const int TX_PIN = 12;
const int RX_PIN = 13;
const int SAMPLES = 50;
const int TIMEOUT = 2000;  // much lower ceiling than 20000

long readCapacitance() {
  long total = 0;
  for (int i = 0; i < SAMPLES; i++) {
    // discharge
    pinMode(RX_PIN, OUTPUT);
    digitalWrite(RX_PIN, LOW);
    digitalWrite(TX_PIN, LOW);
    delayMicroseconds(10);

    // pulse TX, count until RX goes high
    pinMode(RX_PIN, INPUT);
    digitalWrite(TX_PIN, HIGH);

    long count = 0;
    while (digitalRead(RX_PIN) == LOW && count < TIMEOUT) {
      count++;
    }
    total += count;

    digitalWrite(TX_PIN, LOW);
    delayMicroseconds(10);
  }
  return total / SAMPLES;
}

long baseline = 0;

void setup() {
  Serial.begin(115200);
  pinMode(TX_PIN, OUTPUT);
  digitalWrite(TX_PIN, LOW);
  delay(100);

  // calibrate — don't touch pads during startup
  long sum = 0;
  for (int i = 0; i < 100; i++) {
    sum += readCapacitance();
  }
  baseline = sum / 100;
  Serial.print("Baseline: ");
  Serial.println(baseline);
}

void loop() {
  long val = readCapacitance();
  long delta = val - baseline;  // positive = more capacitance

  Serial.print(val);
  Serial.print("\t");
  Serial.println(delta);

  delay(10);
}