| 摘要 |
A telephone ring detector system which finds particular utility in a telephone answering unit, since it responds only to ring signals on the telephone line to activate the system, and it is unresponsive to dialing pulses which would otherwise cause a spurious activation of the telephone answering unit. The system of the invention differentiates the ring signals from dialing pulses by detecting whether or not the telephone line to which the telephone answering unit is connected is in an "on-hook" or "off-hook" hook condition, and by providing an output in response to an incoming signal only when the telephone is in the "on-hook" condition. This assures that the signal detected on the telephone line is not a series of dialing pulses, since the dialing pulses occur only when the telephone line is in the "off-hook" condition. The system of the invention has the feature in that it is effectively direct-current isolated from the telephone line in accordance with the requirements of the Federal Communications Commission. The system of the invention also has the capability of sensing positive or negative voltage deviations when the receiver is taken off the hook, so as to be effective with all types of telephone systems. The system has the added capability of sensing true ring signals outside the normal frequency range and which would be rejected by many prior art systems.
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