| 摘要 |
PURPOSE:To enable secret talk, even in the case of a master/slave telephone, by providing a means which disconnects the telephone set from a line when a user does not call by the telephone set connected at the time of using the line. CONSTITUTION:When a telephone set A is connected to another telephone receptacle 1 at the time of using the line, an LD (LED) in a display circuit is lit to display that the line is busy, but a calling current does not flow between telephone circuits L1 and L2 and a modular jack 2 since the receiver is not hooked up. Therefore, the phototransistor of a photocoupler PC is turned off and the output of the PC is kept in the high level as it is. Consequently, the output of an AND gate G1 goes to the high level and a transistor Q3 is turned on to drive a relay Ry1, and then, a normally closed contact r1 is opened to disconnect the jack 2 from the line. Thus, when another telephone set is used, calling by on another telephone set cannot be heard even when the telephone set A connected to the receptacle A is hooked up.
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