With the new smaller microcontrollers and some simple circuitry, it's actually very easy to build your own digital tachometer. This tutorial will make use of an infraded diode and detector to build the digital tachometer/rpm counter. Below are some pictures of the final setup.
Purpose & Overview of this tutorial
This tutorial will build a working digital tachometer that keeps track of the RPMs (rotations per minute) of a computer fan. So it's really a sophisticated RPM counter module. An IR emitter diode will always be sendind a blanket signal to the IR detector. If a fan blade interrupts the signal the microcontroller will see the change and keep track of each change.
The total cost of parts for this tutorial is around $10 provided you already have a PIC programmer. The RPM counter will be limited to 1200 -> 3200 RPM because this is the computer fan specification for min and max speeds.