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Photocell dropping resistor size for photocell input to a Arduino UNO Answered

I purchased an Arduino UNO. The plan is to use a photocell

as and input to energize a relay as an output. The photocell I have has a value of 2.3K ohms when it sees light and 30K ohms when dark.

What value dropping resistor should I put in series with this photocell across 5 volts dc to gnd?

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Will it need a series resistor? 2.3K is already large enough when it sees light (2mA current). Maybe the question is not understood ...

Cdad's link contains exactly what you need to know.