[chbot] Anyone tried LEDs as light sensors

[QtuTrains]

On 10/11/2008, at 7:54 , Richard Jones wrote:

> Any idea what the equivalent circuit of the reverse biased green  
> LED photo
> detector might be?

The same as all semiconductor junctions.

Remember the OC71 transistor (a while ago now :-) in the black  
plastic package. Scrape off the black paint and you had made a photo  
transistor.

A photon hitting a semiconductor (or probably any material) releases  
an electron (I seem to remember an otherwise stable electron receives  
energy from the photon causing it to move to a higher orbit, or even  
escaping the atom).

The free electron becomes a leakage current in a reverse biased  
diode, or a base current in a transistor (and hence amplified).

In a reverse biased state an LED is just a (very low breakdown  
voltage) semiconductor diode in a transparent or translucent case.

So I guess an equivalent circuit (for reverse bias only) might be:

1. perfect diode

in parallel with

2. Low voltage single use zener diode (reverse bias destructive  
breakdown condition). Somewhere between 5 and 12V I guess.

in parallel with

3. constant current small value leakage

in parallel with

4. variable current leakage directly proportional to the number of  
photons (visible or not, as filtered by LED housing) impacting the  
junction with sufficient energy received to release an electron.

I wonder if one can use a trio of LEDs (red, green, blue all with  
coloured housings) to make a colour sensitive light detector....

Howard
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