What's that resistor for?


Very new at electronics and while I have an engineering degree this is far from my area of specialty. I'm attempting to create a prototype for a vibration sensor. I'm using a piezo film element for this so the arduino "Knock" tutorial (https://www.arduino.cc/en/tutorial/knock) seems great for outlining the basics and setting up the foundation for a more complex application.

Why is there a resistor in parallel with the piezo element? Is it a pull down resistor or is it used to regulate voltage going to the IC or something else I may not be seeing?

Would love to have a better understanding of its purpose before I start deviating from the beaten path.

Without the resistor a piezo can produce more than 5 volts, the 1meg loads this so to limit this to a safe voltage level.


It sets the gain, peizo's are charge sources, ie like a current source with high-pass characteristic,
putting a resistor across it turns it into a usable voltage source, the higher the resistor the
higher the gain. Without such a resistive load you might get 1000's of volts (except you wouldn't,
the protection diodes would clamp the waveform to a rectangular wave unless they burnt out).

More important, the resistor keeps the pin at ground level. So the A/D reads zero when idle.
The piezo is a capacitor. Without the resistor, the pin will be “floating” for DC.
The A/D will read some random number over time. Usually ~300.

I think you mean that Piezoelectric Effect in Ceramic Capacitors A peizo's hit hard can put out very high voltage the resistor keeps that from happing.
like was said in post #1

A piezo with a 1Meg resistor can still produce a high voltage when hit hard.
Just tested it on the scope. Easy to get 50volt peaks with 1Meg and light tapping.
The resistor will only consume 5.5uA of a positive spike.
Worse for negative spikes. The resistor will only take 500nA.
The input protection diodes do most of the work.

The 50V indicates its generating 50uA, so the protection diodes aren't in danger, whether there's
a 1M resistor or not to soak up 5.5uA!

The only function I can see is highpass filter (remove DC).
Zero A/D when not activated.