An approach to the design of active window shades is developed to control the direct solar gain through a window. Using simple actuation hardware and sensors, a control algorithm has been developed that will automatically adjust window shades to save energy by controlling direct solar radiation passing through a window. The control strategy is based on a simple approach that admits light in the winter and blocks light in the summer. Cloudy skies or indirect sun result in horizontal placement of the shades, and the shades close at night. The control algorithm uses two thin-film photovoltaic cells as sensors and an algorithm that is independent of the window orientation, latitude, or solar time, so that it operates properly in an installation location. Analytic and test results show significant energy savings when the automatic window shades are compared with a window without shades, and with a window outfitted with fixed horizontal shades.