Touchless soap dispenser on an Arduino Uno

// Touchless soap dispenser: an FC-51 infrared obstacle sensor under the spout,
// and a small pump on a transistor.
//
//   pin 2   the FC-51's OUT (low when a hand is in range)
//   pin 9   through 1 k into the transistor's base; the pump hangs off 5 V
//           into the collector, with a diode across it, band to 5 V
//   5V/GND  the FC-51's VCC and GND
//
// The FC-51 shines infrared forward and watches for it to come back. A hand
// a few centimeters under the spout sends plenty back and OUT goes low; with
// the trimmer in the middle a white card trips it at about 13 cm. Skin is
// somewhere between white and black, so the spout's sensor is set to see a
// hand only when it is right underneath.
//
// # One dose per hand
//
// A dispenser that pumps for as long as a hand is there empties the bottle.
// So the sketch pumps one dose, then waits until OUT has been high for half a
// second (the hand has gone) before it will pump again.

const int SENSOR = 2;
const int PUMP = 9;
const unsigned long DOSE_MS = 700;
const unsigned long AWAY_MS = 500;

int doses = 0;

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: touchless soap dispenser");
  pinMode(SENSOR, INPUT);
  pinMode(PUMP, OUTPUT);
  digitalWrite(PUMP, LOW);
}

bool handThere() {
  return digitalRead(SENSOR) == LOW;
}

void loop() {
  if (!handThere()) {
    delay(20);
    return;
  }
  doses++;
  Serial.print("hand: dose ");
  Serial.println(doses);
  digitalWrite(PUMP, HIGH);
  delay(DOSE_MS);
  digitalWrite(PUMP, LOW);

  // Wait for the hand to leave, and stay gone for AWAY_MS.
  unsigned long clear = millis();
  while (millis() - clear < AWAY_MS) {
    if (handThere()) clear = millis();
    delay(10);
  }
  Serial.println("ready");
}