Anger Scintillation Camera

// Youblob -- Anger logic on four channels.
// The whole camera is the two lines marked ANGER LOGIC below. Everything else is
// making four analogue channels trustworthy enough to compare, which is the part
// that actually decides whether the position is right.
//
// Wiring: four photodiodes, each into a transimpedance amp, outputs to A0..A3.
// Detector coordinates in millimetres, measured on YOUR board -- these numbers go
// straight into the answer, so measure them rather than assuming the square is square.
const float DET_X[4] = {   0.0, 100.0,   0.0, 100.0 };
const float DET_Y[4] = {   0.0,   0.0, 100.0, 100.0 };
const int   PIN[4]   = {    A0,    A1,    A2,    A3 };

float dark[4] = {0, 0, 0, 0};      // filled by calibrate(), never guessed
float gain[4] = {1, 1, 1, 1};      // equalised at the centre spot

float readAvg(int pin) {           // 32 reads: the ADC noise is larger than you think
  long sum = 0;
  for (int i = 0; i < 32; i++) sum += analogRead(pin);
  return sum / 32.0;
}

void calibrate() {
  Serial.println(F("# DARK: cover the diffuser completely, then send any character"));
  while (!Serial.available()) delay(50);
  while (Serial.available()) Serial.read();
  for (int i = 0; i < 4; i++) dark[i] = readAvg(PIN[i]);

  Serial.println(F("# CENTRE: put the light spot at the exact centre, then send any character"));
  while (!Serial.available()) delay(50);
  while (Serial.available()) Serial.read();
  float v[4], mean = 0;
  for (int i = 0; i < 4; i++) { v[i] = readAvg(PIN[i]) - dark[i]; mean += v[i]; }
  mean /= 4.0;
  // Equalise the channels. A gain difference between two detectors is indistinguishable
  // from the spot being closer to one of them -- it is read as POSITION, not as gain.
  for (int i = 0; i < 4; i++) gain[i] = (v[i] > 1.0) ? (mean / v[i]) : 1.0;

  Serial.println(F("# calibrated"));
  Serial.println(F("x_mm,y_mm,total,a,b,c,d"));
}

void setup() {
  Serial.begin(115200);
  calibrate();
}

void loop() {
  float sig[4], total = 0, sx = 0, sy = 0;
  for (int i = 0; i < 4; i++) {
    sig[i] = (readAvg(PIN[i]) - dark[i]) * gain[i];
    if (sig[i] < 0) sig[i] = 0;    // a negative signal is noise, not a position hint
    total += sig[i];
  }

  if (total < 8.0) { delay(100); return; }   // nothing there; do not divide by noise

  // ---- ANGER LOGIC ----------------------------------------------------------
  for (int i = 0; i < 4; i++) { sx += sig[i] * DET_X[i]; sy += sig[i] * DET_Y[i]; }
  float x = sx / total, y = sy / total;
  // ---------------------------------------------------------------------------

  Serial.print(x, 2);     Serial.print(',');
  Serial.print(y, 2);     Serial.print(',');
  Serial.print(total, 1);
  for (int i = 0; i < 4; i++) { Serial.print(','); Serial.print(sig[i], 1); }
  Serial.println();
  delay(200);
}