| 摘要 |
In the on-hook state of a set of telephone line-switch contacts (LS-1 to LS-5), an incoming low-frequency ringing signal (A) from the central office is rectified (14) and applied to an oscillator circuit (16), which drives the receiver (11) of the telephone handset (21) as the transducer for the tone ringer. A protective switch (20) is used to disable the oscillator circuit from producing an output to the receiver whenever the handset has been lifted from the cradle (22), so as to prevent the user from being rung in the ear if the handset is removed from the cradle and the line switch is thereafter depressed manually while the handset is in the normal listening position. The switch responds to the physical presence of the handset in the cradle, such as by using a magnetically actuated switch mounted in the cradle and operated by a magnet (24) mounted in the facing surface of the handset.
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