Sensor used in this project is MOC 7811 from Motorola. This is a slotted coupler/interrupter device. In each revolution as the beam is interrupted, the sensor generates a positive pulse. Microcontroller will execute interrupt routine, When a pulse occurs. Interrupt routine measures the total time taken for one revolution, divides it by 120, and stores the result into another timer, Configured in auto-reload mode. Now, the timer overflows after tiny intervals of time. Each time it overflows, next stored value is called from the lookup table, and displayed
