If you want to build a RPM meter project based on Microcontroller AVR? Check out this RPM Meter with rotational speed sensor KMI15/1. For this project, Tomi use AT90S2313 to store the program and control the sensor. As compiler he used Bascom AVR.
The KMI15/1 sensor detects rotational speed of ferrous gear wheels and reference marks. The sensor consists of a magnetoresistive sensor element, a signal conditioning integrated circuit in bipolar technology and a magnetized ferrite magnet. The frequency of the digital current output signal is proportional to the rotational speed of a gear wheel.
Program for MCU can be dividing to some stages: get information about pulses, convert Pulse IN value to frequency (Hz), and recalculate frequency value to PWM value.
Documentation project and source code