Every spinning object can be described with RPM parameter which means revolutions per minute. And there are many ways of measuring it but basically all methods are classified in two groups – direct contact and contact-less. More popular are contact-less methods of capturing spin period. Most popular are magnetic, optical, capacitive. Each of them are used depending on surrounding environment and materials. Pyroelectro suggests to enter this world using optical method of measuring RPM. He uses an IR LED pair (transmitter and receiver) to capture revolution data.
Setup is based on Arduino UNO which takes care of sending signals to IR diode and reading from receiver. Also it displays data on 16×2 LCD. Reading data is simple – spinning fan blades interrupt IR beam that are captured by Arduino. Then algorithm calculates RPM knowing time measured for one rotation. Method can be used to measure fan speed used in your project or other devices that spin and can interrupt optical beam.