| 摘要 |
A two-telephone intercom has a simple station-call signalling system. In the preferred embodiment, each telephone is provided with its own respective signalling unit comprising (a) a dc voltage source, (b) a voltage-sensitive threshold element (preferably, a zener diode), and (c) a signalling device, e.g., a beeper and/or flashing light. Whenever one of the telephones is activated (i.e., taken off hook), the signalling device associated with the other telephone becomes operative and remains operative until the other telephone is answered (i.e., activated) or until the first telephone is hung up. Three embodiments of the signalling-unit circuits are disclosed.
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