| 摘要 |
A telephone extension controller for permitting controlled access of telephone line from one of a given number of extension telephones coupled thereto when each extension telephone is controlled with the same comprising a lock-out circuit adapted to be coupled to a ring wire or a tip wire, the lock-out circuit further adapted for allowing a selected extension telephone of a plurality of extension telephones coupled to a telephone line to control and prevent the other extension telephones from accessing the telephone line when its switch hook is raised, the lock-out circuit including an optoisolator photo-coupled Darlington transistor having a conducting section and a photo-activated section and a Zener diode coupled in series with the conducting section of the Darlington transistor and coupled in parallel with the photo-activated section of the Darlington transistor.
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