Described example user interface control apparatus includes a first structure, with a first side, conductive capacitor plate structures spaced along a first direction on the first side, a movable second structure with an auxiliary conductive structure, and an interface circuit to provide excitation signals to, and receive sense signals from, the conductive capacitor plate structures to perform a mutual capacitance test and a self-capacitance test of individual ones of the conductive capacitor plate structures to determine a position of the second structure or a user's finger relative to the first structure along the first direction.