This is the Arduino code corresponding to my humidity project in the general project of the agriculture robotics, in this case my objective is measure the humidity of a plant and depending on the soil moisture or humidity a red LED will be switch on (low humidity)

/*
  HC-SR04 Ultrasonic Sensor Example.

  Turn the LED on when an object is within 100cm range.

  Copyright (C) 2021, Uri Shaked
*/

#define ECHO_PIN 2
#define TRIG_PIN 3

void setup() {
  Serial.begin(9600);
  pinMode(LED_BUILTIN, OUTPUT);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
}

float readDistanceCM() { //this part is for the speed 
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  int duration = pulseIn(ECHO_PIN, HIGH);
  return duration * 0.034 / 2; 
}

void loop() {
  float distance = readDistanceCM();
Serial.println(distance);
  bool isNearby = distance < 30; //this is to determinate the maximum distance the sensor reads
  digitalWrite(LED_BUILTIN, isNearby);

  Serial.print("Measured distance: ");
  Serial.println(readDistanceCM());

  delay(10); //delay is to determinate the time the light is on when it receives a sign
}