Which of the following devices is typically activated by other systems or manually?

The device that is typically activated by other systems or manually is the input device. Input devices are intended to receive signals or data from users or other systems to process or control operations within a given electrical system. Examples include buttons, switches, or sensors that users can operate to input commands or actuate other components within the system.

Understanding this is crucial because input devices form the initial point of communication within a circuit or system. They provide a means for external interaction, enabling users or other systems to influence operations or behavior. This characteristic highlights their role in control mechanisms and automated systems.

The other options, while related to system functions, do not fit the description of being activated in the same manner. Output devices are designed to convey information from the system to the user, transformers are involved in voltage conversion, and interfaces serve as points of interaction that translate communication between different systems but do not inherently activate themselves.