| 摘要 |
PROBLEM TO BE SOLVED: To additionally connect two or more interphones to an existing house information panel without the need for any modification on the house information panel. SOLUTION: The two extension interphones JB are connected to the house information panel J via a speech signal line L. Each of the extension interphones is provided with a speech use amplifier 4 and a disconnection section 5 connected to the amplifier 4. When the one extension interphone JB2 is used, the disconnection section 5 of the other extension interphone JB1 not in use is controlled to disconnect the amplifier 4 as the control. Further, in this case, a display light 11C of the extension interphone JB1 not in use is lighted. Each of the extension interphones is provided with a voltage electrode 7 for detecting a dropped voltage of the speech signal line and a hook ON/OFF detection section 3 for detecting a state of a hook switch 2 so as to discriminate the state of use of the extension interphones.
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