A self-operated regulator can be configured to control a stream of working fluid, such as of a processing plant. The self-operated regulator can include a control valve and a control actuator configured to receive supply energy for the provision of an operating force exclusively from the stream of working fluid and/or from the working fluid for operating the control valve. A magnetic field source can be coupled to the control valve such that the magnetic field source follows the movement of the control valve to determine the movement using, for example, a magnetic sensor. The position of the control valve can be detected based on the determined movement.