We used to clip a playing card onto a fork or stay with a clothes pin to 
make a motor noise when the spokes flapped by. :) :)

Marcel Birthelmer wrote:
> Could you somehow detect an audio signal (some sort of mechanical
> click or something produced by the wheel) using a dsPIC or something
> like that? Or maybe even use an ultrasound transducer and receiver to
> pick up the spokes moving past? Overengineering FTW!
>   
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