Grabbing candy bowl on an Arduino Uno

// Grabbing candy bowl: reach in for a treat and the bowl cackles at you while
// a rubber hand swings down to grab yours.
//
//   HC-SR04  VCC -> 5 V   GND -> GND   TRIG -> pin 7   ECHO -> pin 8
//   servo    PWM -> pin 9, VCC -> 5 V, GND -> GND   (the arm under the hand)
//   piezo    pin 4 -> piezo -> GND   (a passive piezo, so tone() sets the note)
//
// Mount the ranger on the rim so it looks across the top of the bowl, not
// down into it: aimed into the bowl it would see the candy and never rest.
// Anything closer than GRAB_CM is a hand.
//
// Two readings in a row have to agree before the hand moves. An ultrasonic
// ranger now and then hears a stray echo, and one bad reading should not
// set off the prop.
//
// The grab itself is a short, fixed show, so it is written as plain steps
// with delay(). After it, the bowl waits for the hand to leave and then
// rests a moment, so one reach gets one grab.
//
// On the bench: a hobby servo's start-up surge can sag the Uno's 5 V, so give
// it a separate 5 V supply with its ground joined to the Uno's. Pad the
// rubber hand's swing so it bumps, never pinches.
//
// Things to try. Drag the ranger's slider under 12 cm. Change the cackle's
// notes. Swap the piezo for a DFPlayer and a speaker for a real recorded
// laugh, as the jump-scare box does.

#include <Servo.h>

const int TRIG_PIN = 7;
const int ECHO_PIN = 8;
const int HAND_PIN = 9;
const int PIEZO_PIN = 4;

const int GRAB_CM = 12;       // a hand is closer than this
const int CLEAR_CM = 20;      // the hand has gone once it is past this
const int HAND_UP = 20;       // degrees, hand raised and hidden
const int HAND_DOWN = 150;    // degrees, hand over the bowl

Servo hand;
int closeReadings = 0;
unsigned int grabs = 0;

// Distance in centimeters, or 999 when nothing answers.
int distanceCm() {
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  unsigned long echo = pulseIn(ECHO_PIN, HIGH, 30000UL);
  if (echo == 0) return 999;
  return echo / 58;   // sound goes there and back, about 58 us per cm
}

void cackle() {
  // "Heh heh heh heh-heeeh": short notes falling in pitch, then a long one.
  const int notes[] = {880, 784, 698, 622};
  for (int i = 0; i < 4; i++) {
    tone(PIEZO_PIN, notes[i], 90);
    delay(150);
  }
  tone(PIEZO_PIN, 1047, 400);
}

void grab() {
  grabs++;
  Serial.print("Grab ");
  Serial.println(grabs);
  hand.write(HAND_DOWN);   // as fast as the servo can go
  cackle();
  delay(700);
  // Lift slowly, which looks far creepier than snapping back.
  for (int angle = HAND_DOWN; angle >= HAND_UP; angle -= 5) {
    hand.write(angle);
    delay(25);
  }
}

void setup() {
  Serial.begin(9600);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  hand.attach(HAND_PIN);
  hand.write(HAND_UP);
  Serial.println("Candy bowl ready");
}

void loop() {
  int cm = distanceCm();
  closeReadings = cm < GRAB_CM ? closeReadings + 1 : 0;

  if (closeReadings >= 2) {
    grab();
    // Wait for the hand to leave, then rest a moment.
    while (distanceCm() < CLEAR_CM) {
      delay(60);
    }
    Serial.println("Hand gone");
    delay(1500);
    closeReadings = 0;
  }
  delay(60);
}