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Old 8th March 2007, 09:42 PM   #1
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Default LED matrix input device, how do they do it?

http://cs.nyu.edu/~jhan/ledtouch/index.html

it shows an LED matrix being used as an input device.
i want to make a USB input device using some of these.I have many of them from some LED signs that were thrown out because the cpu boards were toast.

i cant figure out what is actually happening, they say the LED also acts as a light dependant diode, but once the LED is off to measure, where does the light come from to measure?
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Old 8th March 2007, 10:46 PM   #2
SY is offline SY  United States
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The on-off time of LEDs is quite fast. If you run them at a lower duty cycle, you can use the "off" time to photodetect. I haven't read the Mims paper, but ambient light might work. Even better (I think) would be to scan, i.e., turn the LEDs off one at a time and use the remaining light of the others as illumination.
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Old 8th March 2007, 11:15 PM   #3
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The diodes can operate as emitters and detectors simultaneously. There will be small fluctuations in the voltage across the LED if supplied with a constant current, depending on the amount of light hitting the LED. The light may be from another source, or just light from the LED reflected back to the LED.

Several months ago I saw an article where someone used lit LEDs as an input device to a uC. I think it may have been in Circuit Cellar.

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Old 10th March 2007, 04:41 PM   #4
cpemma is offline cpemma  United Kingdom
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http://www.merl.com/publications/TR2003-035/

And some DIY testing here.
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