Switching control apparatus for intercom-telephone sets

A switching control apparatus for intercom-telephone sets having a speaker-phone circuit coupled with a speaker and a microphone, a dial circuit with a keyboard electrically connected to the speaker-phone circuit, a talking circuit with its output connected to an input of the speaker-phone circuit, and a pair of voltage regulating circuits connected in series between an AC power source and the talking circuit, comprising:

• a pair of resistors one end of each being respectively series-connected to a pair of incoming telephone lines at one end and the other end of each resistor being shunted by a first surge absorber for protection against high voltage;
• a rectifying circuit having its input connected to both output terminals of said resistors and its output coupled with a first switch of the intercom-telephone set for allowing low-voltage incoming signals to pass therefrom;
• and
• a silicon controlled switching circuit having its anode connected to an output terminal of said rectifying circuit, its cathode connected to a contact of said first switch, and its gate coupled with a unilateral conductive element of the talking circuit through a second surge absorber and a transistor circuit for performing switching control operations of the intercom-telephone set.