| 摘要 |
PURPOSE:To detect positions surely without the influence of ambient light by providing a period when a light emitting element operates to emit light and a period when said element do not operates, and comparing the reception signals of photodetectors in the period when said element do not operate and the reception signal while said element operates. CONSTITUTION:Light emitting element corresponding respectively to photodetectors 1, 2 which detect indicated position do not operate to emit light in the first half of a light emission time. In the first half of the operation time thereof, the output signals from the photodetectors 1, 2 are conducted to a sample holding circuit 7 via analog switches 3, 4 which are closed successively. The output signal of a differential amplifier 6 and the output of the circuit 7 are introduced to a differential amplifier 11, the level difference thereof are amplified. The output signal of a differential amplifier 11 and the level signal of the ambient light from the sample holding circuit are applied to a comparator 12, by which said signals are compared. Thus the positions are detected surely without the influence of the ambient light. |
