HC-SR04 ultrasonic range finder test code. Read the full blog post on www.teomaragakis.com/hardware/electronics/testing-the-hc-sr04-ultrasonic-range-sensor/

HC-SR04.ino

/*
                                      +-----+
         +----[PWR]-------------------| USB |--+
         |                            +-----+  |
         |         GND/RST2  [ ][ ]            |
         |       MOSI2/SCK2  [ ][ ]  A5/SCL[ ] |   C5 
         |          5V/MISO2 [ ][ ]  A4/SDA[ ] |   C4 
         |                             AREF[ ] |
         |                              GND[ ] |
         | [ ]N/C                    SCK/13[ ] |   B5
         | [ ]v.ref                 MISO/12[ ] |   .
         | [ ]RST                   MOSI/11[ ]~|   .
         | [ ]3V3    +---+               10[ ]~|   .
         | [X]5v     | A |                9[ ]~|   .
         | [X]GND   -| R |-               8[ ] |   B0
         | [ ]GND   -| D |-                    |
         | [ ]Vin   -| U |-               7[ ] |   D7
         |          -| I |-               6[ ]~|   .
         | [ ]A0    -| N |-               5[ ]~|   .
         | [ ]A1    -| O |-               4[ ] |   .
         | [ ]A2     +---+           INT1/3[X]~|   .
         | [ ]A3                     INT0/2[X] |   .
         | [ ]A4/SDA  RST SCK MISO     TX>1[ ] |   .
         | [ ]A5/SCL  [ ] [ ] [ ]      RX<0[ ] |   D0
         |            [ ] [ ] [ ]              |
         |  UNO_R3    GND MOSI 5V  ____________/
          \_______________________/
		  
		  http://busyducks.com/ascii-art-arduinos
*/
#define trig 2
#define echo 3

void setup() {
  Serial.begin (9600);
  pinMode(trig, OUTPUT);
  pinMode(echo, INPUT);
}

void loop() {
  float duration, distance;
  
  digitalWrite(trig, LOW);
  
  delayMicroseconds(2);
  digitalWrite(trig, HIGH);
  delayMicroseconds(10);
  digitalWrite(trig, LOW);
  
  duration = pulseIn(echo, HIGH);

  distance = duration / 58;
  Serial.print(distance);
  Serial.println(" cm");
  delay(100);
}