The Implantable Pacemaker

/*
 * Demand pacing logic — BENCH DEMONSTRATOR
 *
 * Wiring:
 *   A0  <- synthesised cardiac signal from a SIGNAL GENERATOR or recorded trace
 *   D9  -> 1k resistor -> indicator LED -> GND        (the "stimulus")
 *   D8  -> scope probe, mirrors the stimulus for timing measurement
 *
 * THIS IS NOT A PACEMAKER. It drives an LED and a scope.
 * It must never be connected to a person or an animal, by any route,
 * and the sense input must never come from electrodes on a living subject.
 * A real pacemaker is an implanted, sealed, regulated device with an
 * energy budget, a lead system and a clinician behind it.
 */

const uint8_t PIN_SENSE = A0;
const uint8_t PIN_PACE  = 9;
const uint8_t PIN_MARK  = 8;

// --- the two intervals that define demand pacing ---
const uint16_t BASE_RATE_PPM   = 70;                      // pace this slowly if nothing is sensed
const uint16_t ESCAPE_MS       = 60000UL / BASE_RATE_PPM; // 857 ms at 70 ppm
const uint16_t BLANK_MS        = 250;                     // deaf after ANY event
const uint16_t PULSE_WIDTH_US  = 500;                     // 0.5 ms stimulus

const uint16_t SENSE_THRESHOLD = 120;   // above baseline, in ADC counts

uint32_t lastEventMs = 0;      // sensed OR paced — both restart the escape timer
uint32_t pacedCount  = 0;
uint32_t sensedCount = 0;

void setup() {
  Serial.begin(115200);
  pinMode(PIN_PACE, OUTPUT);
  pinMode(PIN_MARK, OUTPUT);
  digitalWrite(PIN_PACE, LOW);
  digitalWrite(PIN_MARK, LOW);
  lastEventMs = millis();
  Serial.println(F("# event  interval_ms  paced  sensed"));
}

void emitStimulus() {
  digitalWrite(PIN_MARK, HIGH);
  digitalWrite(PIN_PACE, HIGH);
  delayMicroseconds(PULSE_WIDTH_US);    // short on purpose: the pulse is the energy cost
  digitalWrite(PIN_PACE, LOW);
  digitalWrite(PIN_MARK, LOW);
}

void loop() {
  uint32_t now = millis();
  uint32_t sinceEvent = now - lastEventMs;

  // Blanking: refuse to sense anything at all for a fixed window after any
  // event. Without this the circuit hears its own stimulus, calls it a
  // heartbeat, inhibits itself, and never paces again.
  bool listening = (sinceEvent > BLANK_MS);

  if (listening) {
    int v = analogRead(PIN_SENSE) - 512;
    if (abs(v) > SENSE_THRESHOLD) {
      // Natural beat arrived in time — stay quiet. This is the whole point.
      sensedCount++;
      Serial.print(F("SENSE ")); Serial.print(sinceEvent);
      Serial.print(' '); Serial.print(pacedCount);
      Serial.print(' '); Serial.println(sensedCount);
      lastEventMs = now;
      return;
    }
  }

  // Escape: nothing arrived within the interval, so pace.
  if (sinceEvent >= ESCAPE_MS) {
    emitStimulus();
    pacedCount++;
    Serial.print(F("PACE  ")); Serial.print(sinceEvent);
    Serial.print(' '); Serial.print(pacedCount);
    Serial.print(' '); Serial.println(sensedCount);
    lastEventMs = millis();
  }
}