[sdiy] super-nice LED interface ..

[Ian Fritz]

At 09:23 AM 10/13/05, Jay Vaughan wrote:
>>The array has alternating emitters and detectors, both make from
>>LEDs.  The finger causes light from the emitters to hit the
>>detectors.
>
>but .. all LED's are on in that demo, so does this mean that the array has 
>LED's that are 'off', but still being used as a detector, or does it mean 
>that LED's can be used as detectors even when lit?

Well, yes, it *looks* like they are all "on" in the photo.  But in the 
cartoon they are alternating.  Maybe the detectors are covered or just not 
visible because of the lighting conditions?

An emitter has to be forward biased and a detector has to be reverse biased 
(or unbiased), so there is no way I can see that an LED can be a detector 
while lit.  Another way to think of it is that a detector has a small 
current and an emitter a large one, so it would be impractical to measure a 
current component associated with detection in the presence of the forward 
bias current.

   Ian