Tachometer is an instrument that measures the rotational speed of any shaft or disc in revolutions per minute (RPM) unit. One traditional method of finding the RPM of a rotating shaft is to hook a dynamo with the shaft, and the voltage induced by the dynamo is proportional to the RPM of the shaft. However, this technique may not be applicable every time as it requires a physical contact with the rotating shaft.
Here’s a project that describes a contact-less tachometer based on PIC16F628A microcontroller. The rotation of a disc is measured through a set of IR module that converts a rotation into a pulse. All you have to do is to stick a reflective surface (a piece of plain white paper) at one place on the shaft and bring the IR module facing to the shaft. This will reflect the incident IR beam. In every rotation, the reflective surface will once passes through the module and generate a pulse. These pulses will be counted by the PIC microcontroller, and displayed in to a multiplexed seven segment display. The firmware is written in C and is available for free.