Meant to answer this earlier....... The photo of an Otto chassis you posted reminded me....
You may already know all this, but just in case you don't, here it is. In HO the 'cam' is often a disc on the back of a driving wheel. Soundtraxx sell a pack of them for their DCC sound decoders that are etched on very thin printed circuit board.
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This shows one fitted to a wheel set. It takes some care to fit, as the wheel has to be removed from the axle and if the axle isn't splined, it is easy to screw up the quartering when reassembling.
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The biggest available cams in HO are only 20mm or so in diameter, but it should give you the idea. For your application, you'd need two wipers, one from the sound board cam connection and one to the common return.
There should be enough space between the wheel and the frame on an Otto to make a cam to fit. Cut a disc of shim metal, preferably phos bronze, and glue it to the back of the wheel. The insulation between the contact areas could be made by cutting segments of self adhesive plastic (the stuff used on school books), or possibly a a better solution is to use clear nail varnish to paint the insulating segments on the metal. The wipers would be best made out of phos bronze strip, rather than wire, as flat strip doesn't move around as much when the loco changes direction.
A friend working in HO makes his chuff 'cam' by applying areas of super glue to the back of one of the driving wheel tyres on the uninsulated side, but he only has one wiper to deal with, as the return is via the axle. One of his locos has the characteristic off beat exhaust of that loco class, as he was able to vary the chuff timing by painting different length insulated sections.
Regards,
Graeme
