It does take a definite tap on a cheap piezo disk to make a led blink and doesn't seem to harm it. Connect a led directly to the piezo wires and tap the disk, the led flashes. It also works with led through a diode bridge then a cap to flatten the spike, flashes twice and shines longer.
The tap on the piezo can exceed 5V for a short time. The led/circuit eats the spike and makes light that the Arduino can detect safely with few parts.
I'm wondering if opto-isolator leds can take it. Connect the other side to digital pins and time the HIGH pulse = strength of tap totally pin safe. Will signal slowly eat the chip?