| 摘要 |
PURPOSE:To eliminate the need for a special control circuit for releasing privacy call by providing an optical coupling thyristor SCR to a privacy call circuit SC to control the conduction by light irradiation not only by its gate current, thereby releasing privacy call by a DC signal. CONSTITUTION:In hooking off a handset provided to a talking circuit network NW for outgoing to a trunk line in a telephone set TEL1, since a trunk line voltage higher than the Zener voltage is fed to a Zener diode ZD, the ZD is conducted. A voltage drop is produced across a resistor R1 and the optical coupling thyristor SCR is conducted. The privacy call circuit SC is driven in this way, a trunk line channel of the telephone set TEL1 is closed and the trunk line is acquired. When another telephone set TEL2 or a general telephone set TEL3 hooks off a handset to acquire a trunk line and activates the contact of a hook switch in this state, since a line voltage between trunk line connecting terminals L1 and L2 is lowered considerably by the telephone set TEL1 being acquired before, the Zener diode is not conducted and then no gate current of the optical coupling thyristor SCR flows, the optical coupling thyristor is not conducted. |
