| 摘要 |
PURPOSE:To decrease the frequency of cleaning by smoothing and comparing the output of a photodetector with a reference voltage, and amplifying the error voltage and then controlling the quantity of light of a projector so that the output voltage of the photodetector becomes equal to the reference voltage. CONSTITUTION:The output voltage of a smoothing circuit 14 is supplied to an error amplifying circuit 15 and compared with the reference voltage VRef1. Then a voltage V3 is outputted as an output proportional to the error voltage between the reference voltage and an output voltage V2. In this case, a current IL flowing to a projector 12 is proportional to the output of a differential amplifier IC2, i.e. voltage V3. The quantity of light of the projector 12 is therefore controlled according to the voltage V3 at all times. Consequently, the amplification factor of the photodetector 13 is held constant, so the S/N is good and a bank coupon ticket can be detected stably for a long period. Therefore, the number of times of cleaning is reduced in body detection where the projector and photodetector need to be cleaned frequently so as to prevent deterioration in performance due to dust and dirt. |
