Lie detector, science fair style, on an Arduino Uno

// Lie detector, science fair style: skin response and heart rate against a
// calm baseline, on a ten-segment stress bar.
//
// A science demo, not a lie detector. Nothing can tell a lie from the
// truth by measuring the body: sweat and heart rate rise with surprise,
// effort, laughter or a cold room just as readily as with a fib. What this
// shows, honestly, is the body's stress response, and how it changes from a
// calm baseline.
//
// The skin sensor is a Grove GSR module with two finger straps. Its reading
// x converts to resistance with the module's own formula,
//   R = (1024 + 2x) * 10 k / (512 - x)
// and conductance, in microsiemens, is one over that. Sweatier skin conducts
// more.
//
// Wiring (Arduino Uno):
//   GSR SIG -> A0 (the module runs from 5 V)
//   LED bar segments 1 to 10 -> D2 to D11, each through 220 ohms
//   calibrate button -> A3, to ground
//   MAX30102 on I2C: SDA A4, SCL A5, VIN 5 V

#include <Wire.h>
#include "MAX30105.h"
#include "heartRate.h"

const uint8_t GSR_PIN = A0;
const uint8_t FIRST_BAR = 2;   // segments on D2 to D11
const uint8_t CALIBRATE = A3;  // the button, to ground
const unsigned long CALIBRATE_MS = 10000;

MAX30105 heart;
const byte RATE_SIZE = 4;      // beats averaged
byte rates[RATE_SIZE];
byte rateSpot = 0;
byte ratesSeen = 0;
unsigned long lastBeat = 0;
int bpm = 0;

float baseSkin = 0;    // microsiemens, the calm baseline
int baseBpm = 0;
bool calibrating = false;
bool calibrated = false;
unsigned long calibrateAt = 0;
float skinSum = 0;
long bpmSum = 0;
int samples = 0;
bool buttonDown = false;
unsigned long printAt = 0;

// Skin conductance in microsiemens, or 0 with no fingers on the straps.
float readSkin() {
  int x = analogRead(GSR_PIN);
  if (x >= 505) return 0;  // open straps read 512
  float ohms = (1024.0 + 2.0 * x) * 10000.0 / (512.0 - x);
  return 1000000.0 / ohms;
}

void showBar(int segments) {
  for (uint8_t i = 0; i < 10; i++) {
    digitalWrite(FIRST_BAR + i, i < segments ? HIGH : LOW);
  }
}

// The SparkFun example's way: a beat is a rise in the infrared reflection,
// and the gap between two beats is the heart rate, averaged over four.
void readHeart() {
  long ir = heart.getIR();
  if (ir < 50000) {  // no finger on the sensor
    bpm = 0;
    ratesSeen = 0;
    return;
  }
  if (checkForBeat(ir)) {
    unsigned long now = millis();
    unsigned long gap = now - lastBeat;
    lastBeat = now;
    float rate = 60000.0 / gap;
    if (rate > 30 && rate < 220) {
      rates[rateSpot++] = (byte)rate;
      rateSpot %= RATE_SIZE;
      if (ratesSeen < RATE_SIZE) ratesSeen++;
      int sum = 0;
      for (byte i = 0; i < ratesSeen; i++) sum += rates[i];
      bpm = sum / ratesSeen;
    }
  }
}

void startCalibration() {
  calibrating = true;
  calibrateAt = millis();
  skinSum = 0;
  bpmSum = 0;
  samples = 0;
  Serial.println("Calibrating: sit still and breathe slowly for 10 seconds.");
}

void readButton() {
  bool down = digitalRead(CALIBRATE) == LOW;
  if (down && !buttonDown) startCalibration();
  buttonDown = down;
}

void setup() {
  Serial.begin(115200);
  for (uint8_t i = 0; i < 10; i++) pinMode(FIRST_BAR + i, OUTPUT);
  pinMode(CALIBRATE, INPUT_PULLUP);
  if (!heart.begin(Wire, I2C_SPEED_FAST)) {
    Serial.println("MAX30102 not found: check its wiring.");
  }
  heart.setup();
  heart.setPulseAmplitudeRed(0x0A);  // a dim red lamp to say it is on
  heart.setPulseAmplitudeGreen(0);
  Serial.println("Science fair stress meter ready.");
  startCalibration();
}

void loop() {
  readHeart();
  readButton();
  float skin = readSkin();

  if (calibrating) {
    // Ten seconds of quiet readings make the baseline. The bar fills as a
    // progress meter while it does.
    unsigned long spent = millis() - calibrateAt;
    if (skin > 0 && millis() - printAt >= 200) {
      printAt = millis();
      skinSum += skin;
      bpmSum += bpm;
      samples++;
    }
    showBar(spent * 10 / CALIBRATE_MS);
    if (spent >= CALIBRATE_MS) {
      calibrating = false;
      if (samples == 0) {
        Serial.println("No fingers on the straps. Press the button to try again.");
        return;
      }
      calibrated = true;
      baseSkin = skinSum / samples;
      baseBpm = bpmSum / samples;
      Serial.print("Baseline: skin ");
      Serial.print(baseSkin, 2);
      Serial.print(" uS");
      Serial.print(", heart ");
      Serial.print(baseBpm);
      Serial.print(" bpm");
      Serial.println(". Now ask a question.");
    }
    return;
  }

  if (millis() - printAt < 500) return;
  printAt = millis();

  if (!calibrated) return;
  if (skin == 0) {
    showBar(0);
    Serial.println("skin no fingers on the straps");
    return;
  }
  // Stress is a change from the baseline, not a level: one person's calm is
  // another's excited. The scale is ours, not a medical one: every 5% more
  // conductance is one segment
  float skinChange = (skin - baseSkin) * 100.0 / baseSkin;
  float score = skinChange / 5.0;
  // and every 3 beats a minute above the baseline is another.
  int heartChange = bpm > 0 && baseBpm > 0 ? bpm - baseBpm : 0;
  score += heartChange / 3.0;
  int segments = constrain((int)(score + 0.5), 0, 10);
  showBar(segments);
  Serial.print("skin ");
  Serial.print(skin, 2);
  Serial.print(" uS (");
  if (skinChange >= 0) Serial.print('+');
  Serial.print(skinChange, 0);
  Serial.print("%)");
  Serial.print("  heart ");
  if (bpm > 0) {
    Serial.print(bpm);
    Serial.print(" bpm (");
    if (heartChange >= 0) Serial.print('+');
    Serial.print(heartChange);
    Serial.print(")");
  } else {
    Serial.print("--");
  }
  Serial.print("  stress ");
  Serial.print(segments);
  Serial.println("/10");
}