HC-SR04 Ultrasonic sensor on Uno won't read beyond 6-7 cm

Other Hardware Sensors
1

Hi - newbie here. I have set up my SainSmart HC-SR04 from Amazon using the NewPing library and the simple example sketch that came with it (thanks Tim Eckel!). I am having trouble with the output, though. At distances greater than 6cm or 7cm the serial monitor shows a steady 6cm. If I move an object 6cm or closer it will read the distance accurately. As I point the sensor at different spots around my workspace (e.g., the wall and ceiling, the output occasionally (once or twice a minute maybe?) will spike to what seems to be a correct distance, but only for one or two echos.
Anyone have a suggestion as to what might be wrong and how I can get it running right? Thanks!

2

What is the supply voltage to the HCSR04? Post your code, please.

3

Thanks for the quick response!

VCC is connected to the 5v on the Uno. The Uno is powered via USB - only the HC-SR04 is hooked up.

Code is as follows (from http://playground.arduino.cc/Code/NewPing):

// ---------------------------------------------------------------------------
// Example NewPing library sketch that does a ping about 20 times per second.
// ---------------------------------------------------------------------------

#include <NewPing.h>

#define TRIGGER_PIN 12 // Arduino pin tied to trigger pin on the ultrasonic sensor.
#define ECHO_PIN 11 // Arduino pin tied to echo pin on the ultrasonic sensor.
#define MAX_DISTANCE 200 // Maximum distance we want to ping for (in centimeters). Maximum sensor distance is rated at 400-500cm.

NewPing sonar(TRIGGER_PIN, ECHO_PIN, MAX_DISTANCE); // NewPing setup of pins and maximum distance.

void setup() {
Serial.begin(115200); // Open serial monitor at 115200 baud to see ping results.
}

void loop() {
delay(50); // Wait 50ms between pings (about 20 pings/sec). 29ms should be the shortest delay between pings.
unsigned int uS = sonar.ping(); // Send ping, get ping time in microseconds (uS).
Serial.print("Ping: ");
Serial.print(uS / US_ROUNDTRIP_CM); // Convert ping time to distance in cm and print result (0 = outside set distance range)
Serial.println("cm");
}