| 摘要 |
<p>PURPOSE: To automatically adjust a load of a sensor element by changing an input signal of a constant current circuit, obtaining an optimum current for the sensor element, setting the input signal in accordance with the optimum current, and comparing an output signal of the sensor element with a threshold. CONSTITUTION: A microprocessor 12 varies a control signal to a D/A converter 13 stepwise from a minimum value to a maximum value in a set range, thereby changing an input voltage Vi of a constant current circuit CI from a minimum value to a maximum value and changing a sensor current Is from a minimum value to a maximum value. While monitoring an output signal voltage Vs of the circuit CI, the microprocessor 12 measures the current Is as a sensor current value of a phototransistor PT2 when the voltage Vs exceeds a threshold Vth. Moreover, the microprocessor 12 compares the measured sensor current with an initial sensor current, detects a deterioration degree of a sensor element, and sets a set current value of the circuit CI to be optimum in accordance with the deterioration degree. The presence/absence of a card C and a position of the card are detected by comparing the voltage Vs with the threshold Vth.</p> |
