I recently started a project that utilizes a piezo sensor to sense vibrations. The piezo signal is fed to a uC in order to detect vibrations above a certain threshold. However, the piezo's high voltage spikes may damage the pin so I added a schottky diode for reverse voltage protection and a zener diode for spike suppression.
The initial circuit looked like this:
(The buffer is not necesarry, but it protects the uC)
Now here's the problem: When the uC is turned off, the piezo may still exhibit voltage when hit. Since Vcc is at 0V, the buffer or uC's internal protection diodes will be killed and damage the IC. Therefore, I need a circuit that isolates the piezo from the uC when there's no supply voltage.
Is there a simple solution to deal with voltages when Vcc=0V? It cannot draw more than 50uA when turned on because everything is powered by a coin cell.