Abstract: The lowering and raising of slats of a venetian blind are controlled by control signals from a photodetector circuit which is mounted adjacent to the venetian blind and which acts responsive to predetermined limit values of the background brightness being exceeded. Following lowering of the venetian blind, control of the closing or screening angle of the slats is provided by a sun-tracking photodetector affixed to a slat and operating in dependence upon the angular alignment of the slat. In order to fix the closing angle of the slats at a selected value, subsequent adjustment of the slats is controlled by the sun-tracking photodetector which can, in turn, be controlled externally by a manually operated keying mechanism. The photodetector used in measurement of background brightness comprises three aligned photodiodes which are directed skyward in three different directions and which together detect the incident light over the full range of 180.degree. in front of the venetian blind.