Pulse sensor and input voltage

Good morning,
I am building a system with stm32 wl55jc1 to count water flow with a pulse flow meter.
So I created two internal clocks in stm32 one to create a time window and the other that counts each pulse received from the sensor. At the trigger of the first clock I check how many pulses the second clock counted and get the instantaneous flow rate.
I put an LED in parallel with the signal to visually see the pulses.

The problem is that the count the clock makes and the pulses I see do not match.
I would like to ask for some documentation on the input voltage and so that I can design my RC circuit as best I can (as I am convinced that the problem is electrical).

thanks for your time,

greetings

Roberto