Tone wheels sizzle in this organ-inspired musical instrument


Young readers may not remember the days when every mall had a music store – not the kind where cassettes and vinyl records were sold, but the kind where you could buy instruments. These places inevitably had an employee singing mall music to everyone on an electric organ. And more often than not, the organist would play a Hammond organ, with the distinct sound of these instruments generated by something similar to this robot phonic wheel organ.

Tone wheels are toothed ferromagnetic wheels that rotate near a pickup coil. This induces a current which can be amplified; modify the tooth profile or change the speed of rotation, and you have control over the sounds produced. While a Hammond organ uses this technique to produce a wide range of sounds, [The Mixed Signal]The effort of is considerably more modest but nonetheless interesting. A stepper motor and a 3D printed 1: 8 ratio gearbox power a pair of shafts that each carry three different tone wheels. The tone wheels themselves are laser cut from mild steel and range from what looks like spur gears to wheels with just a few large lobes. This is a step up from the previous version of this instrument, which used 3D printed tone wheels from magnetic filament.

Each tone wheel has its own microphone, wound up using a winder which [TheMixed Signal] previously built. Each coil has a soft iron core, allowing for the addition of one or more neodymium bias magnets, which dramatically changes the tone. The video below shows the build and a demo; skip to around 4:10 p.m. if you just want to hear the instrument play. It’s interesting. But it’s clearly a work in progress, and we can’t wait to see where it goes.

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