Strobe Disc

How to use: Print at exactly 100% scale - verify with the 100mm scale bar before use - and cut out the disc along the dashed line. Place it on your platter and spin at the speed you want to check. Dim the room lights and illuminate the disc with a mains-powered LED lamp or a neon lamp, and use the rings matching your local mains frequency - 50Hz or 60Hz depending on the country. Consult the legend to see which numbered ring refers to your platter speed and that frequency. It will appear stationary when the platter is running at the correct speed. If the bars drift clockwise the platter is running fast; if they drift anticlockwise it is running slow. If no ring settles at all, try a different lamp.

Technical note: Many mains-powered LED lamps, especially everyday bulbs and desk lamps, flicker twice per AC cycle - 100 times per second on 50Hz mains, or 120 on 60Hz mains - too fast for the eye to notice. Their simple drivers pass some of the mains ripple through to the LEDs. A neon lamp flashes at the same rate, so between flashes the disc turns exactly one bar of its ring and the pattern appears frozen. Higher-quality “flicker-free” LED lamps and incandescent bulbs give a nearly steady light and won't freeze the pattern. Phone strobe apps aren't recommended either: phones can't time their flashes precisely enough, so the bars shimmer or drift even when the speed is correct.