Abstract: The invention relates to a contact display panel circuitry in which the phototransistors are on two adjacent sides of the contact display panel and the diodes emitting infrared radiation to the phototransistors are on the opposite sides, and in which, under the control of shift registers, each phototransistor in turn is coupled to a common line, the logic circuit of the device being capable of computing the location of a possible contact point on the basis of changes in the signal in the common line. According to the invention the use of controllable switches and numerous resistors and capacitors is avoided by coupling in series with the phototransistor a diode in such a manner that the collector of the phototransistor is coupled to the transfer register and the emitter is coupled via the said diode to the common line.

Abstract: The invention relates to a circuitry for compensating the effect of external light on infrared-sensitive phototransistors in a contact display panel, the phototransistors being coupled via diodes to a common line. The compensation circuitry is a voltage-dependent resistor between the common line and the ground of the circuit, the resistance value of the resistor decreasing as the direct-voltage level of the line rises. The resistor is preferably a transistor circuit.