| 摘要 |
PURPOSE:To prevent mixture of noise by connecting a photodetector drive power supply to a series circuit comprising a photodetector and a switching element with a control circuit at reception and turning off the switching element of a power supply for driving a light emitting element. CONSTITUTION:A control signal (e) of a control circuit 8 goes to H level at transmission, a switching element 9 is turned off to turn off a power supply P2 for a photodetector 3, a control signal (c) of the control circuit 8 goes to an L level, a signal (d) goes to an H level, a transistor (TR)6 is turned off, a TR7 is turned on, a current flows to a light emitting diode 2, which is lighted. The signals c, d go to the L level at reception, the TRs 6, 7 are turned off, the signal (e) goes to L level, a TR9 is turned on and the power supply P2 is connected to a common electrode K. A series impedance with a resistor 4 and the diode 2 is much larger than the impedance of the TR9 and the power suppl P1 is turned off substantially. The power supply P2 biases reversely the photodetector 3. Thus, the signal is received without the effect of noise of the power supply P1. |
