */
  if( echotime > MAX_ERROR )
    {
    // Valid number covert to cm
    distance = echotime;
    distance += SCALE_CM_ROUND;  // add in half bit rounding  
    distance /= SCALE_CM;
    }

  /* catch errors first */
  if( echotime <= MAX_ERROR || distance > MAX_RANGE )
    {
  //  digitalWrite( redled, HIGH );               // Range error too large
    if( echotime > 0 && echotime <= MAX_ERROR )
   //   digitalWrite( yellowled, HIGH ); // Light 2nd LED error
   Serial.println("MAX");
    }
  else
    if( distance < MIN_RANGE )
  //    digitalWrite( yellowled, HIGH ); // Range too close
     Serial.println("MIN");
    else
      Serial.println( int( distance ) );        // In range output distance

  next_time = new_time + INTERVAL;       // save next time to run
  }
}

which parameters I need to change to go far? (more than 5 mt)