| 摘要 |
A telephone ringing circuit responds to both conventional high voltage 20Hz ringing signals and low voltage tone alerting signals. A conventional electromechanical ringer is connected across the telephone line and is responsive to high voltage 20Hz ringing signals. A high capacitance capacitor is charged by the dc voltage present on the telephone line and is discharged across the electromechanical ringer when a tone detector circuit connected across the line detects a tone alerting signal, thereby activating the electromechanical ringer.
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