| 摘要 |
<P>PROBLEM TO BE SOLVED: To detect the position of a key without using an A/D converter. <P>SOLUTION: A light emitting diode 201 is driven by using an infrared LED driving current I<SB>LED</SB>whose amplitude is modulated with a specified time function to emit irradiation light 203 to the key 101. Then a phototransistor 202 photodetects reflected light 204 from the key 101 and generates a sensor output voltage V<SB>E</SB>corresponding to the photodetection quantity. A comparator 804 compares the sensor output voltage V<SB>E</SB>with a threshold voltage V<SB>S</SB>and generates a PWM signal having a pulse width corresponding to the level of the sensor output voltage V<SB>E</SB>. A PCM modulator 805 converts the PWM signal into a PCM signal. A driving circuit 806 controls the distance (u) between a photosensor 103 and the key 101 toward a target value according to the PCM signal. Consequently, a digital signal for detecting the position of the key 101 can be generated without using any A/D converter. <P>COPYRIGHT: (C)2005,JPO&NCIPI |
