Automatic soap dispenser: ultrasonic on 8 and 7, pump on 2, on an Arduino Uno

// An automatic soap dispenser: a hand under the spout runs the pump.
//
//   HC-SR04 VCC -> 5V   TRIG -> pin 8   ECHO -> pin 7   GND -> GND
//   L293D 1,2EN, VCC1 and VCC2 -> 5V   GND -> GND
//   L293D 1A -> pin 2   1Y -> the pump's + lead; the pump's - lead -> GND
//
// A pump is a small motor, so like the fan it needs a driver: pin 2 HIGH
// puts 5 V on 1Y and the pump runs. It runs for half a second, then waits
// for the hand to go away before it will run again, so one wave is one dose.
//
// Drag the distance slider on the sensor below 10 cm.

const int TRIG = 8;
const int ECHO = 7;
const int PUMP = 2;
const int HAND_CM = 10;

bool served = false;

long distanceCm() {
  digitalWrite(TRIG, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG, LOW);
  long echo = pulseIn(ECHO, HIGH, 30000UL);
  return echo / 58;
}

void setup() {
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT);
  pinMode(PUMP, OUTPUT);
  digitalWrite(PUMP, LOW);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: soap dispenser, sensor on 8 and 7, pump on 2");
}

void loop() {
  long cm = distanceCm();
  bool hand = cm > 0 && cm < HAND_CM;
  if (hand && !served) {
    Serial.println("hand: one dose of soap");
    digitalWrite(PUMP, HIGH);
    delay(500);
    digitalWrite(PUMP, LOW);
    served = true;
  } else if (!hand) {
    served = false;
  }
  delay(100);
}