You can use a digital input. The switching might not be clean since there is a undefined state between 0V and 5V where the input isn't guaranteed to be 0 and isn't guaranteed to be 1. As your variable resistor changes and the voltage passws through that region you may get random switching. I think you can use a Schmidt Trigger inverter to clean up the input. It provides solid switching on a slowly changing signal. Each 74HC14 chip will do 6 channels.