Abstract: Aspects of the disclosure relate to methods, apparatus, and systems for tracking a presence of a body part via a theremin-based sensor and outputting a usable signal. For example, a tracking system includes a signal processor and a theremin circuit including at least one antenna. The theremin circuit detects a varying position of a body part relative to the at least one antenna and outputs an analog signal to the signal processor having a varying frequency or amplitude based on the varying position of the body part relative to the at least one antenna. The digital processor determines a rate of change of the varying frequency or amplitude of the analog signal, determines at least a velocity of the body part based on the rate of change, and outputs a digital signal corresponding to at least the velocity to a system or device coupled to the theremin-based tracking system.