There were plans for one in a back issue of Model Engineer (late '80s, maybe???) I couldn't figure out the drawings, dolt that I am, but you might have better luck.
That one used permanent magnets. I wonder about making a DC-powered one, winding coils for the electromagnets, but I don't know enough about how it ought to be designed. (n.b. if you do it, use DC only -- with AC, an electromagnet turns off 60 times a second!) (50 if you're in the UK.... ;-) )
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