A retractable screen door for drawing a flexible screen panel across an opening, including a flexible screen wound around a spring biased take-up shaft, and guide rails for guiding the screen as it is moved across the opening. For example, the signal sourcing device may be a digital to analog converter having a plurality of current sources that output currents which may have an error. When the mains supply unit is cut off, the switch unit is automatically switched to connect the generator unit with the storage unit.