Bike Speedometer Using Reed Switch

//This code was made by Alex Cockman with inspiration from the theory from the websites linked below.
//Adruino.com's link: https://www.arduino.cc/en/Tutorial/LibraryExamples/HelloWorld
//danielleamya's linked: https://forum.arduino.cc/t/bike-speedometer-using-reed-switch/514581
//Arduino.com's linked: https://www.arduino.cc/reference/en/language/functions/external-interrupts/attachinterrupt/
//Adminastrator from Eletronics Hub eletronic - set up and understanding of reed switches: https://www.electronicshub.org/how-to-interface-reed-switch-with-arduino/#:~:text=Connect%20one%20end%20of%20the,of%20a%20Pull%2Dup%20Resistor


#include <LiquidCrystal.h>

int reedSwitch = 2;  //Declaring pin for reed switch to be used. Pin 2 or 3 must be used for the reed switch on the Uno
int radius = 12.5;   //hard coding in the radius of the wheel for future conversion into circumference 
int circumference;   //declaring the circumference variable
int totaltime;       //declaring the total time until the next trip on the reed switch variable
int Speed;           //declaring the Speed variable
int timer;           //declaring the timer to calculate the total time variable

const int rs = 12, en = 11, d4 = 5, d5 = 4, d6 = 3, d7 = 6;   // Declaring pins for LCD Display
LiquidCrystal lcd(rs, en, d4, d5, d6, d7);

void isr()           
 {
      timer++;                        // Begin the timer and start counting
      delayMicroseconds(9600);        // Delay to start after a 0.0096 seconds
 }
void setup(){
  Serial.begin(9600);                 // Start serial monitor after a few seconds. Mainly for testing code to get it to work.

  lcd.begin(16,2);                    // Declare the scope that the LCD screen displays on
  lcd.setCursor(6, 0);                // Set Speed in the middle of the lCD display
  lcd.print("Speed");                 // Print on LCD screen what is inbetween the quotation marks
   
   attachInterrupt(digitalPinToInterrupt(2), isr, RISING);    //Begins the count of seconds when the reed switch in pin 2 is set off

   pinMode(reedSwitch, INPUT_PULLUP);                         // Sets the reed switch to be an input
   timer = 0;                                                 // Sets timer to 0 seconds
   Speed = 0;                                                 // sets the speed to 0 seconds
   totaltime = 0;                                             // sets the total time to start off at 0
   circumference = 6.28 * radius;                             // calculated the circumference
}
void loop() {

   if (millis() - totaltime >= 1000) {                                      // Gets varibales to go into the loop everytime the millis() - the total time elapsed is less than 1ms
                                                                            // So unless I become superman then no human will be able to achieve this. 
        detachInterrupt(digitalPinToInterrupt(2));                          // Stops the timer from the reading from pin 2 aka the reed switch

        Speed = ((8.181818182*circumference)/(millis() - totaltime)*timer); // Calculates the speed of the bike. The 8.18 number is converting the time in seconds into hours and inches into feet.

        timer = 0;                                                          // Makes the timer go back to zero
        totaltime = millis();                                               // Makes the total time the new time
        attachInterrupt(digitalPinToInterrupt(2), isr, RISING);             // Make the ISR loop attach back

        lcd.setCursor(8,1);                                                 // Sets cursor to the bottom left 
        lcd.print(Speed);                                                   // Prints the speed to the LCD display
        Serial.print("Speed");                                              // Prints "Speed" to serial monitor. Mainly for troubleshooting.  
        Serial.print(Speed);                                                // Prints the speed to the serial monitor. Mainly for troubleshooting.

   }
}