You can connect a piezo through a fullwave rectifier (4 diodes) to a BJT gate. The BJT charges a wire with no pulldown that goes to a digital pin in proportion to the piezo tap. The digital pin eats 1 uA per read, I made one that showed a very light touch taking 20 reads to change HIGH to LOW while a hard smack with a screwdriver butt took over 2000 reads.