Plodding through the net I came across an thought provoking little snippit..
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In 1977, Forrest M. Mims reminds us in one of his "Engineer's Notebooks" that LEDs can also be used as photodiodes...
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Then I found this and my jaw dropped, open mouthed scratching the stubble on my chin... OOOOOOOOOhhhhhh yes I said to myself, I like that.
I'm guessing he's using the reflected light as a proximity sensor. It's just like all the LED/phototrasistor paired proximity sensors, only using the LED as both the light source and the detector. It's interesting, but it's obviously limited just like a regular Prox sensor. Ambient light and changes in ambient light have to be adjusted for. Shadows, anything like that can cause unwanted readings, or make it unusable.
I like how he's using actual proximity reading and not just on/off sensing.
there's a couple threads from some guy who spent months experimenting with this. he says its' capacitive. he wound up concluding that it's not very cost effective. I'm just telling ya what he said .
Well, it looks like the Mad Prof added a PDF file which I assume is related to the video. The PDF describes using the LED's as photo emmiters and detectors, so in this case, I'm assuming it's not ussing capacitive sensing.
It's funny this topic comes up, I recently obtained a collection of most of Mims' Engineer's Mini-Notebooks, and although now 20-30 years old they hold up well and are full of great ideas and circuit designs. I don't have the one that details the detection experiment, but he does make references to led light sensitivity.
I got interested and did some Googling, here are some relavent links I have found...
Ambient light according to the text is less of a problem than anybody thought as the peak wavelength responce of the of the LED in "detector" mode is closely related to its emission characteristics. Sadly the the responce times may be a bit slow for high speed data exchange , but it certainly looks like a keypad or dare I say keyboard ,could be done.
I would be interested to hear what our resident PIC programers have to say,
and wonder how the process holds up when a Bi-Colour LED is used?
I was thinking that another application of using a LED as a light sensor may be to generate 'random' data to reseed a random number generater using the tapped shift register method. You could have it check the light level every so often, and continously reseed the shift registers on the fly. This should eliminate generating the same pattern of random bits (unless the light level is always exactly the same). Cheap randomizer...
