The signal comming from a piezo is really uneven. First of all in order to be able to compare the signals, they need to have some similarities. I

found that the signal is almost a sinewave (with decreasing amplitude). Here is a screenshot of my (little) oscilloscope :

My idea was to rectify the signal so that I only have to care about amplitude.

I used a rectifying bridge (Gretz Bridge) in order to do that.
What a noisy signal !

After passing it through an amplifier and an RS latch :

Now we are talking !