Grooving in the right direction using motors driven backward

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The backdriven stepper motors produce notes over four octaves(Credit: Josh Sheldon)

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After using an oscilloscope to confirm that stepper motors driven backward produced an alternating current, University of Maryland senior Josh Sheldon decided to turn the resulting waveforms into audible sounds. He attached a series of steppers to a frame, had them backdriven by a large spinning cone and then lowered the motors one at a time using a piano keyboard to produce a tune or two. The result is the magical Stepper Organ, and the output is just a little bit eerie.

Sheldon discovered that the waveform he managed to generate from a backdriven stepper motor was, more or less, sinusoidal, meaning that running it at different speeds would potentially sound similar to a sine wave synth. He lined up 49 steppers, each sporting a rubber wheel, and mounted them to the top of a wooden frame in a staggered formation.

A large wooden cone sits below, made up of different-sized MDF disks threaded on a rod. Each CNC-machined disk corresponds to the position of a stepper, the largest disk is 25 inches (640 mm) in diameter, while the smallest is 1.57 in (40 mm). A note frequency chart was used to make sure each stepper and disk interaction would produce a singing note over four octaves.

The assembled cone tipped the scales at around 90 lb (40 kg), and Sheldon says that if he'd plumped for an 88 key setup, the largest disk would have been 16 ft (4.8 m) in diameter.

Each oak/walnut key on the piano keyboard to the front lowers a stepper attached to a lever onto the spinning cone. The thicker the cone, the faster the stepper is backdriven and the higher the "note" produced. Motors driven faster are also louder, so each note has its own volume knob to make sure the output is even.

Unlike Martin Molin's wonderful Musical Marble Machine, which is cited as an inspiration for the project, the Stepper Organ is not hand-cranked. The large cone in the center is driven by a separate electric motor and the audio signal output via a jack to the left, sent to a speaker and synth-like sounds produced. The speed of the cone, controlled using a dial to the left of the keyboard, determines the key of the played notes.

You can see and hear this bizarre instrument in the video below. If you want to see how the Stepper Organ was made, Sheldon has uploaded a number of build videos to his YouTube channel.

Source: Josh Sheldon

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