[G6Tanuki]

Maybe you could also consider an 'electret' mic insert [as fitted to both landline and mobile telephones since the 1980s] with a suitable bias-battery?

There again, it's amazing the output of some piezo transducers: a friend's bathroom-scales use four of them. Not only do they sense the avoirdupois of the person standing on the scales, but they generate enough voltage to power the scales. There's no battery, you just tap the platen on top of the scales a few times to wake them up, then you climb on board and the energy of your wakeup-knocking powers the LCD and its associated microprocessorage for long enough to tell you just how overweight you are.