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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:

schematic

simulate this circuit – Schematic created using CircuitLab

(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.

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    \$\begingroup\$ Your schematic is readable, no doubt, but a word of advice for the future - input comes from the left and output signals exit to the right of a schematic. \$\endgroup\$ – winny May 15 '19 at 12:40
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Add another Schottky diode from the non-inverting input to the positive power supply rail. The Schottky diode will be reverse-biased in normal operation, but when the power is off it will conduct any spikes to the positive rail where the bypass capacitors will kill it off, and even if the user pumps it the zener will prevent the supply rail from going too high.

Since Schottky diodes have lower forward drop than regular diodes, almost all the current will go through the Schottky diode. You can use a single BAT54S dual diode to replace both D1 and the new diode.

Right now you have a 1N4148 for the negative side, which is not a Schottky and significant current will flow through the IC when the input goes negative, which could lead to latch-up and thence destruction of the IC.

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I would say this is industry standard. Works both in powered and unpowered state. A small resistor in series with any input comes to mind too unless you are aiming for MHz reacion speed. Often good for EMI purposes but need to be tested in circuit.

You can use the same BAT54S on the input pins of the MCU. They are dirt cheap.

schematic

simulate this circuit – Schematic created using CircuitLab

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You have already implemented clamp circuits that restrict the voltage at the input to within the range -0.7 volts to about +5.1 volts so, to protect the sensitive input when power is removed you need to place a resistor in series with the gate input.

To calculate the value you need to extract, from the data sheet, the maximum input current allowed for the device. That may be typically 1 mA and this would then lead you to choose a resistor of 5k1 in value. Going for something a little bit bigger should't be a show-stopper so choose 10 kohm (for this numerical example).

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