This economical sensor HCSR04 provides 2cm to 400cm of non-contact measurement functionality with a ranging accuracy that can reach up to 3mm. Each HC-SR04 module includes an ultrasonic transmitter, a receiver and a control circuit.
There are only four pins that you need to worry about on the HC-SR04: VCC (Power), Trig (Trigger), Echo (Receive), and GND (Ground).

Here a possible connection schema:

Also in this case we can found an interesting library written for Arduino able to drive and take measurement from this sensor. The library is the newPing which can be downloaded in our Download page)
If you have problems installing libraries into your arduino framweork you can read my tutorial

Let's startincluding the ping library:

            #include 
          

Using this library you will need just to declare a sonar object passing the pin parameters into the constructor

            NewPing sonar(TRIGGER_PIN, ECHO_PIN, MaxDistance);
          

Where TRIGGER_PIN and ECHO_PIN are the two pins you defined (and connected) for the related function. Be carefull to not invert those two pins. One of the most common errors is to invert them so to not read any value.
MaxDistance is the distance you want to use as reference point as maximum that can be encountered. This value is used to tune the sonic sensor and the algorithm.
In order to read constantly this value as a dynamic parameter I installed on the left side of the multidimensional box a potentiometer. Rotating the potentiometer I can vary the max distance parameter.
This is also the reason why it is a variable and not a costant in the code you saw.
If you want to learn how to connect and use a potentiometer with Arduino you can read this simple tutorial

You can retrieve the distance measured in cm using the function:

            sprintf(Centimeter, "%lu", sonar.ping_cm());
          

It returns an unsigned long that you can use to display something in your debug area or, like in my case, in the oled screen NewPing library supports also some other functions. Here attached a list of all methods available:

- sonar.ping(); - Send a ping, returns the echo time in microseconds or 0 (zero) if no ping echo within set distance limit
- sonar.ping_in(); - Send a ping, returns the distance in inches or 0 (zero) if no ping echo within set distance limit
- sonar.ping_cm(); - Send a ping, returns the distance in centimeters or 0 (zero) if no ping echo within set distance limit
- sonar.ping_median(iterations); - Do multiple pings (default=5), discard out of range pings and return median in microseconds
- sonar.convert_in(echoTime); - Converts microseconds to distance in inches
- sonar.convert_cm(echoTime); - Converts microseconds to distance in centimeters
- sonar.ping_timer(function); - Send a ping and call function to test if ping is complete.
- sonar.check_timer(); - Check if ping has returned within the set distance limit.
- timer_us(frequency, function); - Call function every frequency microseconds.
- timer_ms(frequency, function); - Call function every frequency milliseconds.
- timer_stop(); - Stop the timer.

In my specific case I decided to give in the oled a graphical representation of the distance measured by the sonic sensor.
For this reason I draw a view area and an y line proportional to the measure of the conical area. The proportion, is based on the max distance read from the potentiomenter.

In the picture, the max value=50 is the value in centimeters evaluated based on the potentiometer current value.
The screen you've seen is very easy to obtain using the adafruit graphcal library i described in the page I dedicated to the gps and to the oled functions
In order to display the right view on the panel (GPS, Accellerometer or distances) I build a draw function and this is the section related to the SONAR view.

else if (Display == DISPLAY_SONAR)  {
  int BarLong;
  
  //display.drawCircle((display.width() - WIDTH_SIGNAL_AREA) / 2, display.height() / 2, display.height() / 2-2, WHITE);
  display.drawLine((display.width() - WIDTH_SIGNAL_AREA) / 2 - 30, 0, (display.width() - WIDTH_SIGNAL_AREA) / 2, display.height(), WHITE);
  display.drawLine((display.width() - WIDTH_SIGNAL_AREA) / 2 + 30, 0, (display.width() - WIDTH_SIGNAL_AREA) / 2, display.height(), WHITE);

  Serial.print("Distanza:");
  Serial.println(atoi(stringGPS));
  BarLong = map(atoi(stringGPS), 0, MaxDistance/2, 0, display.height());
  Serial.print("BarLong:");
  Serial.println(BarLong);
  
  display.drawLine((display.width() - WIDTH_SIGNAL_AREA) / 2, display.height(), (display.width() - WIDTH_SIGNAL_AREA) / 2, display.height() - BarLong, Colour);
  display.setCursor(5, 5);
  display.print(stringGPS);
  display.setCursor((display.width() - WIDTH_SIGNAL_AREA) / 2 + 10, display.height() - 8);
  
  //Fix the Max Value
  display.setTextColor(WHITE);
  display.print("Max:");
  display.print(MaxDistance/2);
  display.setTextColor(Colour);
  
}
          
            

In this function you will see some parameters which depend on the dimension of your screen and on the dimension of the working area you decided to implement in your available dinamic screen (I mean the space available for non service objects like the blutooth signal presence indicator or like the gps signal indicator).