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Several years ago I had a requirement to measure the
rotational speed of some cooling fans. Ordinarily I would have put a drop of
white-out on a fan blade, shone a bright light on it, and measured the
frequency of reflected light with a phototransistor. Unfortunately the fan
guard and filter screen prevented me from doing this. I decided to use the
stroboscopic effect to 'freeze' the motion of the fan by varying the pulse rate
of a bright light source. When the fans motion apparently
stopped, I could measure the frequency of the light source, divide that number
by 60 and I would have the RPM of the fan. Since ultra bright LEDs were cheap I
designed a simple, battery-powered, circuit based on a 555 timer to make a
strobe light. The schematic is shown below.
To accommodate a wide RPM range, I had two switch selectable
frequency ranges: 0.3 – 300 Hz and 300 – 30 kHz. The frequency is adjustable
using a 10 turn potentiometer, which gives fairly fine control. (The pot was
the most expensive part of the project, but a single turn one was just too coarse.) The capacitors are
polystyrene for stability.
I noticed that it was sometimes difficult to find the
'freeze' point using the square wave output of the 555, so I reduced the duty cycle from 50% to
20% using a 4017 decade counter. It takes a little practice to find the
fundamental freeze frequency (both harmonics and sub-harmonics will freeze the
blades), but after playing with it for a few minutes it's obvious when you've
hit the fundamental.
  
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