| 摘要 |
PROBLEM TO BE SOLVED: To prevent a speech of a general telephone set illegally in branch connection to a telephone line to which public telephone sets are connected. SOLUTION: A voltage of lines L1, L2 closed as a loop based on an off-hook operation of a public telephone set is detected and when it is 40 V or below, a transistor(TR) Q1 is turned off to open a DC loop of the lines L1, L2, and in the case that the line voltage after opening the DC loop is 40 V or below, it is judged that a general telephone set 100 is busy and an anti-parallel circuit 12 is used to short-circuit the lines L1, L2. Upon the receipt of a PB signal by a PB receiver during off-hook of the public telephone set, it is judged that the general telephone set makes dialing, the lines L1, L2 are similarly shortcircuited.
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