It was an analog rpm meter. I used a 555 in monostable mode. A comparator was placed at the output of hall sensor, so that the output of 12V would arrive only when the magnet came in a certain proximity of that hall sensor (the negative peak didn't trigger the 555 circuit). The hall sensor required a magnet to be placed on the shaft. So when the shaft rotates once, the 555 receives the 12V input for the corresponding period of time, and produces a very small unit step output of a time we pre-set. A capacitor at the output smoothed the voltage. An analog (or digital) voltmeter was calibrated and RPMs were read
. I also tried it in Multisim and it worked there as well.
The thing is that the IR LED + photodiode didn't have such a good response to a rotating fan above like 300 rpm or so (tried it by checking it through oscilloscope). So Hall sensor was a great alternative because it could read like 80000 rpms easily.