The Nixie Tube

// Nixie driver: display, anti-poisoning sweep, and a cathode health check.
//
// Ten MPSA42 low-side switches, one per cathode. One 22k anode resistor sets the
// current for whichever cathode is pulled low. NEVER pull two low at once.
//
// The health check is the useful part: a clean cathode strikes within a few hundred
// microseconds of its drive going low, a poisoned one takes milliseconds or needs
// several tries. Sensing the strike is done by watching the anode node collapse
// from ~180 V to ~140 V through a high-value divider.
//
// Divider:  ANODE ---- 10M ---- GPIO35 ---- 100k ---- GND
//           180 V at the anode gives 1.78 V at the pin; 140 V gives 1.39 V.
//           Both are inside the ADC range. Use 1% resistors and 1/2 W on the 10M
//           for the voltage rating, not the power.
//
// HIGH VOLTAGE. One hand behind your back, insulated bench, 220k bleeder across the
// rail, and verify with a meter that it has actually discharged before touching.

const int CATHODE[10] = {13, 12, 14, 27, 26, 25, 33, 32, 4, 2};  // digit 0..9
const int PIN_ANODE_SENSE = 35;    // ADC1, input only

// ADC counts corresponding to "the tube has struck". Calibrate once: read the pin
// with the tube dark, then with it lit, and put the midpoint here.
int strikeThreshold = 1900;

// --- Anti-poisoning ---------------------------------------------------------
// Every cathode gets run regularly whether the application needs it or not.
const unsigned long POISON_INTERVAL_MS = 3600UL * 1000UL;   // once an hour
const unsigned long POISON_DWELL_MS    = 800;               // per digit
unsigned long lastPoisonRun = 0;

void allOff() {
  for (int i = 0; i < 10; i++) digitalWrite(CATHODE[i], LOW);   // NPN base low = off
}

void showDigit(int d) {
  if (d < 0 || d > 9) { allOff(); return; }
  allOff();                       // off FIRST. Never two cathodes at once.
  delayMicroseconds(200);         // let the old discharge extinguish
  digitalWrite(CATHODE[d], HIGH);
}

// Time from drive asserted to the anode node collapsing. Returns micros, or 0 if
// it never struck within the timeout.
unsigned long measureStrikeTime(int d) {
  allOff();
  delay(50);                      // fully extinguished and de-ionised

  unsigned long t0 = micros();
  digitalWrite(CATHODE[d], HIGH);
  while (micros() - t0 < 20000UL) {
    if (analogRead(PIN_ANODE_SENSE) < strikeThreshold) {
      unsigned long dt = micros() - t0;
      delay(5);                   // let it settle so the eye sees it too
      allOff();
      return dt;
    }
  }
  allOff();
  return 0;                       // never struck - badly poisoned, or wiring fault
}

void healthCheck() {
  Serial.println("\n# cathode health check");
  Serial.println("# digit\tstrike_us\tverdict");
  for (int d = 0; d < 10; d++) {
    unsigned long dt = measureStrikeTime(d);
    Serial.print(d); Serial.print('\t');
    if (dt == 0) {
      Serial.println("----\t\tDID NOT STRIKE - heavily poisoned or open");
    } else {
      Serial.print(dt); Serial.print("\t\t");
      if      (dt <  600) Serial.println("clean");
      else if (dt < 3000) Serial.println("mild poisoning - run the sweep more often");
      else                Serial.println("HEAVY - may not recover");
    }
    delay(200);
  }
  Serial.println("# A tube where digits 6-9 are all slow has been a clock.");
}

void antiPoisonSweep() {
  Serial.println("# anti-poisoning sweep");
  // Three passes. Mild sputtered film comes off when the cathode is run properly.
  for (int pass = 0; pass < 3; pass++) {
    for (int d = 0; d < 10; d++) {
      showDigit(d);
      delay(POISON_DWELL_MS);
    }
  }
  allOff();
  lastPoisonRun = millis();
}

void setup() {
  Serial.begin(115200);
  delay(500);

  for (int i = 0; i < 10; i++) { pinMode(CATHODE[i], OUTPUT); digitalWrite(CATHODE[i], LOW); }
  analogReadResolution(12);
  analogSetPinAttenuation(PIN_ANODE_SENSE, ADC_11db);

  Serial.println("# Nixie driver");
  Serial.print("# anode sense with tube dark: ");
  Serial.println(analogRead(PIN_ANODE_SENSE));
  Serial.println("# (light digit 0 by hand and read it again to set strikeThreshold)");

  healthCheck();
  antiPoisonSweep();
}

void loop() {
  // Ordinary use: count 0..9 slowly.
  for (int d = 0; d < 10; d++) {
    showDigit(d);
    delay(700);
  }

  // Scheduled maintenance. Not optional on a tube that shows one digit most of the day.
  if (millis() - lastPoisonRun > POISON_INTERVAL_MS) {
    antiPoisonSweep();
  }
}