Capacitative touch operated keyboard immune to interference - uses reception electrode signal difference to generate noise free output signal and interference level detection to inhibit key pad operation

摘要

The signals from the readout lines are taken to a subtractor which calculates the difference between the readout and a reference line in the keyboard. The difference output signal is used, together with the parasitic signals which have entered the keyboard from external electromagnetic sources removed. The keyboard remains sensitive to operation by a low capacitance touch. The keyboard consists of keypads surrounded by an earthed screen backed by pairs of emitting electrodes excited in sequence by an alternating signal and receiving electrodes (B) coupled capacitatively by the presence of a finger near the emitter electrodes. The reference electrode (21) may be one or the combination of several of the other readout electrodes such as the adjacent pair in a keypad matrix. Each readout line is integrated and compared with two reference levels. The comparator outputs are OR-gated and used to inhibit keyboard operation whilst interference persists.