> A piezo disc has a current that is too low to light an LED
FYI, a 1" diamater piezo directly connected to a high brightness red LED will definitely produce a visible (quite strong actually) flash if you tap it with your finger (or hit it against the table) ; the more violent you get, the more light !
Anyway, to use it as a switch you'd need a rather strong debouncing (since the thing resonates a lot) and something like a flip-flop to switch states (on/off).