The cam consists of two parts: blue round disk and yellow cam.
Green follower moves one time during two revolutions of the cam.
The weight forces the follower toward the cam. Spring force is another possible way. There must be sufficient friction between the yellow cam and the blue pin to avoid accidental motion of the yellow cam.
The idea of this video is taken from
http://www.youtube.com/watch?v=M7H-wnHxxXU
by the introduction of a Youtube user, TheWindGinProject.
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