Supplies

Arduino Leonardo
breadboard
jumper wires
ultrasonic sensor
servo motor
DC motor
plastic fan
container
cork

Step 1: INTRO

Instead of blowing with no purpose, this fan blows when it senses something or someone before blowing

Step 2: STEPS:

gather the materials
servo motor to Dpin #3
ultrasonic sensor: Trig to Apin #5, Echo to Apin #4
connect L298n to Arduino Board
connect DC motor to L298n
reset the servo motor to 0 degrees
glue the cork to the servo motor
glue the ultrasonic to the cork facing 0 degrees
glue the DC motor on to the cork, also pointing at 0 degrees
put the following code in:

//to use servo motor
#include <Servo.h>
//ultrasonic pins
int Echo = A4;
int Trig = A5;
//setup servo motor to "Servo1"
Servo Servo1;
//servo to number 3
int servoPin = 3;
//sensor to number 2
int Sensor_pin = 2;
//set angle to 0 degrees
int angle = 0;
int angle_step = 6;
int Distance_test()//set up the ultrasonic sensor
{
  digitalWrite(Trig, LOW);
  delayMicroseconds(2);
  digitalWrite(Trig, HIGH);
  delayMicroseconds(20);
  digitalWrite(Trig, LOW);
  float Fdistance = pulseIn(Echo, HIGH);
  Fdistance = Fdistance / 29 / 2;
  return (int)Fdistance;
}
void setup() {
  pinMode(Sensor_pin, INPUT);//set up servo motor
  Servo1.attach(servoPin);
  Servo1.write(0);
  delay(2000);
  pinMode(Echo, INPUT);
  pinMode(Trig, OUTPUT);
  Serial.begin(9600);
  pinMode(8, OUTPUT);
  pinMode(7, OUTPUT);
  pinMode(6, OUTPUT);
  Serial.begin(9600);
  angle = 0;
  Servo1.write(angle);
}
void loop()//non stop loop
{
  Servo1.write(0);//reset servo
  Servo1.write(10);//servo to 10 degrees
  delay(200);//delay(wait) 200 millisecends
  Servo1.write(0);//reset servo
  delay(500);//delay(wait) 500 millisecends
  angle = 0;
  for (int i = 0; i <= 180; i += angle_step)
  {
    Servo1.write(i);
    delay(200);
    int dis = Distance_test();
    Serial.println(dis);
    if (dis < 20)
    {
      angle = i;
      Serial.println(angle);
      break;
    }
  }
  //spin motor
  if (angle > 0)
  {
    Servo1.write(angle);
    digitalWrite(7, 1);
    digitalWrite(6, 0);
    delay(10000);
    digitalWrite(7, 0);
    delay(2000);
  }
  // go back home
  for (int i = angle; i >= 0; i -= 5)
  {
    Servo1.write(i);
    delay(100);
  }
  delay(5000);
  angle = 0;
}

Step 3: TRIAL and ERROR

Originally, I was going to use the passive infrared sensor(PIR) to control the DC motor, but after using it, I found out a huge problem...The PIR was a 180 degrees sensor which can only be used digitally instead of analogically. Which means that, instead of telling you how far or where the object is, it can only tell you whether if there is movement going on within its 180 degrees radius. After hundreds and thousands of times of experimenting, I finally decided that the PIR was not effective enough for this project, so I came to the conclusion that the ultrasonic sensor would be better to use.