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.
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.
Leo..
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.
Leo..
