Chris at PyroElectro built this simple Arduin-controlled IR pair that can be used to measure the RPMs of a CPU fan.
A tachometer is a useful tool for counting the RPM (rotations per minute) of a wheel or basically anything that spins. The easiest way to build a tachometer is using a transmitter and receiver. When the link between them is broken, you know that something is spinning and can execute some code that calculates the current RPM of whatever is spinning to break the transmitter/receiver link.
In this article we will explore how to use an IR transmitter and receiver break-beam pair similar to the PIC Tachometer project I built a few months ago, but because of popular demand, the Arduino system will be used for all the processing and break-beam interruption counting. The end result will be a 16×2 LCD displaying the RPM of some computer fans.
[via Embedded Projects]