Why is a Resistor shunted on an Piezo touch sensor circuit.

Other Hardware General Electronics
1

Hello,
I saw that in nearly all circuits for piezo touch sensors there is a resistor shunted. Why? I thought there is no resistor needed.

2

Look at the VALUE .
Think about it.
It's obviously not a current limiting resistor so what is it ?
Is a piezo a resistor or a capacitor ?
What is the functikn of a pRallel RC circuit ?

Seek and Ye shall find.
Could it be related to impedance ?
Does a piezo have a rated frequency?

3

raschemmel:
Look at the VALUE .
Think about it.
It's obviously not a current limiting resistor so what is it ?
Is a piezo a resistor or a capacitor ?
What is the functikn of a pRallel RC circuit ?

Seek and Ye shall find.
Could it be related to impedance ?
Does a piezo have a rated frequency?

Maybe it acts as a Filter for the natural generated frequencies?

4

electronicsnoob:
Hello,
I saw that in nearly all circuits for piezo touch sensors there is a resistor shunted. Why? I thought there is no resistor needed.

Piezo elements are current sources, or more properly mechanical charge sources, and also capacitors. A resistor is used to modify the amplitude by converting a current to a voltage, as well as prevent voltage drift with time (act as high-pass filter in conjunction with the piezo's capacitance).

Varying the resistance will affect the sensitivity when used as a knock sensor.

5

It's a pull down resistor which insures the input is low/ground with no signal from the piezo.

An unconnected (floating) input will "float" to an undefined value. It will also float with a capacitor-only or a "capacitive" component like a piezo which does not provide a path for DC current.

6

Why is there a 1M resistor across the piezo ?

7

If I am understanding it right, the resistor acts as a Filter and prevents the arduino pin from "floating"?

8

electronicsnoob:
If I am understanding it right, the resistor acts as a Filter and prevents the arduino pin from "floating"?

And also sets the sensitivity. Note that that circuit assumes the input protection diode on the Arduino pin can handle the worst-case current from the piezo element, which is hard to judge without measurements/checking.

9

Looking at a capacitor it will hold its charge until it is drained off either by some external means or by internal leakage. Remove the resistor and see what happens to your performance.
Good Luck & Have Fun!
Gil