| 摘要 |
A line switch control circuit for a telephone subset comprising a line switch (2), a hook switch mode detector (TR4), a voltage sensing circuit (TR5) having at least two inputs (D and F), and a dialler chip (3) having a CMOS push-pull pulse output (DP). The circuit ensures that when the subset is in the on-hook mode, on the one hand, an appropriate level of current is drawn from the exchange line (L1, L2) to energise the dialler chip (3), and on the other hand, the line switch (2) remains non-conducting during on-hook mode. In the off-hook mode, the circuit renders the line switch non-conducting during breaks in dialling signals as well as during flashes by diverting current from the line switch into the pulse output (D) of the dialler chip.
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