i've been trying to do a velocimeter with millis function but i just can't get a time interval to measure the frequency of the activation of the sensor, is there any way to measure this value?
Detect the state change of the sensor (the IDE comes with an example for a button but the principle can also be used fir a sensor).
Record the millis when the sensor reading goes from e.g high to low; this is the first start time.
Repeat and you will have a second start time.
From there, you can calculate duration and frequency.