Make Your Own Stroboscopic Tachometer

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.