The String Galvanometer and the ECG

/*
 * Single-lead ECG acquisition — AD8232 front end
 *
 * Wiring:
 *   AD8232 OUTPUT -> A0
 *   AD8232 LO+    -> D11        (leads-off detect, positive electrode)
 *   AD8232 LO-    -> D10        (leads-off detect, negative electrode)
 *   AD8232 3.3V   -> 3.3V       AD8232 GND -> GND
 *
 * BATTERY POWER ONLY while electrodes are on skin.
 * This is a physiology demonstrator. It is not a diagnostic device and
 * nothing it prints may be used to make a decision about anyone's health.
 */

const uint8_t PIN_ECG = A0;
const uint8_t PIN_LO_P = 11;
const uint8_t PIN_LO_N = 10;

const uint16_t FS_HZ = 500;                     // sample rate
const uint32_t SAMPLE_US = 1000000UL / FS_HZ;   // 2000 us

// R-wave detection
const uint16_t REFRACTORY_MS = 200;   // no second R can occur inside this
const float    THRESH_DECAY  = 0.999f;

uint32_t nextSampleUs = 0;
uint32_t lastRMs      = 0;
uint32_t lastRRMs     = 0;
float    peakEstimate = 0.0f;
bool     armed        = true;

void setup() {
  Serial.begin(115200);
  pinMode(PIN_LO_P, INPUT);
  pinMode(PIN_LO_N, INPUT);
  analogReference(DEFAULT);
  nextSampleUs = micros();
  Serial.println(F("# ecg_raw  bpm  (-1 = leads off)"));
}

void loop() {
  // Fixed-interval sampling. Busy-wait on the timer rather than delay(),
  // so the interval does not drift with the work done in the loop.
  uint32_t now = micros();
  if ((int32_t)(now - nextSampleUs) < 0) return;
  nextSampleUs += SAMPLE_US;

  // Leads-off takes absolute priority over any signal processing.
  if (digitalRead(PIN_LO_P) == HIGH || digitalRead(PIN_LO_N) == HIGH) {
    peakEstimate = 0.0f;
    armed = true;
    Serial.println(F("-1 -1"));
    return;
  }

  int raw = analogRead(PIN_ECG);          // 0..1023
  float centred = (float)(raw - 512);
  float mag = fabs(centred);

  // Adaptive threshold: track the running peak, decay it slowly so the
  // detector follows changing signal amplitude without manual tuning.
  if (mag > peakEstimate) peakEstimate = mag;
  else                    peakEstimate *= THRESH_DECAY;

  float threshold = peakEstimate * 0.6f;
  uint32_t ms = millis();

  if (armed && mag > threshold && threshold > 20.0f) {
    if (ms - lastRMs > REFRACTORY_MS) {
      if (lastRMs != 0) lastRRMs = ms - lastRMs;
      lastRMs = ms;
      armed = false;                       // blank until signal falls back
    }
  }
  if (!armed && mag < threshold * 0.5f) armed = true;

  uint16_t bpm = (lastRRMs > 0) ? (uint16_t)(60000UL / lastRRMs) : 0;

  Serial.print(raw);
  Serial.print(' ');
  Serial.println(bpm);
}