Measuring Distance with an SR04 Ultrasonic Rangefinder

These are widely available on eBay. I sell them here.

Sample Routine - PICAXE

#picaxe 20X2
#terminal 9600

' SR04.Bas (PICAXE-20X2)
'
' Illustrates an interface with the SR04

' Launches pulse using PulsOut Command and measures the width of the Echo
' pulse.
'
' PICAXE-20X2                      SR04
'
' Trig (B.0)  --------------------- Trigger
' Echo (B.1) <--------------------- Echo
'
'                       GRD ------- GRD
'                       +5 VDC ---- 5VDC
'
' copyright, Peter Anderson, Baltimore, MD, Nov 9, '10

Symbol Tm = W0
Symbol RInches_10 = W1
  
   dirsB = %00000001
   low B.0

   Pause 1000			' allow system to settle when booting

   Do
      PulsOut B.0, 1	' sonar output, 10 us
      PulsIn B.1, 1, Tm
      If Tm = 0 Then
          SerTxD("OverRange", CR, LF)
		  
	  Else	  
          RInches_10 = 6 * Tm /10
          RInches_10 = 7 * Tm / 100 + RInches_10
          RInches_10 = 8 * Tm / 1000 + RInches_10
          SerTxD (#Tm, "  ", #RInches_10, CR, LF)
      Endif
      Pause 1000

   Loop

Sample Routine - Arduino

// SR04 Ultrasonic Ranging Module HC SR04 - Arduino
//
// SC SR04 is available at eBay or http://www.satistronics.com in China
//
// copyright, Peter Anderson, Baltimore, MD, Nov 3, '10

void display_q(long x);
long microsecondsToInches_10(long d_microseconds);
long InchesToCentimeters_10(long inches_10);

#define trigPin 13
#define echoPin 12

void setup()
{
   pinMode(trigPin, OUTPUT);
   digitalWrite(trigPin, LOW);
   pinMode(echoPin, INPUT);
   Serial.begin(9600);
   delay(2000);
}

void loop()
{

  long duration, inches_10, cm_10;

  digitalWrite(trigPin, HIGH); // output a pulse on trigPin
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);


  duration = pulseIn(echoPin, HIGH);


  inches_10 = microsecondsToInches_10(duration);
  cm_10 = InchesToCentimeters_10(inches_10);

  display_q(inches_10);
  Serial.print(" in, ");
  display_q(cm_10);
  Serial.print(" cm");
  Serial.println();
  delay(1000);

}

long microsecondsToInches_10(long d_microseconds)
{
  long inches_10;
  // 1130 feet / second = 0.1356 tenths of inches / us
  // dividing by 2 for the round trip;
  //
  // inches_10 = 0.0678 * d_microseconds

  inches_10 = 6 * d_microseconds / 100
            + 7 * d_microseconds / 1000
            + 8 * d_microseconds / 1000;
  return inches_10;
}

long InchesToCentimeters_10(long inches_10)
{
  long cm_10;

  cm_10 = 2 * inches_10 + 5 * inches_10 / 10 + 4 * inches_10 / 100; // cm = 2.54 * in
  return cm_10;
}

void display_q(long x)
{
   int whole, fract;
   whole = x / 10;
   fract = x % 10;
   Serial.print(whole, DEC);
   Serial.print(".");
   Serial.print(fract, DEC);
}