Piezo vibration sensor works only if i touch it with my fingers

Projects General Guidance
1

Hey,
I connected a MEAS piezo vibration sensor as a knock detector. It works only with I touch it with my fingers, like a Makey-Makey. It doesn't work when I knock on it with something else.
Please help me figure out what the problem is.

schematic attached
Screenshot 2024-05-12 at 19.41.15

the code is:

const byte piezoPin = A0, ledPin = 13;
int rawValue, maxValue, minValue = 1023;
const int samples = 250;
const int threshold = 100; // sensitivity
int randVal = 0;

void setup() {
  Serial.begin(9600);
  pinMode(ledPin, OUTPUT);
}

void loop() {
  maxValue = 0; // reset
  minValue = 1023; // reset
  randVal = random(1000);
  for (int i = 0; i < samples; i++) {
    rawValue = analogRead(piezoPin);
    if (rawValue < minValue) minValue = rawValue;
    if (rawValue > maxValue) maxValue = rawValue;
  }
  if (maxValue - minValue > threshold) {
    Serial.println(randVal);
    digitalWrite(ledPin, HIGH);
    delay(30); // to see LED
    digitalWrite(ledPin, LOW);
  }
}
2

Possibly you have an open connection. Please post a close up, focused picture of the setup.

In any case, the circuit should work better if you add a 100 nF capacitor from the junction of the 100K voltage divider to ground.

3

Thanks.
I just noticed that I used 100ohm instead of 1Mohm. I guess it caused some problems. Unfortunately I don't have a 1Mohm resistor.

4

Resistors can be salvaged from junked electronics.

5

Those will always need their values checked! Age, excess current, both affect the resistance.

6

Even out of tolerances, would be good improvement in this case :grinning:

7

I suspect that when you touch it you are coupling noise into the circuit that is detected having nothing to do with the sensor.

8

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