Thanks Geoytex, I saw the White Paper earlier but hurried by a little. I will read it a little closer, but it doesn't seem to lend itself to a significant number of sensors due to the component count.
Chipwich said:
What do you mean that the LED could sense it's own output? Laser diodes use a separate photodiode to sense output power and provide feedback. But I don't see how you could use just a single diode to perform both input and output simultaneously.
Did I actually say that . . . seemingly I was very close. I meant that the environment could be sensed at the same time/alternating with periods of illumination , but failed to express myself clearly; sorry!
More a case of sequentially being an input or output device in a manner that appears to be simultaneous to the viewer/user. The depletion layer of the LED PN junction can be charged and will act as a very small capacitor. The discharge time can be measured giving an indication of the capacitance magnitude and by extrapolation the amount of the charge on the junction. This magnitude could then be used to pwm the diode and so mimic the degree of illumination to some degree. Not the effect that we are actually using, but another possibility I guess, but would necessitate the use of opamps etc, become very complicated and not want to scale well for nefarious arrays of wizardry that someone will doubtless wish to introduce. I just dream about these things
Varicap/varactor diodes use the same reverse biased PN junction, depletion layer capacitance to good effect. Like all specialised diodes, the junction is tailored to the task. In the same way that a 1N4001 rectifier diode is a poor man's varicap diode, then our LED is being recognised as a potentially economic light sensor.
You will have to forgive me the fractional precision nature of my description, but I believe that I am somewhere close if not exactly in the target zone
Good picture at the bottoml
Chipwich, I will digest your link in the light of day tomorrow as it is a bit late here and good lads should be inspecting eyelids from the inside about now.
The advice regarding the connection of the serial cable is probably invaluable and possibly also indicates that using another chip to drive multiple LED/sensors could be less than plain sailing. I appreciate the variable mileage from different types of LED and again take from your experience of establishing a light sensitivity threshold.
I imagine that white LEDs are enhanced with phosphors in the envelope that attenuate light in the reverse direction due to their presence.