You don’t need to use ADC … however, using ADC gives you a second dimension to your data. Not only can sense location, you can sense distance. I use the ADC to read the state of charge of the led junction. You can also do this with a digital input pin, by timing how long it takes for the charge to decay. For me, programming the ADC routines was easier than writing a precision timing loop.

It’s been a few years since I’ve read the MERL work, but I’m guessing the used one of the 509′s schmitt-trigger inputs to read the state of the LED. From the 509 datasheet, the ST input buffer recognizes 0 to 0.15v as a “low” and 0.85 to Vdd as a “high”. A good red led will generate about 1 volt when illuminated by another red led at close range.

I’ve just been reading up on your LED as sensors posts which is really really cool. I started off by reading this research article (http://www.merl.com/reports/docs/TR2003-35.pdf) explaining the basics. One thing I noticed was that they don’t really mention that they’re using an ADC they just suggest you can use any I/O pins, which was one of my first issues, as I know you encountered, since many PICs only have a few if any ADC inputs. In the demo ckt they show they are using the pic12c509 which has no ADC inputs!! So that leaves me to think they must be using some more clever way to utilize all I/O pins? I havent read all of your logs but have you investigated that? Possibly using a simple interrupt to detect when the voltage is a logic ’0′ input and use that to indicate the LED capacitance has discharged? Just some thoughts. I’d love to hear what you think. I havent really had a chance to investigate myself. Just curious. Thanks!!!