Contactless keyboard

Contactless keyboard comprising a plurality of keys;

• a plurality of key transformers each associated with one of said keys and including a primary and a secondary winding and a plunger-type core movable relative to the respective key transformer between a retracted and an extended position in response to the operation of the associated key;
• a plurality of individual first electric branches each connecting said primary windings of a first group of said key transformers in series with one another;
• a plurality of individual second electric branches each connecting said secondary windings of a second group of said key transformers in series with one another such that each of said key transformers of any one of said first groups is a member of a different one of said second groups;
• a source of alternating electric current;
• means for distributing the alternating electric current in succession to said first branches for the electric current to flow through said primary windings and induce voltages of a first direction in said secondary windings of said key transformers of the respective first group, such voltages being of a higher first magnitude with the respective core in said extended position and of a lower second magnitude with the respective core in said retracted position, said distributing means including a connecting member;
• a compensating transformer common to said key transformers and including a primary winding connected in series with all of said first branches and a secondary winding connected in series with all of said second branches, said primary and secondary windings of said compensating transformer being so arranged relative to one another that flow of the electric current through said primary winding thereof induces in said secondary winding thereof a voltage of an opposite second direction and of a magnitude between said first and second magnitudes with attendant phase reversal in the respective second branch upon movement of said core between said extended and retracted positions;
• selecting means including a selecting member having an output and operative for connecting said second branches in succession to said output to supply output signals thereto;
• and evaluating means connected to said output and operative for evaluating the phase of the output signals, including phase comparison means operative for comparing the phase of the respective output signal with that of a reference signal.