#define Echopin 26
      #define Triggerpin 25
      int distance;
      long duration;
      
      void setup() {
        // put your setup code here, to run once:
        pinMode(Echopin,INPUT);
        pinMode(Triggerpin,OUTPUT);
        Serial.begin(9600);
      }
      
      void loop() {
        // put your main code here, to run repeatedly:
        digitalWrite(Triggerpin,LOW);
        delay(20);
        digitalWrite(Triggerpin, HIGH);
        delayMicroseconds(100);
        digitalWrite(Triggerpin, LOW);
        
        duration = pulseIn(Echopin,HIGH);
        distance = ((duration*0.034)/2);
        Serial.println("distance = ");
        Serial.print(distance);
        Serial.println("cm");
      
      }
  
  //esp32:
  //press and hold the button of boot button
  //press and release the reset button
  //relase this boot button