On the back of the masonite disk is an old CD, shiny side out. The black cylinder with the wire coming out of it has an LED and a light sensor in it. Every time the black taped part of the wheel goes by, it sends a pulse to the mechanical counter on the far left to count the turns. (A friend made that part for me. His web site is http://cafewalter.com.) When it really gets going, it makes a nice CLACKITY-CLACK sound.
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