Can I measure multiple photoresistors from one pin?
If response time is critical (on the order of about a quarter of a second) consider using photo diodes instead of Cadmium sulfide cells.

Also, consider using an Arduino with more ADC ports.  The number of ADC ports is usually a constraint of the processor used.  Different Arduinos use different processors.  Some may have more ADC ports.

Finally, to utilize a single ADC port for multiple unknown resistances, consider using a resistor network where all the unknown resistances and a known pull down resistance are connected to the ADC port. Then, in turn, using as many Arduino digital pins as there are unknown resistance, apply a digital high to only 1 unknown resistance. Keeping the digital pins to all the other unknown resistance in a high impedance state. That is, change them from OUTPUT to INPUT digital ports. Then take an ADC sample. Apply this same procedure for all the other unknown resistances. Consider adding a delay after changing the state of the digital pins to allow for the ADC input to acclimate to the new voltage.







simulate this circuit &ndash; Schematic created using CircuitLab