| 摘要 |
PROBLEM TO BE SOLVED: To provide a video intercom which attains a stable operation even when the impedance of a power line is fluctuated. SOLUTION: When a call switch of a slave unit is closed, a control power supply VA on the power line 9 gradually increases from a voltage of 8V. When control power supply VA exceeds a prescribed first level V1, a hysteresis circuit 18 is activated and a discrimination voltage VB rises to a voltage of 5V. An ON/OFF discrimination circuit 19 activates an internal power supply circuit 17 after a lapse of a prescribed time by a delay circuit 21 to supply the internal power supply voltage VC of 5V to each circuit in the slave unit. After that, when the video intercom is turned off, the control power supply VA on the power line 9 is gradually decreased and when the control power supply VA reaches a prescribed second level V2 or below, the internal power supply circuit 17 is turned off and the ON/OFF discrimination circuit 19 is also turned off. COPYRIGHT: (C)2004,JPO
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