Referring to FIGS. 9 to 11, a slot pin 118 may project from the base 110. The slot pin 118 may be disposed in the guide slot 124. The slot pin 118 may move in the guide slot 124. A washer 118w and/or a fastening element 118f may be coupled to the slot pin 118. The diameter of the washer 118w may be larger than the diameter of the slot pin 118. For example, the fastening element 118f may be engaged in the slot pin 118. The diameter of the fastening element 118f may be larger than the diameter of the slot pin 118.

When the rotating plate 120 is rotated on the base 110, the slot pin 118 may move relative thereto in the guide slot 124, and may limit the movement of the rotating plate 120. The movement of the rotating plate 120 may be pivoting or rotation.

An indicator 119 may extend from the outer circumferential surface of the rotating plate 120 in the radial direction of the rotating plate 120. A sensor 140 may be mounted on the base 110 adjacent to the outer circumferential surface of the rotating plate 120. For example, the sensor 140 may be a photo sensor. The indicator 119 may pass through the sensor 140 when the rotating plate 120 is rotated.