| 摘要 |
PURPOSE:To discriminate easily whether shielding a photodetector from a light emitting element is completed, by using a PLL circuit that is driven by photodetection and then drives the light emitting element. CONSTITUTION:One signal photodetected by photodetector 1 is input to multiplying circuit 2, and the other drives PLL circuit 6 through 90 deg. phase-shifting circuit 5. One output of the PLL circuit is input to multiplying circuit 2 and the other drives light emitting diode 8 through light-source driving circuit 9. Assuming a body to reside between the photodetector and light emitting diode for shielding, no output signal is sent out because any output of the photodetector does not arrive, but when no body resides, two signals 90 deg. out of phase with each other are input to multiplying circuit 2, which outputs a low-frequency signal. Then, providing the light source and photodetection part in the loop of the PLL circuit can attain synchronization between the both, so that malfunction caused by disturbing light will not occur. |
