Hacking a Prosthetic Arm to Control a Music Synthesizer

Prosthetic limbs have come a long way since the days of solid hunks of plastic that vaguely resembled flesh. While those were designed mostly for aesthetic purposes, modern robotic prosthetics are actually intended to be functional. Special sensors placed on the user’s residual limb make it possible to control the prosthesis’s actuators. When the user flexes a bicep muscle, for example, the sensors can pick that up and tell the fingers of a prosthetic hand to close. Bertolt Meyer is a musician who has a prosthesis that works that way, and hacked it to control a music synthesizer.

Meyer was born without the lower half of his left arm, and has been using an advanced robotic prosthetic arm for many years now. Despite how capable that prosthesis is, the actuators still respond too slowly to easily control a synthesizer. The hand of that prosthesis was upgraded a while ago, and Meyer still had the older version in his possession. The old prosthetic hand was no longer functional, but Meyer thought it might be possible to hack it and repurpose it to control his synthesizer. That hand, along with the new replacement, has a special collar connector that makes it easy to snap it onto the rest of the prosthetic arm. Importantly, that connector also passes through the electrical signals coming from the electrode sensors on Meyer's residual limb.

Most synthesizers, except for some inexpensive entry-level models, have CV (Control Voltage) inputs that allow for external control. Meyer, along with his husband Daniel and Chrisi from KOMA Elektronik, converted his old prosthetic hand into “SynLimb” to interface with the synthesizer through those CV inputs. All of the robotic components were removed from the prosthesis and replaced with a custom circuit board that Meyer built. That takes the electrode signals and translates them into CV signals. Meyer's husband 3D-printed a frame to hold that in place on the original prosthesis’s collar connector. Now Meyer can simply “think” a command, which is actually him subconsciously flexing the muscles in his residual limb, and the synthesizer will respond immediately.