Apparatus for converting a change in fluid-pressure into mechanical motion of a part, which motion can be used to generate a suitable output signal. Two bands affixed to a sheave are wound thereon, side by side, in opposite directions. The free ends of the bands are connected to a pushrod which is in the same plane and at right angle to the axis of the sheave. The clockwise wound band is connected to the right of the axis of the sheave and other band is connected to the left thereof. The pushrod is connected at one of its ends to a pressure capsule which deflects when the fluid-pressure acting thereon is changed. This deflection causes the sheave to rotate and generate the desired output effect.