3D printer piezo board leveling Arduino test sketch

SimplePiezoBoardTest.cpp

/*
   Simple test program for 3D printer piezo
   bed leveling adapter board described at
   https://www.tspi.at/2019/09/11/piezobedlevel.html
   (using Arduino IDE and libraries).
   
   This does not provide any I2C connectivity, etc.
   and just periodically reads analog samples, dumps
   them to the serial port and does some moving average
   filtering (as well as using a inductive probe as
   veto to suppress triggering while moving)
*/

#define ANALOGPIN_IN0           A0
#define ANALOGPIN_IN1           A1
#define ANALOGPIN_IN2           A2
#define ANALOGPIN_IN3           A3
#define DIGITAL_OUT             9
#define INDUC_IN                10

#define TRIGGERDEVIATION        15

static int movingAverageState[4];
static int measuredCenterlines[4];
static int initsamples;

void setup() {
  Serial.begin(9600);

  // Set output pin
  pinMode(DIGITAL_OUT, OUTPUT);
  pinMode(INDUC_IN, INPUT);
  movingAverageState[0] = analogRead(ANALOGPIN_IN0);
  movingAverageState[1] = analogRead(ANALOGPIN_IN1);
  movingAverageState[2] = analogRead(ANALOGPIN_IN2);
  movingAverageState[3] = analogRead(ANALOGPIN_IN3);

  double avg[4];

  avg[0] = 0.0;
  avg[1] = 0.0;
  avg[2] = 0.0;
  avg[3] = 0.0;
  for(initsamples = 0; initsamples < 1000; initsamples = initsamples  + 1) {
    avg[0] = avg[0] + ((double)analogRead(ANALOGPIN_IN0)) / ((double)1000);
    avg[1] = avg[1] + ((double)analogRead(ANALOGPIN_IN1)) / ((double)1000);
    avg[2] = avg[2] + ((double)analogRead(ANALOGPIN_IN2)) / ((double)1000);
    avg[3] = avg[3] + ((double)analogRead(ANALOGPIN_IN3)) / ((double)1000);
  }

  Serial.print("Init samples (averages): ");
  Serial.print((int)avg[0]);
  Serial.print(" ");
  Serial.print((int)avg[1]);
  Serial.print(" ");
  Serial.print((int)avg[2]);
  Serial.print(" ");
  Serial.println((int)avg[3]);

  measuredCenterlines[0] = (int)avg[0];
  measuredCenterlines[1] = (int)avg[1];
  measuredCenterlines[2] = (int)avg[2];
  measuredCenterlines[3] = (int)avg[3];
}

static int debounceCounter = 12;
#define movingAverageAlpha 0.4f

void loop() {
  int aValue[4];

  aValue[0] = analogRead(ANALOGPIN_IN0);
  aValue[1] = analogRead(ANALOGPIN_IN1);
  aValue[2] = analogRead(ANALOGPIN_IN2);
  aValue[3] = analogRead(ANALOGPIN_IN3);

  movingAverageState[0] = movingAverageAlpha*aValue[0] + (1.0f - movingAverageAlpha)*movingAverageState[0];
  movingAverageState[1] = movingAverageAlpha*aValue[1] + (1.0f - movingAverageAlpha)*movingAverageState[1];
  movingAverageState[2] = movingAverageAlpha*aValue[2] + (1.0f - movingAverageAlpha)*movingAverageState[2];
  movingAverageState[3] = movingAverageAlpha*aValue[3] + (1.0f - movingAverageAlpha)*movingAverageState[3];

  int trigCount = 0;

  int measValues[4];
  for(int i = 0; i < 4; i++) {
    measValues[i] = movingAverageState[i] - measuredCenterlines[i];
  }

  Serial.print(measValues[0]);
  Serial.print(" ");
  Serial.print(measValues[1]);
  Serial.print(" ");
  Serial.print(measValues[2]);
  Serial.print(" ");
  Serial.println(measValues[3]);

  for(int i = 0; i < 4; i++) {
    if(measValues[i] < 0) { measValues[i] = measValues[i] * -1; }
  }

  for(int i = 0; i < 4; i++) {
    if(measValues[i] > TRIGGERDEVIATION) { trigCount++; }
  }

  if((trigCount >= 1) && digitalRead(INDUC_IN)) {
    debounceCounter = 12;
  }

  if(debounceCounter > 0) {
    digitalWrite(DIGITAL_OUT, HIGH);
    debounceCounter--;
  } else {
    digitalWrite(DIGITAL_OUT, LOW);
  }

  delay(5);
}

Just to add that: Even that code should work unmodified if one pulls the input for the unconnected piezo to GND or VCC so no random noise is measured - I've actually done that during testing a few times.

Thanks for replying. I have remixed your board for using the Attiny85 instead. When I have it done I'll post my testings here.
I'm planning to disable the Attiny85 reset pin, and get 2 extra pins to create a threshold setup pins to easily tune the board without reprogramming the attiny85. Of course, I need to change the code to matches the t85 pinout. I have done this for the JohnSL
/ FSR_Endstop too, but the FSR sensors it's not cheap here where I live (it's about US$16 each, where the Piezo costs only $1.6 each).

Yeah having a reset method is pretty useful - I've added this (but not uploaded here up until now, maybe I should do this) to my own implementation too using I2C (since most components of my custom machine communicate via I2C anyways). I'd be really interested in how well that approach works for your machine too - since the only "experience" I made with this is unfortunately my own custom designed machine.

Yep, the price and also the constraints by having an capacitive sensor mounted onto the printhead also was a reason I tried to operate without one - I think it should be possible with more fancy signal processing (for example if I hear correctly I have an fan mounted onto my printhead that seems to change frequency in case of a touch event - it should be possible to detect that) - and of course also to discriminate for noise so one doesn't need any other additional sensor - I didn't follow that route though since the capacitive sensor already had been in place so there was no need for that though it would be really interesting if this works well (and after print has started one could of course disable or ignore endstop triggering as usual; Maybe it would also be possible to simply do some statistics to remove noisy triggering by the piezos to by modifying the tramming process ...)

Maybe just another comment about "biasing" the piezos that way - depending on the piezos this works pretty well but when one drives the input of the AVR directly with them one relies on the protection diodes to catch voltage spikes that might arise when larger forces are acting on the piezos - also one cannot use typical industrial sensors that way, it works pretty well with piezo disks that are thought as guitar pickups or speakers though usually but of course there is no guarantee that a specific type doesn't deliver voltages too large for an AVR to handle. It's never been a problem for my setups (even when dropping tools on the print surface, etc.) with different disks but I think it's worth to mention