| 摘要 |
PURPOSE:To eliminate the need for accurate operating sequence of a hook switch by using two sets of diode bridges, two DC blocking capacitors and a hook switch having one set of make contact point. CONSTITUTION:In the standby state, a hook switch 6 is opened and a talking circuit 7 is disconnected. When a bell signal formed by superimposing a sinusoidal wave on a DC is impressed and the level of a terminal L1 is higher than that of a terminal L2, the DC block capacitor 1 is charged, a current flows to a diode bridge 2, a tone ringer circuit 3, a diode bridge 4, and the terminal L2, the DC block diode 5 is short-circuited to the terminal L2 via the bridges 2, 4 and discharged. When the level of the terminal L2 is higher, the capacitors 1, 2 are discharged/charged respectively. In the talking state where the switch 6 is closed, the terminals L1, L2 are connected to the circuit 1 via the bridge 4 and the circuit 3 is disconnected. Thus, the accurate operation sequence of the hook switch is not required and a polar simple electronic switch is used and a polar DC block capacitor is used.
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