(*Nasser Abbasi Feb 19, 2008 *)
(*updated 8/12/14*)
Manipulate[
 tick;
 Module[{xStep, yStep},
  xStep = signX*RandomReal[{0, dx}];
  yStep = signY*RandomReal[{0, dy}];
  x = x + xStep;
  y = y + yStep;
  
  If[x > 1 - r || x < r,
    If[x > 1 - r, signX = -1, signX = 1]
   ];
  
  If[y > 1 - r || y < r,
    If[y > 1 - r, signY = -1, signY = 1]
   ];
  
  If[state == "RUN",
   tick = Not[tick]
   ];
  
  Graphics[Disk[{x, y}, r], ImageMargins -> 0, Frame -> True, ImagePadding -> 1, PlotRangeClipping -> False, 
   PlotRange -> {{0, 1}, {0, 1}}]
  ],
 
 {{dx, 0.01, "x step size"}, .001, 0.1, .001, AppearanceElements -> All},
 {{dy, 0.01, "y step size"}, .001, 0.1, .001, AppearanceElements -> All},
 {{r, 0.1, "radius of ball"}, .05, 1, .01, AppearanceElements -> All},
 Grid[{
   {Button["run", state = "RUN"; tick = Not[tick]], Button["stop", state = "STOP"]}
   }],
 {{state, "STOP"}, None},
 {{tick, False}, None},
 {{signX, 1}, None},
 {{signY, 1}, None},
 {{x, .5}, None},
 {{y, .5}, None},
 ContinuousAction -> False,
 TrackedSymbols :> {tick}
 ]