Importance

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Abstract

Disclosed is a dialing apparatus used for a power failure extension telephone set 30 in a key telephone system. The power failure extension telephone set 30 is driven by power supplied from a key service unit 2 so as to function as an extension telephone set when power is supplied to the key service unit 2, but directly connected to an office telephone line 1 so as to function as an individual telephone set when power is not supplied to the key service unit 2. The dialing apparatus comprises a dial switch 31 of single contact-layer structure and a control switch circuit 21 connected to the dial switch 31 and changed over according to the power supply or power failure condition of the key service unit. That is, when power is supplied to the key service unit 2, the control switch circuit 21 connects the dial switch 31 to a control circuit 19. This control circuit 19 is activated by power supplied from the key service unit 2 and detects the state of the dial switch to transmit a control signal to the key service unit. On the other hand, when power is not supplied to the key service unit 2, the control switch circuit 21 connects the dial switch 31 to a dial signal transmit circuit 20. This dial signal transmit circuit 20 is activated by power supplied through an office telephone line 1 and detects the state of the dial switch whenever a handset 16 is off-hooked, to transmit a call to the office telephone line 1.

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First Claim