There are also number of transducer technologies that permit RPM
measurement, contactless, handheld, non optical, like -
how to get pulses from optical sensor?
tell me working principal?
Imagine you have got an airplane modell and want to measure rpms of the propeller. The difference of light reflections can be the signal to count (and divide by two).
a reflecting mark on a wheel could be detected. Usually you take a phototransistor or a photodiode for light detection. To further increase the insensitiveness against environment light you cold use an LED emitting at a higher frequency (kHz) and pick-up and filter your signal appropiately.
Slotted disk approaches -
If shaft drives a generator/alternator, detect pole piece ripple frequency to compute RPM.
So may approaches, so little time.......
Dumb questions -
1) W what are you trying to measure the RPM of ?
2) Range of RPM you are trying to measure ?
3) Absolute accuracy you are trying to achive for measurment ? Or is relative accuracy more important ?
4) Environment the measurment is taking place in ? Temp, Pressure, Shock.......
5) Measurement device distance you desire from shaft that is rotating ?
6) Power available for measuring device ?
So many questions, so little time.........