X=Y graphics tablet using conductive film - which is contacted to resistive plate to give voltage differentials dependent on distance of contact from centre of plate

摘要

The graphics tablet converts the pressure exerted by a pointed marker to analog electrical signals which define the pressure point in X,Y coordinates. A flexible conducting film (2) with a potential (V) applied to it through a resistor (R1) is stretched a few tenths of a millimetre above a resistive plate (1) which has terminals along each of its four sides linked through adjustable resistors (p1-p12) so that the resistors on any side are all connected via a resistor (R2) to earth and to an input of a differential amplifier (3,4). When the marker connects the conducting film to the resistive plate a voltage (V) is applied at that point in the plate causing a voltage to appear at the input of a differential amplifier. The voltage will depend on how far the contact point is from the centre of the resistive plate. Two differential amplifiers give X,Y coordinates as analog output voltages. The resistors connected to the edge of the plate (p1-p12) may be adjusted to compensate for material variations. A Schmitt trigger circuit (5) connected to one amplifier input can be used to signal data availability.