Any system or device that carries out operations automatically in response to the output of other electronic devices, such as sensing devices, computers, or discriminator circuits. Feedback of the output signal is frequently employed to provide the controlling signal.

An automatic controller measures any variable quantity or condition and produces an output designed to correct any deviation from the desired value. The threshold signal is the minimum input signal to which the control system responds by producing a corrective signal. A system that is actuated by electrical signals is known as an electric controller. If the electrical signals are used to excite electromagnets that determine all the basic functions of the device, it is termed a magnetic controller. In electrical switching, a desired performance is maintained by automatically opening and closing switches in a given sequence.