Measuring veloctiy by detecting objects with an analog sensor
Expanding to an answer,

The normal approach would be to use the microcontrollers analog comparator (built in to most 8/16 bit chips), one leg has an RC to represent the average voltage with a slight load to reduce the voltage on the RC by a little, so the signal needs to fall a certain amount before triggering, the other is the input possibly through the ADC mux for the older AVR style microcontrollers

Example circuit: Falstad Circuit Simulator Link

This is relying on the assumtion your sensors are fairly well matched to one another, if they are not, you would either need a dedicated external comparator per input, or an analog switch to move the input to AIN0's RC network as you switch ADC mux inputs.