| 摘要 |
A door annunciator briefly sounds an alarm when a door opens but not when the door closes. When the door begins to open, a reed switch makes a connection so that an electrical current resulting from a change in the amount of electrical charge stored in a capacitor triggers an electronic alarm chip and thereby causes the alarm to sound. When the door closes the switch connects the capacitor to restore its original level of charge. The components are installed in a small self-contained enclosure adjacent the door, and a permanent magnet to operate the reed switch as the door opens and closes is installed on the door.
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