A surgical instrument includes a surgical end effector coupled to an elongate tube and a rod that passes through the elongate tube. A control mechanism coupled to the elongate tube includes an input axle having a first axis of rotation, an idler pulley having a second axis of rotation perpendicular to the first axis, and a lever coupled to the rod and having a third axis of rotation parallel to the second axis. First and second capstans are fixed to the input axle. Cables pass over the idler pulley and are coupled to the lever. Rotating the input axle in a first direction winds a first cable onto the first capstan, rotates the lever, and moves the rod in a second direction. Rotating the input axle opposite the first direction winds a second cable onto the second capstan, rotates the lever, and moves the rod opposite the second direction.