| 摘要 |
An optical sensing device for use on a mobile telephone or the like comprises an LED, a photodetector and a rotary key. The rotary key surface has alternating regions of high and low reflectivity from which LED light is reflected to the photodetector. The output of the photodetector has a first dynamic state and a second substantially static state for each reflectivity characteristic as a function of time. A switching apparatus pulses the voltage supplied to the LED and the photodetector at a rate sufficient to ensure that each alternate region on the key is exposed to a pulse of light, each pulse being sufficiently short that the photodetector output only relates to the dynamic state. An output threshold is applied which, when reached, indicates that the region of the surface being viewed is of high reflectivity. The output relating to each pulse is compared to that of the previous pulse to determine movement. Upon the threshold being reached, the voltage may cease to be applied for the remainder of the pulse, to save current consumption. Further regions of intermediate reflectivity may also be incorporated on the key surface.
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