GB1311713A - Multiple choice display systems - Google Patents

Abstract

1311713 Cathode-ray tube displays; touch operated switches SANDERS ASSOCIATES Inc 23 June 1970 [28 July 1969] 30534/70 Headings H3B and H4T Sensing wires affixed to the screen of a data display cathode-ray tube 14 (Fig. 1) are connected to respective detectors 20a-20d which respond selectively to the body capacitance of an observer touching one of the wires and emit a signal for controlling the actual data displayed via a data processing unit 10 and a display unit 12 containing D./A. converters and the deflection yoke for the tube. The data comprises a question and a number of (four) answers, the latter being displayed in register with a respective sensing wire and the processing unit operates such that the content of a particular question (and multiple answers) displayed may be determined by the answer given to a previous question. The detectors are identical and comprise, as shown in respect of detector 20d, a "Programmable Unijunction Transistor (PUT)" 24 which is biased by means of battery 28 and resistor 26 to the region B (Fig. 2) and the anode threshold voltage at which the diode between the anode 24a and a gate electrode 24g becomes forwardbiased to fire the transistor is controlled by a voltage divider comprising resistors 34, 36. In operation, when the circuit is loaded by the operator body capacitance (in series with capacitor 35) to provide a "timing capacitor" the latter charges and reduces the current flowing through the transistor whilst the voltage across the timing capacitor increases and the operating point moves in the negative resistance region from B towards the point A. When the timing capacitor becomes fully charged the current flow through the transistor increases and the operating point returns towards point B. This cycle then repeats, the transistor operating as a relaxation oscillator, until the "timing capacitance" is removed by the operator removing his finger from the selected wire. The output of the circuit is connected to the data processing unit 10 through an impedance matching transistor amplifier 42.