Ibrahim writes –

how to build a contact -less tachometer (device used to count the revolutions per minute of a rotating shaft) using a 8051 micro controller and a proximity sensor.

As the name implies, what makes this device special, is that it can very accurately measure the rotational speed of a shaft without even touching it. This is very interesting when making direct contact with the rotating shaft is not an option or will reduce the velocity of the shaft, giving faulty readings.

HOW TO – Make a 99000 RPM digital contact-less tachometer – Link.