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A circuit assembly detects a touch point within a display area. A plurality of light emitters are disposed around a periphery of the display area. A plurality of detectors are disposed around the periphery, each detector being opposed to a respective light emitter. Switching circuitry for connecting

A circuit assembly detects a touch point within a display area. A plurality of light emitters are disposed around a periphery of the display area. A plurality of detectors are disposed around the periphery, each detector being opposed to a respective light emitter. Switching circuitry for connecting the light emitters and detectors in a switching matrix has a plurality of matrix rows and matrix columns. The switching circuitry includes a plurality of row lines, each row line corresponding to a respective matrix row and a plurality of column lines, each column line corresponding to a respective matrix column. Low-side switching transistors selectably couple the light emitters and detectors to the row and column lines, respectively. A common modulated signal is coupled to the anodes of all the light emitters. A detection signal is provided by connecting the collector terminals for the detector phototransistors in common. Consequently, high-side multiplexing circuits are avoided.

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1. A circuit assembly for detecting a touch point within a display area, comprising:a plurality of light emitters disposed around a periphery of said display area;a plurality of detectors disposed around said periphery, each detector being opposed to a respective light emitter; andswitching circuitr

1. A circuit assembly for detecting a touch point within a display area, comprising:a plurality of light emitters disposed around a periphery of said display area;a plurality of detectors disposed around said periphery, each detector being opposed to a respective light emitter; andswitching circuitry for connecting said light emitters and detectors in a switching matrix having a plurality of matrix rows and matrix columns, wherein said switching circuitry includes:a plurality of row lines, each row line corresponding to a respective matrix row;a plurality of column lines, each column line corresponding to a respective matrix column;a plurality of light emitter transistors each coupled between a respective light emitter and a respective row line, each light emitter transistor having a control input coupled to a respective column line and responsive to a respective column select signal for connecting its respective light emitter to said respective row line;a plurality of pull-up resistances each coupling a respective one of said row lines to a predetermined voltage; anda plurality of row transistors each coupled between a respective row line and ground, each row transistor having a control input coupled to a respective row line and responsive to a row select signal for selectably connecting its respective row line to ground.2. A touch screen display comprising:an electronically-controlled visual display panel for presenting visual information within a display area;a touch-point circuit assembly including:a plurality of light emitters disposed around a periphery of said display area;a plurality of detectors disposed around said periphery, each detector being opposed to a respective light emitter, said detectors having respective output terminals connected in common to provide a detection signal; andswitching circuitry for connecting said light emitters and detectors in a switching matrix having a plurality of matrix rows and matrix columns, wherein said switching circuitry includes:a plurality of row lines, each row line corresponding to a respective matrix row;a plurality of column lines, each column line corresponding to a respective matrix column;a plurality of light emitter transistors each coupled between a respective light emitter and a respective row line, each light emitter transistor having a control input coupled to a respective column line and responsive to a respective column select signal for connecting its respective light emitter to said respective row line;a plurality of pull-up resistances each coupling a respective one of said row lines to a predetermined voltage; anda plurality of row transistors each coupled between a respective row line and ground, each row transistor having a control input coupled to a respective row line and responsive to a row select signal for selectably connecting its respective row line to ground; anda controller for selectably activating pairs of said light emitters and detectors in a sequence to detect said touch point.3. The touch screen display of claim 2 wherein said light emitters are comprised of light emitting diodes having their anodes connected in common.4. The touch screen display of claim 3 further comprising a power modulator for supplying modulated power having a predetermined modulation frequency to said commonly connected anodes of said light emitting diodes.5. The touch screen display of claim 2 wherein said switching circuitry further includes:a plurality of detector transistors each coupled between a respective detector and a respective row line, each detector transistor having a control input coupled to a respective column line and responsive to a respective column select signal for selectably connecting its respective detector to said respective row line.6. The touch screen display of claim 5 wherein said touch-point circuit assembly further comprises:a ring-shaped printed wiring board (PWB) having a central aperture coinciding with said display area, said PWB having said light emitters and said detectors mounted around a periphery thereof, wherein each of said respective light emitter transistors is mounted on said PWB proximate to its respective light emitter, and wherein each of said respective detector transistors is mounted on said PWB proximate to its respective detector.7. The touch screen display of claim 6 wherein said PWB includes conductor traces routed around said periphery to provide said row lines and said column lines.8. The touch screen display of claim 2 wherein said detectors are comprised of phototransistors having their collector terminals connected in common for providing a detection signal.9. The touch screen display of claim 8 further comprising an active clamp coupled to said detectors to receive said detection signal, said active clamp eliminating residual charge between activation of said selectably activated pairs.10. The touch screen display of claim 2 further comprising an active inductor coupled to each of said detectors, said active inductor providing a highpass filter.11. The touch screen display of claim 10 further comprising a differential peak detector coupled to said active inductor and responsive to said reception signal for detecting when a touch point between a selected light emitter and a selected detector is obstructed.12. A circuit assembly for detecting a touch point within a display area, comprising:a plurality of light emitters disposed around a periphery of said display area;a plurality of detectors disposed around said periphery, each detector being opposed to a respective light emitter; andswitching circuitry for connecting said light emitters and detectors in a switching matrix having a plurality of matrix rows and matrix columns, wherein said switching circuitry includes:a plurality of row lines, each row line corresponding to a respective matrix row;a plurality of column lines, each column line corresponding to a respective matrix column;a plurality of light emitter transistors each coupled between a respective light emitter and a respective row line, each light emitter transistor having a control input coupled to a respect column line and responsive to a respective column select signal for connecting its respective light emitter to said respective row line;a plurality of pull-up resistances each coupling a respective one of said row lines to a predetermined voltage;a plurality of row transistors each coupled between a respective row line and ground, each row transistor having a control input coupled to a respective row line and responsive to a row select signal for selectably connecting its respective row line to ground; anda plurality of detector transistors each coupled between a respective detector and a respective row line, each detector transistor having a control input coupled to a respective column line and responsive to a respective column select signal for selectably connecting its respective detector to said respective row line.13. The circuit assembly of claim 12 further comprising a ring-shaped printed wiring board (PWB) having a central aperture coinciding with said display area, said PWB having said light emitters and said detectors mounted around a periphery thereof, wherein each of said respective light emitter transistors is mounted on said PWB proximate to its respective light emitter, and wherein each of said respective detector transistors is mounted on said PWB proximate to its respective detector.14. The circuit assembly of claim 13 wherein said PWB includes conductor traces routed around said periphery to provide said row lines and said column lines.15. The circuit assembly of claim 12 wherein said light emitters are comprised of light emitting diodes having their anodes connected in common for connecting to a current source.16. The circuit assembly of claim 12 wherein said detectors are comprised of phototransistors having their collector terminals connected in common for providing a detection signal.