| 摘要 |
PURPOSE:To detect the current of a telephone circuit without deteriorating the speech quality by putting a resistance into the telephone circuit, connecting a voltage controlled oscillator in parallel to said resistance and supplying the output of said oscillator to a comparator to extract the ON/OFF signals. CONSTITUTION:When a current flows to a telephone circuit 8, a difference of potential is produced between points before and after a resistance 10. A voltage controlled oscillator 11 oscillates an alternating current of a prescribed level of voltage in response to said difference of potential. Then the alternating current is supplied to a comparator 13 via a transformer 12. In this case, the voltage is set at a level higher than the reference voltage VCC that is supplied to a comparator 13. Therefore the comparator 13 outputs an ON signal. When no current flows to the circuit 8, the oscillator 11 produces no alternating current. Thus the comparator 13 delivers an OFF signal and therefore the pres ence or absence of the current of the circuit 8 is decided by the output signal of the comparator 13. Then the off-hook state of a telephone is detected.
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