Electronic identification assemblies of capacitive keyboards

Claim 1

An electronic assembly associated with a capacitive keyboard for identifying keys of this keyboard which have been depressed, said keys being distributed in a matrix arrangement so as to be disposed at intersection points of a checkerboard pattern formed of m lines and n columns, said assembly comprising means for successively applying to the inputs of the m lines trains of n successive identical pulses spaced apart mutually in time by identical time intervals t, means for successively collecting from the outputs of the n columns the signals created in response to said pulses including echoes produced when keys are depressed, the n outputs being for this purpose scanned successively at the time intervals t and this scanning being renewed m times in succession, means for detecting and identifying, among the collected signals, echoes which exceed a predetermined threshold and which correspond to depressed keys, comparator means, having a first input and a second input, for comparing the amplitude of said signals, received at said first input, with that of a reference voltage, received at said second input, and for selecting only the echoes whose amplitude exceeds that of the reference voltage, and means for automatically giving at all times to the reference voltge an amplitude related to the mean of the amplitudes of the signals outputted at that time at at least two of the outputs of the columns, said assembly further comprising averaging means for giving at all times to said amplitude of the reference voltage a value which is an average over a time period of said mean of the amplitudes of said signals.