// NEEDS debouncing

int opto = 3;             // opto-interrupter
int rpm = 0;              // rev per min
float speed, dia = 0.021; // speed and diameter in meters
unsigned long dtime;      // previous "start time"
volatile int rotation;    // counter

void setup() {
  Serial.begin(115200);
  // pinMode(opto, INPUT_PULLUP); // xfpd using button tied high
  pinMode(opto, INPUT);
  // attachInterrupt(digitalPinToInterrupt (opto), ISR1, FALLING); // xfpd using button tied high
  attachInterrupt(digitalPinToInterrupt (opto), ISR1, RISING);
}

void loop() {
  if (millis() - dtime >= 1000) { // timeout
    dtime = millis();             // reset timer

    // calculate outside the ISR because calculating is not fast
    rpm = rotation * 60;          // rotations per second x 60 seconds per minute
    speed = (PI * dia) * (rpm);   // "linear circumferences" per "time measured"

    // print outside the ISR because printing is very slow
    Serial.print("Bogie Speed is ");
    Serial.print(speed);
    Serial.print(" m/s at ");
    Serial.print(rpm);
    Serial.print(" RPM over");
    Serial.print(rotation);
    Serial.print(" rotation");
     if (rotation != 1) Serial.print("s");
    Serial.println(".");

    rpm = speed = rotation = 0; // reset counters
  }
}

void ISR1() {
  rotation++; // count a rotation only, then calculate outside the ISR
}