| 摘要 |
<p>PURPOSE:To discriminate whether or not there is a body to be discriminated even if the light emission intensity of standard light deteriorates by comparing a detected voltage with a reference value in a discrimination stand-by mode and shifting the detected voltage to a proper level if there is deterioration in the quantity of light emission. CONSTITUTION:A selection switch 5 is changed over with a switching signal in the discrimination stand-by mode to connect an in-calibration current limiting resistance R1 to an LED 1. Then a value to be discriminated (detected voltage when paper or the like P0 is not inserted) is inputted. The value Sx is the difference between the collector and emitter voltages of a phototransistor 2. The value Sx is compared with an in-calibration reference value S0=Vcc-V0 (V0<Vcc/2). When the the value Sx is smaller than the value S0, i.e. discrimination margin width W=(Vcc/2)-V0, an operation point change signal is generated. Consequently, a gain control signal is supplied to a resistance selecting multiplexer 7 to select a next resistance r2 larger than a resistance r1 being selected currently, thereby changing the operation point. Lastly, a resistance R0 is selected with a switching signal and calibration is completed. When the LED 1 further deteriorates, the operation point further moves.</p> |
