For the Optical Illusions Secretary’s Challenge I decided to re-create a stroboscope. Some years ago I made one to determine the rotational speed of my E20R motor, spinning a black/white segmented disc under tungsten lighting. I remember it worked but the definition of the Stop motion was not very good and only slightly better under fluorescent lighting.

Eventually I realised the poor definition was due to the afterglow of the lamp filament, i.e. the mains powered lamp does not actually go-out fifty times per second!

This time I decided to experiment with LED illumination using a variable speed driven multi-segmented disc. This should allow a range of image synchronisation possibilities.

A disc was drawn with ten sets of black/white segmented phases ranging from one cycle at the centre through to ten cycles at the outside. Then a 12V DC motor was mounted vertically in a frame powered via a Darlington pair speed controller.

A 240V AC LED mains powered (50Hz frequency) lamp was suspended over the motor mounted disc. The effect at variable speeds proved better than under tugsten or fluorescent illumination. At 50Hz illumination, the speeds of each phase on the disc could be observed as stopped from 300 RPM at the outer segment through to 3000 RPM at the inner segment.

The disc speed in RPM is calculated from:

60 x Flash frequency / Number of strobe phases

i.e. at 50Hz, ten strobe phases indicates 300 RPM.

Hoping for even better clarity I tried using a Velleman variable flash circuit driving a pair of low voltage LEDs, powered by a 9V battery. This gave a slightly better stopped image, but it was difficult to control the high frequency flash range.

A speed chart and strobe discs are shown in the photos, right.

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