Measure RPM from a piece of tape spinning on a fast thing.

rpm_test.ino

// These aren't special, just use ints.
const int IR = 4;
const int PHOTO = 2;
// These are coming from the "micros()" function, so that dictates an unsigned long
volatile unsigned long rpm_value;
volatile unsigned long prev_rpm_time = 0;
volatile unsigned long current_rpm_time = 0;

float rpmmath;
float smooth_rpm = 0.0; // Be sure you initialize this.

void setup() {

    // Power the LED
    pinMode(IR, OUTPUT);
    digitalWrite(IR, HIGH);

    pinMode(PHOTO, INPUT_PULLUP);

    attachInterrupt(digitalPinToInterrupt(PHOTO), rpm_falling, FALLING);

    Serial.begin(9600);
}

const float MICROS_PER_MINUTE = 60.0 * 1000 * 1000;
const float ALPHA = 0.10;

void loop() {
    rpmmath = MICROS_PER_MINUTE / rpm_value;
    smooth_rpm += ALPHA * (rpmmath - smooth_rpm);
    Serial.print("RPM In = ");
    Serial.print(smooth_rpm);
    Serial.print(" raw = ");
    Serial.println(rpmmath);
    delay(200);
}

void rpm_falling() {

    // Store the time of the rising.
    current_rpm_time = micros();

    // Compute the delta time.
    rpm_value = current_rpm_time - prev_rpm_time;

    // Store the time of the rising for the next time.
    prev_rpm_time = current_rpm_time;

    // Now, add the interrupt.
    attachInterrupt(digitalPinToInterrupt(PHOTO), rpm_falling, FALLING);
}