RPM disc

This routine draws circles of specified radii that are divided into a definable number of alternating black and white segments. These discs can used to measure the speed of rotation of motors etc. When viewed under a light that is powered by mains electricity the patterns will appear stationary at certain rotation speeds. In Europe these speeds are given by 6000/N rpm - where N is the number of black segments. In the US and other areas that use a 60Hz mains frequency the formula becomes: 7200/N rpm.

You can read about one application of discs of this type at: http://www.jeffree.co.uk/pages/speedmeasurement.html

Edit and compile if you like:

% Routine to draw segmented circles for use as timing strobes
% Author: Simon Martin
\documentclass{article}
\usepackage{tikz}
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{tikzpicture}
\setlength\PreviewBorder{5pt}%

\newcommand{\segments}[3] {% draw circle divided into segments
  % #1 = outer radius
  % #2 = inner radius
  % #3 = no of segments
  % note number of segments is total number of black and white segments
  % need to pass units with radius.
  \def\radius{#1}
  \def\nsegs{#3}
  \def\segangle{360/\nsegs}
  %draw dotted circle as guide for cutting out
  \draw [dashed](0,0) circle (\radius);
  \foreach \x in {1,3,...,\nsegs} %draw alternate segments
      \filldraw[fill=black,draw=black] (0:0mm) -- ({(\x-1)*\segangle}:\radius )
      arc ({(\x-1)*\segangle}:\x*\segangle:\radius ) -- cycle;
  % draw an inner circle 
  \draw[fill=white] circle(#2);}

\begin{document}
\begin{tikzpicture}
  \matrix [column sep=-5mm, row sep=-5mm] {
    \segments{7mm}{1mm}{12} & & \segments{7mm}{1mm}{18} \\
          & \segments{21mm}{2mm}{6} & \\
    \segments{7mm}{1mm}{24} & & \segments{7mm}{1mm}{30} \\};
\end{tikzpicture}
\end{document}