In a bunch of tutorials using photocells you see a resistor being used like a voltage divider.
When I first started plating with photocells I wasn't using an additional resistor, I was just plugging straight into Analog on the Arduino and measuring that.
So why the additional resistor - does it make the measurement more accurate?
A photocell sources current, which translates into a voltage when routed through an resistor. The open-circuit voltage is not very precisely related to the illumination of the cell.
The photocell in the link is a LDR. This does not source current.
The second resistor from 5v to the analogue input and the LDR to ground gives the voltage divider to be read.
Why it worked without the fixed R, I don't know but it would have to be something to do with the internal circuitry.
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