Abstract: A circuit for apparatus to be used with a key system line card for providing electrical isolation between the line circuit and the central office. The adapter responds to ring current from the central office to close a path to the line circuit to cause the line circuit to respond. The circuit provides two-way supervision responding to change of conditions either at the station or at the central office, and also provides a time delay to prevent the circuit from entering the hold condition improperly.

Abstract: The specification discloses a voltage supply system particularly adapted for use as an automatic brightness control for an image intensifier tube. The system includes a power oscillator for applying a constant A.C. voltage to a first multiplier which generates at least one constant D.C. voltage for application to the screen of the image intensifier tube. A regulator circuit includes a low power oscillator which generates a control A.C. signal and which further includes circuitry responsive to the screen current of the image intensifier tube for varying the amplitude of the control A.C. signal. The frequency of the control A.C. signal is substantially greater than the frequency of the constant A.C. voltage. A bilateral attenuator receives the constant A.C. voltage and is operable in response to the control A.C. signal for varying the amplitude of the constant A.C. voltage. A second multiplier is connected to the output of the attenuator for generating a variable D.C.