Since there isn’t really much Information on any multitouch LED display, we started off with some background research on the subject. The paper by Paul Dietz, William Yerazunis, and Darren Leigh talks in detail about using LEDs as both input and an output.
Part A of the above diagram illustrate the noramal operating condition a typical LED driver. The current is flowing from the cathode to the anode of the LED and the LED is emitting light. However LED is also a photodiode and therefore it is sensitive to light.
Part B of the diagram illustrate a LED that is reverse biased. Under such conditions, the LED acts as a capacitor and the reverse biases charges the capacitance.
In Part C of the diagram, the I/O pin that is previously at VCC is switched to input mode. This allows the photocurrent to discharge the capacitance built up in the LED. By timing how long it takes the LED to discharge we can measure the amount of photocurrent. (Paul Dietz, William Yerazunis, Darren Leigh) If we extend this to a LED matrix, then it is possible to create a multitouch LED Display.
References:
E. Fred Schubert, Light-Emitting Diodes
Paul Dietz, William Yerazunis, Darren Leigh, Very Low-Cost Sensing and Communication Using Bidrectional LEDs
Jonathan Pak, The Light Matrix: An Interface for musical expression and performance.
Scott E. Hudson, Using Light Emitting Diode Arrays as Touch-Sensitive Input and Output Devices
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