// https://create.arduino.cc/projecthub/madhav133/light-gate-using-arduino-de846c?ref=platform&ref_id=424_trending___&offset=21

int timer = 0; //For the counting of the breaked time of light.
bool checking = false; //Checks if the the object is being monitored.
float speed = 0; //The final speed of the object.
float lengthOfBreaker = 0.04; //The length of the breaker used in the object.

void setup() {
  pinMode(8, OUTPUT); //An LED for showing the 'checking' variable.
  Serial.begin(9600); //Begin the serial communication.
}

void loop() {
  if (analogRead(A0) < 900) { //Checks if the light intensity is below 900. That means the light is breaked.
    checking = true;
    digitalWrite(8, checking);
    timer = 0;
    while (analogRead(A0) <= 900) { //Increment the timer until the object is gone.
      timer++;
      delay(1); //A fixed time of 1 millisecond is given for counting.
    }
    if (timer > 0) { //Check if the timer value is greater that 0 milliseconds.
      checking = false; //Set the completed variable to true since the checking is completed.
      digitalWrite(8, checking);
      float timeInSec = (float(timer) / float(1000)); //Convert the time in milliseconds to seconds. Output is a float value.
      speed = lengthOfBreaker / timeInSec; //Calculates the speed of the object by using this formula: Speed = Distance / Time
      Serial.print("Object Instantaneous Speed: "); //
      Serial.print(speed);                          //  Print the object speed
      Serial.print(" m/s");                         //  in the Serial monitor.
      Serial.print("\n");                           //
      timeInSec = 0; // Reset the seconds value.
    }
    speed = 0;           //
    checking = false;    //  Reset other values too.
    timer = 0;           //
  }
}