Arrangement for voice-frequency signalling in telephone systems

Claim 1

In an automatic telephone system, the combination comprising: at least one subscriber instrument, said subscriber instrument including first and second terminals, a speech circuit having an input terminal and an output terminal, one of said terminals of said speech circuit being connected to said first terminal, a series circuit including at least one PNPN-diode and a voicefrequency transmitter, one end of said series circuit being connected to said first terminal, a key set including at least one key-operated switch which normally connects said second terminal to the other terminal of said speech circuit and when operated connects said second terminal to the other end of said series circuit; a subscriber''s line pair connected to said terminals of said subscriber''s instrument; and a telephone exchange comprising at least two terminals connected to said subscriber''s line pair, a voice-frequency receiver, a relay means actuated for a given period of time when said key-operated switch is operated, said relay means including means for transmitting to said subscriber''s instrument a voltage surge when said relay means is actuated, said voltage surge being sufficient to cause said PNPN-diode to conduct and remain conducting until said keyoperated switch is released and said relay means further including means for connecting said voice-frequency receiver to said terminals of said exchange during said given period of time.