| 摘要 |
A garage door opener is retrofitted with a device that causes a garage door to close after it has been open for a predetermined amount of time. In a garage door of the type having a light that is activated when the door is opened or closed, a photocell is mounted in close proximity to the light to detect the presence or absence of light. When light is detected, the photocell conducts and prevents a first automatic switch, such as a silicon-controlled rectifier, from conducting. When absence of light is detected, the first automatic switch closes and a capacitor discharges through the first automatic switch and throws a second automatic switch, such as a transistor, that is electrically connected in parallel to a manually-operated switch for opening and closing the garage door. When the second automatic switch conducts, it mimics closing of the manually-operated switch and thus effects closing of the garage door. The circuit is enabled by a spring-loaded plunger that holds a pair of electrical contacts apart from one another only when the door is open.
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