| 摘要 |
PURPOSE:To detect the talking state by allowing a talking state detector to detect a loop current directly and display it when a subscriber hooks off a handset so as to display that the telephone set is in the hook-off state. CONSTITUTION:At the time of the handset of a telephone set 3 is hooked off and the off-hook state is attained, a loop current I flows through the telephone set 3 via a talking state detector 2 from a station 1 and a Zener voltage V is generated between terminals of the 2nd Zener diode 26. Thus, a voltage (V-Vbe) is generated across a resistor 23 via an emitter of a transistor (TR) 22 and a current of (V-Vbe)/R1 (resistance of resistor 23) flows to an emitter circuit of the TR 22, then, the collector circuit, supposing that the current amplification factor beta of the TR 22 is sufficiently large. Since the V in equation is a terminal voltage of the Zener diode 26, the current is constant and the current flowing to a light emitting diode 21 is kept within a prescribed range decided by the brightness and rated power of the light emitting diode 21.
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