This technique is rather simple, but probably not many are using it. Kathy Yang shared his experiments where LED matrix is turned in to drawing board. Arduino UNO and 74LS138 ic’s take care of addressing each pixel on LED matrix. Phototransistor is used to read pixel coordinates and so the light corresponding LED.
Whole trick lies upon scanning matrix lines with LEDs dimmed so it wouldn’t be looking as lit, but enough to be detected by phototransistor with comparator circuit. With a bit more polishing it could be really fun project to do with kids or even great learning kit to teach writing letters or numbers. Another use could be user interface, where one’s could change settings directly on display.