ADC preamp using Schmitt trigger, opamps and transistor reset
As you see, it works in the simulated case. You should do practical tests before building these more than a couple. If you are in noisy environment (vibrations, really LOUD sounds, radio transmitters, phones) you can get into troubles. 

Fast and easy to use sensitivity and decay time constant settings with proper visual indicators are a must if this will not be a part of some absolutely fixed installation.

If you have several of these monitoring adjacent targets, the dynamics can be a problem. Say target A gets weak and strong hits and the adjacent target B, too. If the spilled vibration from a strong hit to B is high enough to be noticed as a weak hit to A, you get a false triggering. Consider to do some spillage reasoning in the software. For example rise the treshold in A for few milliseconds if B has got strong hit. Some clever programming is needed to avoid too long calculation delays.