Piezo Help

Projects General Guidance
1

I am trying to make a piezo-based door lock. I have a pretty good idea of how to go about it, except the piezo sensor(s) that I ordered from sparkfun are not doing what they are supposed to. I am just trying to test out the tutorial from http://arduino.cc/en/Tutorial/Knock but for some reason, I can't get any output in the Serial Monitor.

This is my circuit:

This is my code:

// these constants won't change:
const int knockSensor = A0; // the piezo is connected to analog pin 0
const int threshold = 100; // threshold value to decide when the detected sound is a knock or not

// these variables will change:
int sensorReading = 0; // variable to store the value read from the sensor pin

void setup() {
Serial.begin(9600); // use the serial port
}

void loop() {
// read the sensor and store it in the variable sensorReading:
sensorReading = analogRead(knockSensor);

// if the sensor reading is greater than the threshold:
if (sensorReading >= threshold) {
// update the LED pin itself:
digitalWrite(ledPin, ledState);
// send the string "Knock!" back to the computer, followed by newline
Serial.println("Knock!");
}
delay(100); // delay to avoid overloading the serial port buffer
}

I am using this piezo: Piezo Vibration Sensor - Large - SEN-09196 - SparkFun Electronics and a 1 megohm resistor.

When I knock on my table, weird things happen, like "Knock!" is displayed 10 seconds after I actually Knock or it just doesn't display at all.

When I disconnect the wires, and the analog port is empty, the Serial Monitor displays "Knock!" every 100ms. I know my board is fine, I just don't know why the piezo isn't working. Any suggestions would be GREATLY appreciated =]

2

When I disconnect the wires, and the analog port is empty,

the output from analogRead() is meaningless. That's what you meant to say.

A piezo detects vibration. Taped down like that, how is it supposed to vibrate?

3

PaulS:
A piezo detects vibration. Taped down like that, how is it supposed to vibrate?

Do you think that packaging tape is much of a damper? If the sensor is in contact with the table, and the table is transmitting sound waves, doesn't the sensor perform better if it's kept in good contact?

4

Even if I untape it, I don't get regular results

5

The blue wire looks like it is connected to ground. Is it?

6

Yes it is. The piezo responds if I tap it, but knocks don't work whatsoever.

7

The piezo responds if I tap it

Responds how?