Voltage Dividers and Photocells

A voltage divider is the basis of many sensor circuits, such as photocells, pressure sensors, and thermal sensors:

Picking a Value for the Fixed Resistor:

Measure the highest value(R max)and lowest value (R min)of the photocell or other resistive sensor.

The optimal fixed resistor will be the square root of (Rmin x R max)

Another way to look at it is that the output divider Thevenin resistance is 0.833 times the lower resistor value, so as long as that value is no more than about 1% of the atmega input resistance you should be okay.