| 摘要 |
PURPOSE:To remove an effect of surrounding light, by sampling a level of surrounding light immediately before the reception of photosignal and holding the same simultaneously, taking the difference between the holded value and the value during photsignal reception, and juding the detection of photosignal only when it is over a certain value. CONSTITUTION:As the photodetecting output of a light pen 1, surrounding light only is given to the circuit at sampling time and any light among the light emitting diodes D1-D9 and the surrounding light are additionally given to the circuit at holding time. A voltage comparator 4 holds the state immediately before the light emission of light emitting diodes at a phase inversion type sample hold circuit 3, and the surrounding ligth, which is held and phase-enverter in the circuit 3, is subtracted from the sum output of the signal light of light emitting diode and the surrounding light of a photosignal amplifier 2 due to the light emission of the light emitting d iode, to output only the signal light of the light emitting diode. R4 shown in figure is a level shifting variable resistor. |
