| 摘要 |
PURPOSE:To eliminate the need for a line supervising relay by waveform-shaping a voltage corresponding to a voltage of a telephone line to detect a bell signal from the ratio of the length of high voltage period to low voltage period. CONSTITUTION:A voltage corresponding to a voltage of telephone lines L1 and L2 is waveform-shaped by a comparator circuit 10. Then a pulse width comparator circuit 11 calculates the ratio of the length of high voltage period to low voltage period of an output of the comparator circuit 10 so as to detect that this operating value is a prescribed vlaue corresponding to the bell signal. Further, an output of the pulse width comparator circuit 11 is outputted as a bell detecting signal. |
