| 摘要 |
PURPOSE:To detect a channel stably and surely, by storing a momentary value of the AC signal under the connection state of the line between a calling subscriber and a called subscriber and by comparing the stored value with the succeeding state. CONSTITUTION:An oscillator 8 is connected to a line. For a call, the polarity of the line is inverted. When detecting the inversion, a relay 1 controls a contact 21 so that the contact 21 is opened after closing for a constant time. When the contact 21 is closed, the output voltage of an amplifier 12 rectified by a rectifier 13 is stored in a capacitor 17. Since the capacitor 17 is connected to the gate of an FET 16 operated as a source follower, the capacitor 17 holds the voltage even if the contact 21 is opened. Consequently, the source voltage is equal to the voltage supplied from the oscillator under the connection state of the line. If the calling subscriber stops the call, the impedance of the line becomes high, and as the result, the voltage in one terminal of a comparator 14 is lowered to generate an output from the comparator 14, thus detecting the stop of the call. |
