Using op amps like that is producing AC on the output
you could double the output by putting 2 LEDs parallel but revers one of the LEDs so that light is produced on both halfs if the cycle
or run the output through a bridge rectifier before a single LED would make it brighter too
Actually, since it's a single ended supply, and no coupling cap at the output before the LED, the LED only sees strictly fluctuating DC, so one of the two LED's will never light up because it will always be reverse biased.
The bipolar LED or bridge idea would work with the split supply version earlier in the thread, however, and in fact actually is drawn as such.
As far as a peizo generating 1000V, while this is entirely true, it only really applies to a specially designed type of peizo crystal assembly. A buzzer element would shatter with the force it would take to get that spike.
But, even with that being said, if it were to ever see 1000V from a peizo at the input, a 1K resistor would represent little more than a short, dropping next to nothing from it (the op-amp's input impedance is MUCH higher than 1K, so there would be little if any drop across the resistor).
If you're worried about it, a 1K might be prudent in
parallel (ie from pin 2 of the op-amp to ground).
EDIT - On second glance, it a resistor there will probably do more harm than good, as it will have a significant effect on the feedback provided by the gain pot. I'd just leave it out.
Steve