An electronic device includes a base, a multi-stage sensor, a top cover, and a side cover. The multi-stage sensor is configured to sense a sensed area. The top cover includes a top-sensed element, and the top cover is detachably connected to a top side of the base, so that the top-sensed element can be selectively in or not in the sensed area. The side cover includes a side-sensed element, and the side cover is detachably adjacent to a front side of the base, so that the side-sensed element can be selectively in or not in the sensed area. In response to that only the top-sensed element is in the sensed area, the multi-stage sensor outputs a first signal. In response to that neither the top-sensed element nor the side-sensed element is in the sensed area, the multi-stage sensor outputs a second signal.