Here's what's going on in this circuit, from left to right: First there is an op-amp (LF353) which is supplying a virtual ground at a little over 2.5v. The next op-amp buffers the hi-z output from the piezos and provide a little gain. Next a pair of 10k pots set a lower and upper threshold voltage. The next chip is a quad comparator (LM339) which triggers when the sensor voltage crosses the threshold. The last chip on the breadboard is an "or" gate (7432) which combines the outputs from the comparator, so I end up with just one output per sensor. Which is good, because my microcontroller has only two external interrupts. At bottom is the circuit drawing I made while I was building this circuit, which I don't expect is detailed enough to be useful unless you are used to looking at circuit diagrams.