Jump-scare box on an Arduino Uno

// Jump-scare box: someone walks up to the box on the porch, the lid bursts
// open with a scream and two red eyes flicker inside. Then it shuts, rests,
// and waits for the porch to clear before it arms again.
//
//   RCWL-0516  VIN -> 5 V   GND -> GND   OUT -> pin 2   (high while it sees movement)
//   servo      PWM -> pin 9, VCC -> 5 V, GND -> GND     (the arm that lifts the lid)
//   DFPlayer   VCC -> 5 V   GND -> GND   TX -> pin 10   RX <- 1 k <- pin 11
//              a small speaker on SPK_1 and SPK_2; track 001.mp3 is the scream
//   eyes       two red LEDs on pins 5 and 6, each through 220 ohms to GND
//
// Why a radar: the RCWL-0516 sees movement through a thin plastic or cardboard
// wall, so it can hide inside the box. It also sees through doors and thin
// walls, so aim it away from the house. A PIR sensor works too, on the same
// pin, but it needs a hole to look out of.
//
// The sketch never waits in delay(). It keeps one state, ARMED, SCARING or
// RESTING, and the time it entered it, so the eyes can flicker while the lid
// is open and the sensor is read the whole time.
//
// On the bench: give the servo its own 5 V supply (a USB power bank works)
// with its ground joined to the Uno's. A servo that jumps hard can pull the
// Uno's 5 V down far enough to reset it, right at the scary moment.
//
// Things to try. Click the radar to walk up. Change LID_OPEN for a bigger
// throw, or SCARE_MS for a longer scream. Put a second track on the card and
// pick one with random(1, 3) so repeat visitors get a surprise.

#include <SoftwareSerial.h>
#include <DFRobotDFPlayerMini.h>
#include <Servo.h>

const int SENSOR_PIN = 2;
const int LID_PIN = 9;
const int EYE_LEFT = 5;
const int EYE_RIGHT = 6;

const int LID_SHUT = 10;               // degrees, lid resting on the box
const int LID_OPEN = 110;              // degrees, lid thrown open
const int EYE_IDLE = 12;               // a faint glow while armed
const unsigned long SCARE_MS = 3000;   // how long the lid stays open
const unsigned long REST_MS = 8000;    // quiet time before it can fire again

SoftwareSerial link(10, 11);           // RX, TX
DFRobotDFPlayerMini player;
Servo lid;
bool haveSound = false;

enum State { ARMED, SCARING, RESTING };
State state = ARMED;
unsigned long since = 0;
unsigned long nextFlicker = 0;
unsigned int scares = 0;

void eyes(int level) {
  analogWrite(EYE_LEFT, level);
  analogWrite(EYE_RIGHT, level);
}

void setup() {
  Serial.begin(9600);
  link.begin(9600);
  pinMode(SENSOR_PIN, INPUT);
  lid.attach(LID_PIN);
  lid.write(LID_SHUT);
  haveSound = player.begin(link);
  if (haveSound) {
    player.volume(28);
  } else {
    Serial.println("No DFPlayer answered: the box will scare in silence");
  }
  eyes(EYE_IDLE);
  Serial.println("Armed");
}

void loop() {
  unsigned long now = millis();
  bool someone = digitalRead(SENSOR_PIN) == HIGH;

  switch (state) {
    case ARMED:
      if (someone) {
        state = SCARING;
        since = now;
        scares++;
        lid.write(LID_OPEN);
        if (haveSound) player.play(1);
        Serial.print("BOO! Scare ");
        Serial.println(scares);
      }
      break;

    case SCARING:
      if (now - since >= SCARE_MS) {
        lid.write(LID_SHUT);
        eyes(0);
        state = RESTING;
        since = now;
        Serial.println("Lid shut, resting");
      } else if (now >= nextFlicker) {
        // A candle-like flicker: a random level, held for a random moment.
        eyes(random(40, 256));
        nextFlicker = now + random(30, 90);
      }
      break;

    case RESTING:
      // Rearm only once the rest is over and nobody is standing there, so
      // one visitor gets one scare.
      if (now - since >= REST_MS && !someone) {
        state = ARMED;
        eyes(EYE_IDLE);
        Serial.println("Armed");
      }
      break;
  }
}