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Just a suggestion to your "UI".
On Power Up - if the gain is CORRECT - have the device "chirp" twice.
This way you get positive confirmation that the "gain monitor" is operational, and that your settings are correct. Otherwise if the board fails (broken wire to the Piezo anyone??), you may never know until it is too late.
Since the current is so low you could skip the diodes (which would have to be configured for each system) and put a small micropower LDO linear regulator in there. Then it would be a universal solution.
You may need to put a current limit resistors on the I/O line(s) as well, I have no idea what the output impedance of the servo signal line is, but if it can source more than 1ma of current it may fry the I/O line because the voltage will be over the MCU's VCC from the diode or the regulator.
Mike is that piezo device a buzzer (you just apply DC) or does the PIC drive the buzzer with a timer?