| 摘要 |
A ringback circuit in a security protection system, of the type which monitors a dc current, automatically reports to remote premises that the central station has, at an appropriate time, received an indication that the dc current has been changed to the normal current level for night operation. At the central station, a change in the current level from normal daytime to normal nighttime current is sensed, activating an oscillator. An electronic current limiter in the communications line responds to the oscillator through an isolation circuit, oscillating the current in the communications line, which activates a buzzer at the remote premises.
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