I designed and 3D printed wheels to fit the keys, so that they would spin from an arrangement of belts driven by a common shaft, powered by a 12v DC motor. Originally I played around with striking them while they spun, but I grew to prefer the sound they would make simply rattling around from momentum.
A PWM motor controller is mounted on the side and the speed can be finely controlled using a dial. With this control, I can ramp the speed up to a point where the centrifugal force silences the rattling of the keys. Bringing the speed back down gently reintroduces the clinking sounds of the keys in a cascade of sound. It reminds me of the mix of percussion and chimes you hear from old Pachinko machines, or the flurry of clacking made of train station split-flap displays.
This uncut performance was recorded on three cameras, using two microphones (one stereo, one mono). When I discovered that the stereo recording picked up a lot of low-end rumble that resonated into the floor it shared with the instrument, I further split that recording into low frequency and high-frequency recording, which were treated separately and recombined. The mono recording was treated with a delay and oscillating low pass filter and layered back into the recording. With the benefit of two standard microphones and a contact mic, I'm confident that this same performance and sonic characteristics could be achieved live, without the use of post-processing.