Irgendwie habe ich wohl mit meinen Antworten die Luft herausgenommen, normalerweise müßten sich die Vorschläge nur so drängen.
Dann sollte ich noch die Methode zur Messung kleiner Photoströme ansprechen, bzw zitieren.
http://www.thebox.myzen.co.uk/Workshop/LED_Sensing.html
Practice
... The photo current is very small in an LED because the geometry of the junction is not designed to maximise it like it is in a photo diode. What we need to do is to find a way to integrate (a posh way of saying add up) all the little photo currents over a set time. To do this we use a trick, we make the diode look like a capacitor, those conducing regions separated by an insulator look like a capacitor. So we charge it up and then time how long it takes the photo current to discharge it, just like this:-
This involves a bit of electronic gymnastics with the ends of the LED, but as the Arduino has configurable pins this is easily done in software. The other trick is instead of waiting for the capacitor to discharge enough to give a logic zero, which can be a long time, simply measure the voltage across it, after you have given it a short time to discharge. I call this the integration time because it is integrating (adding up) all the small photon currents.
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