> a 100µA load that's 6250 hours, or about 260 days.
If 260 isn't enough, you can go even further by taking a small cheap MCU
like a picmicro, use it's internal Watchdog timer to handle the timing, and
then drive the peizo from the pic using a FET like the bss123.
The only tricky part is getting a low power Regulator to power the pic. I
used the tps715xx from TI, although it's quite a challange to put them on
your PCB by hand :). Maybe you can use a zener for this, but my personal
experience with them is quite limited and the experience I do have usually
involves needing alot more current then the 4uA from the LDO.
All this should not draw more then 4 to 5 uA when idle. So it could work for
many years on a single 9V block. Assuming of course your piezo doesn't take
up too much energy.
Goodluck to you.