A simple Tachometer written for the Arduino. Uses a 'photo interruptor' on pin 3 as the input. The photo interruptor used was: https://www.sparkfun.com/products/9299 In combination with this breakout board: https://www.sparkfun.com/products/9322

tachometer.c

#include <SoftwareSerial.h>
#include <serLCD.h>

int lcdTxPin = 2;
int tachPin = 3;

volatile int pulseCount = 0;
volatile unsigned long samplePeriodStart = 0;
volatile unsigned long pulsePeriod = 0;
unsigned long rpm = 0;

int pulsesPerRevolution = 50;
int pulseSampleSize = 20;
int minRpm = 10;

serLCD lcd(lcdTxPin);

void setup(){
  lcd.clear();
  lcd.setBrightness(30);
  pinMode(tachPin, INPUT);
  attachInterrupt(1, registerTachPulse, FALLING);
  
  samplePeriodStart = micros();
}

void loop(){
  if(pulsePeriod > 0){
    rpm = 60000000 / pulsePeriod;
    rpm = rpm / pulsesPerRevolution;
  }
  
  if((micros() - samplePeriodStart) > ((60000000 / (minRpm * pulsesPerRevolution)) * pulseSampleSize)){
    rpm = 0;
  }
  
  lcd.clear();
  lcd.print("RPM: ");
  lcd.print(rpm);
  delay(100);
}

void registerTachPulse(){
  pulseCount++;
  
  if(pulseCount == pulseSampleSize){
    pulsePeriod = micros() - samplePeriodStart;
    pulsePeriod = pulsePeriod / pulseSampleSize;
    pulseCount = 0;
    samplePeriodStart = micros();  
  }
}