Well Bobbo_ZA, you must have quite a few magnets attached to the main shaft if you are expecting to get pulses at a 20 Millisecond interval. I would start with a slotted disk with about 8 or possibly 16 slots on it. Pass it thru an opto interrupter so as not to get any switch bounce as well as not have any drag from the magnets. Then look at the period of the waveform. that should give you good resolution on the LOW end of the spectrum as well as good resolution on the HIGH end of the spectrum. The math should be simple. You should be able to get 1 to 10 hz. update rate for the lcd display.

Dave Purola,
N8NTA