Smart trash can: ultrasonic on 8 and 7, servo lid on 9, on an Arduino Uno

// A smart trash can: wave a hand over it and the lid opens.
//
//   HC-SR04 VCC -> 5V   TRIG -> pin 8   ECHO -> pin 7   GND -> GND
//   servo signal -> pin 9   red -> 5V   brown -> GND
//
// The sensor sends a burst of sound when TRIG is pulsed, and ECHO stays HIGH
// until the echo comes back: about 58 microseconds for every centimeter. When
// something is closer than OPEN_CM, the servo turns to the open angle; once
// nothing has been there for a second, it closes again.
//
// Drag the distance slider on the sensor below 20 cm.

#include <Servo.h>

const int TRIG = 8;
const int ECHO = 7;
const int SERVO_PIN = 9;
const int OPEN_ANGLE = 0;
const int CLOSED_ANGLE = 90;
const int OPEN_CM = 20;

Servo lid;
unsigned long lastSeen = 0;
bool open = 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);
  lid.attach(SERVO_PIN);
  lid.write(CLOSED_ANGLE);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: smart trash can, sensor on 8 and 7, lid on 9");
}

void loop() {
  long cm = distanceCm();
  if (cm > 0 && cm < OPEN_CM) {
    lastSeen = millis();
    if (!open) {
      lid.write(OPEN_ANGLE);
      open = true;
      Serial.println("hand seen: lid open");
    }
  } else if (open && millis() - lastSeen > 1000) {
    lid.write(CLOSED_ANGLE);
    open = false;
    Serial.println("lid closed");
  }
  delay(100);
}