I'm making a light detector. I plan on just using a photodiode and a resistor, tapping off between the two and using an analog pin to sense the voltage...it should be zero volts in the dark and full battery voltage in the bright light. I can use a pot for the resistor so I can adjust it.
I know the Arduino board will only measure between 0 and 5V, but I could hook the photodiode up to the 9V power supply or the 5V regulated arduino supply. What would be the advantage of using one versus the other?
OTOH, depending on the dark/light resistance of your sensor and the resistance of your pullup/pulldown resistor, 9V may result in a more sensitive operation, since you almost certainly won't get a full 0-5V output range with a simple pullup/pulldown resistor arrangement.
Why not? I will use a pot for the resistor so that I can adjust it. It seems like there should always be a resistance that lets me swing from 0 to 5V.