Pushbutton telephone keyboard adaptable to conventional dial appts. - uses electronic switching circuit with transmission inhibit facility for digital dialling pulses

摘要

The keyboard connects to points already present in existing telephone sets without requiring the latter to be modified and without employing a relay. The keyboard has a two-part circuit. The first part (E1) is an electroic switch controlled by a control. The switch is in series between the line (L1) and the transmission circuit (7) of the telephone set. The second part (E2) comprises two subcircuits actuated alternately by the control depending on whether a number is being selected or a conversation is in progress. The first subcircuit (E21) is a high-impedance 2-pole (conversation) and the second subcircuit (E22) is an electronic switch, normally closed but opened in sync. with number selection. The switch is made in V-MOS technology and is controlled by a bipolar voltage transistor.