The 10K or lower impedance is what the analog input 'wants'. According to this data sheet PDF here the DC resistance can be as high as 100M Ohm. But, the 10K or lower is the recommended maximum DC external impedance. Hence the 10K resistor. You could go lower if there was a need, IOW if your photocel required a lower load resistance, perhaps for linearity at a particular wavelength.Because the external recommended resistance is 10K or higher, there is really not a reason to get too hung-up on this part of the circuit. The biggest challenge is to breadboard the photocell, a 10K, and then do some testing with the light-source you intend on using. The ceiling and floor values are going to be experimental ones.Ray
All analog inputs on all analog input [EDIT (such as ADCs) ] semiconductor devices are designed to be high-impedance ("your results may vary"). High Impedance in general would be considered 1 Mohm => 100 M 0hm with 10 Mohm probably being more common. This is why Mr. B. said "forget about it !...." The only thing you need to be careful about is applying any voltage which exceeds the arduino allowed range for Analog Input , which 0 V to 5V . Stay in that range and your golden.
I've been told i need some kind of amplification for the photodiode.
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