| 摘要 |
An optical sensor (10) comprises: a light source (14) located on one side of a transport path (16); a variable current drive (22,24,26), an optical receiver (20) located on an opposite side of the transport path (16) to the light source ans aligned therewith to detect light output therefrom; and a memory (34). The variable current drive (22,24,26) is suitable for energising the light source (14) so that the light intensity from the light source (14) increases with increasing current. The variable current drive (22,24,26) includes (i) a drive circuit (24,26) for applying a pulse of current to the light source (14), during which pulse the light source is energised; and (ii) a counter (22) for increasing the amount of current applied by the drive circuit (24,26) during a pulse of current. The memory (34) may be arranged to store a value from the counter (22) indicative of a number of media items present in the transport path (16).
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