I am using an HC-SRO4 ultrasonic sensor for a wall-avoidance, maze-solving robot. However, I have recently run into an issue where I am getting a whirring noise from the sensor because it is taking measurements way too often.

This is the code I have right now:

const int serialPeriod = 250;       // only print to the serial console every 1/4 second
unsigned long timeSerialDelay = 0;

const int loopPeriod = 20;          // a period of 20ms = a frequency of 50Hz
unsigned long timeLoopDelay   = 0;

// specify the trig & echo pins used for the ultrasonic sensors
const int ultrasonic2TrigPin = 8;
const int ultrasonic2EchoPin = 9;

int ultrasonic2Distance;
int ultrasonic2Duration;

void setup()

    // ultrasonic sensor pin configurations
    pinMode(ultrasonic2TrigPin, OUTPUT);
    pinMode(ultrasonic2EchoPin, INPUT);

void loop()
    debugOutput(); // prints debugging messages to the serial console

    if(millis() - timeLoopDelay >= loopPeriod)
        readUltrasonicSensors(); // read and store the measured distances

        timeLoopDelay = millis();

void readUltrasonicSensors()
    // ultrasonic 2
    digitalWrite(ultrasonic2TrigPin, HIGH);
    delayMicroseconds(10);                  // must keep the trig pin high for at least 10us
    digitalWrite(ultrasonic2TrigPin, LOW);

    ultrasonic2Duration = pulseIn(ultrasonic2EchoPin, HIGH);
    ultrasonic2Distance = (ultrasonic2Duration/2)/29;

void debugOutput()
    Serial.print("timeSerialDelay: ");

    if((millis() - timeSerialDelay) > serialPeriod)
        Serial.print("ultrasonic2Distance: ");
        Serial.print("cm: ");

        timeSerialDelay = millis();

and my wiring:

breadboard wiring

Link to product

Does anyone know why the scanning is happening constantly? It should only be happening every 1/4 of a second, not constantly. The weird thing is that this wasn't happening earlier before I started using the cable.

I believe it has something to do with the 8.5" extension cable I have (needed because I can't mount three sensors, one facing to each side and one in front, on a single breadboard). This is because when I mounted the sensor directly onto the breadboard, the delay was correct and the whirring was not present. However, as I just mentioned, I need the extension cable to be able to have the three sensors.

Does anyone have a solution for this?


  • 1
    If loopPeriod = 20 then it should be sampling at 50Hz, like the comment says – TheDoctor Mar 23 '14 at 15:49
  • 1
    Change loopPeriod to 250 for a quarter second sample rate – TheDoctor Mar 23 '14 at 15:49
  • Okay, did that. It did slow down the rate, but I could still hear a faint clicking which got louder as it got closer to an object and it will pause if the value reaches 0cm. Why is this? – RPiAwesomeness Mar 23 '14 at 16:02
  • Most ultrasonic sensors emit a bit of sound in the audible spectrum. I have noticed this a lot in the Maxbotics EZ line. – TheDoctor Mar 23 '14 at 16:07
  • So, it is just something I have to put up with? Okay. I suppose I can survive that. But why is the scanning pausing (and dropping to 1/4 second rates) when it reaches 0cm? Thanks for the quick response! – RPiAwesomeness Mar 23 '14 at 16:09

If you want to sample at 4Hz (4 times per second), you need to change your headers accordingly:

const int serialPeriod = 250;       // only print to the serial console every 1/4 second
unsigned long timeSerialDelay = 0;

const int loopPeriod = 250;          // a period of 250ms = a frequency of 4Hz
unsigned long timeLoopDelay   = 0;

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