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The auto exposure control (AEC) system in a video camera selects the suitable aperture size, gain setting, and exposure time such that the exposure best fits the scene brightness. Although a few approaches to AEC have been proposed, most of them lack smart mechanisms to quickly estimate the right exposure when users suddenly change the scene. The problem becomes more serious when some regions in the captured video frame are overexposed or underexposed. In this paper, we propose a robust AEC approach to real-time video recording. The proposed system consists of a well-designed control flow as well as some heuristic rules to scene brightness measurement and exposure adjustment. To get a more accurate estimation on scene brightness, the system adaptively selects suitable sensor operational modes. The proposed auto exposure also includes mechanisms to detect occasional moving objects and the flicker of the fluorescent light. Experimental results proved that the proposed AEC system has good performance under various conditions, and it helps to improve the recorded video quality.