| 摘要 |
A membrane switch is actuated by pressure transmitted from a movable key member through two coaxial springs connected end-to-end. A first of the two springs is held by the key member in a partially strained state requiring an initial force to strain it further. The second spring, smaller in diameter and stiffer than the first, has a free end at which force is applied to the membrane switch. As a top of the key member is pressed, the free end of the second spring moves toward, touches, then presses the membrane switch. Force builds rapidly with displacement in the small spring until it reaches the initial force under which the first spring is retained. This initial force is made roughly equal to the force required to actuate the membrane switch. After that point, the force-displacement characteristic is that of the two-spring combination, which is less stiff than the second spring alone. Thus, long key travel is permitted while the actuation force is reached early in the displacement of the key member. In one embodiment, a resilient boot, covering a key stem of the key switch, buckles when the key is pressed to provide a click-like feedback to an operator.
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