| 摘要 |
PURPOSE:To remove an unnecessary sound at the time of a transmitter-receiver and and to prevent the output of a transmitter from being applied to the receiver by cutting off an input signal to the receiver first when a transmission state is entered, and cutting off a speech signal to the transmitter first when a reception state is entered. CONSTITUTION:The output pulse width of a monostable multivibrator 3 and a monostable multivibrator 4 is set wider than the time when chattering is caused. The chattering of the output 51 of an OR circuit 5 and the output 61 of an EXNOR circuit 6 is eliminated. Then when a transmission-reception changeover switch 1 is turned on, an analog switch 8 is turned off and an analog switch 12 is turned on, so a microphone 13 is connected to a transmitting circuit 11 to enter the transmission state. Further, when the reception state is entered, on the other hand, the transmission-reception changeover switch 1 is turned off, the analog switch 12 is turned 'OFF', and the analog switch 8 is turned on. Consequently, the application of an excessive current from the transmitter to the receiver is evaded and an unnecessary sound from a speaker is cut off.
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