| 摘要 |
PURPOSE:To attain the accurate read-out of information despite of the deterioration of an optical sensor in sensitivity by comparing the output of the optical sensor with the reference value and controlling the elecrtric power supplied to a light source based on the result of said comparison. CONSTITUTION:A bit terminal Q1 of a shift register SR15 outputs ''H'', and an SW1 is turned on to supply a current corresponding to a resistance R1 to an LED array 3. If the sensitivity of an optical sensor 7 is deteriorated under such conditions, the level of a current ISI flowing to a photodetecting element b1 is reduced. Then the output voltage -VSI rises up to satisfy -VSI>-VREF. A comparator 20 outputs ''H'', and the output pulse of an oscillator 23 is supplied to the SR15 as a shift pulse SH1 through an AND gate 22. Therefore the SR15 shifts the position of ''H'' to a terminal Q2 from the terminal Q1. Thus switches SW1 and SW2 are turned off and on respectively. The base voltage of a TR12 rises and the current flowing to the array 3 increases since R2-R1 is satisfied. This compensates the deterioration of sensitivity of the sensor 7. Then the above-mentioned action is repeated to select the brightness which satisfy -VSI<=-VREF. |
