| 摘要 |
PURPOSE:To cut off the trunk line current only by depressing one push button, by using one keyboard switch as an independent make contact and by controlling a semiconductor switch, which is inserted in the speaking circuit, by this contact. CONSTITUTION:The make contact of a button which is not used for dial numbers of a push-button dial DL is used as a switch S and is connected to a semiconductor switch HC1 inserted in a speaking circuit TC. When the handset is hooked off, hook switches HS1 and HS2 are turned on, and trunk lines L1 and L2 are connected to a diode bridge DB, and power is applied to the dial DL and the circuit TC through the switch HC1. When the switch S is depressed in case that an erroneous number is depressed on the dial DL, a transistor TRQ3 is turned on, and TRs Q1 and Q2 are turned off, and the trunk line current to the speaking circuit is cut off, and thus the same effect as turning-off of hook switches is obtained. |
