Piggy bank on an Arduino Uno

// Piggy bank: a CH-926 coin acceptor and a TM1637 display.
//
//   pin 2    the acceptor's COIN (white), with the Uno's pull-up
//   GND      the acceptor's GND, shared with its 12 V supply
//   pins 13, 12  the display's CLK and DIO
//
// The acceptor is taught each coin and the pulses it is worth; here a nickel
// sends 1, a dime 2, a quarter 5 and a dollar coin 10. COIN pulls low for
// each pulse, so an interrupt counts falling edges, and a burst is over once
// the line has been quiet for longer than any gap inside one (the acceptor's
// gaps are as long as its 50 ms pulses here, so 200 ms is plenty).

#include <TM1637Display.h>

const int COIN = 2;
const int CLK = 13;
const int DIO = 12;
const unsigned long QUIET_MS = 200;

TM1637Display display(CLK, DIO);

volatile int pulses = 0;
volatile unsigned long lastPulse = 0;
long cents = 0;

void onPulse() {
  pulses++;
  lastPulse = millis();
}

int centsFor(int count) {
  switch (count) {
    case 1: return 5;
    case 2: return 10;
    case 5: return 25;
    case 10: return 100;
    default: return 0;
  }
}

void show() {
  display.showNumberDecEx((int)(cents % 10000), 0b01000000, true);
}

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: piggy bank");
  pinMode(COIN, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(COIN), onPulse, FALLING);
  display.setBrightness(0x0f);
  show();
  Serial.println("saved $0.00");
}

void loop() {
  noInterrupts();
  int count = pulses;
  unsigned long last = lastPulse;
  interrupts();
  if (count > 0 && millis() - last > QUIET_MS) {
    noInterrupts();
    pulses = 0;
    interrupts();
    int value = centsFor(count);
    Serial.print(count);
    Serial.print(" pulses: ");
    if (value == 0) {
      Serial.println("not a coin I know");
    } else {
      cents += value;
      Serial.print(value);
      Serial.print(" cents, saved $");
      Serial.print(cents / 100);
      Serial.print('.');
      if (cents % 100 < 10) Serial.print('0');
      Serial.println(cents % 100);
      show();
    }
  }
  delay(10);
}