Count coins with the photo-interrupter

Coin machines, vending machines and money boxes in shops count coins with light. Each coin rolls past a beam of light and blocks it for a moment. In this workshop project, the photo-interrupter from the Sensor Kit is that light barrier: drop a coin edge-first through its slot, and the UNO counts it, beeps and shows the total on the LCD.

What you'll learn

• How a photo-interrupter (light gate) works
• How to count events without counting the same one twice
• How fast an object passes and why the sketch must never be slower
• How to show a running count on the LCD

What you need

• UNO R3 board and USB cable (UNO R3 Starter Kit)
• Photo-interrupter module (45-in-1 Sensor Kit)
• Active buzzer module (45-in-1 Sensor Kit)
• 1602 LCD display, 10 kΩ potentiometer and 1 resistor, 220 Ω (UNO R3 Starter Kit)
• A few small coins
• Breadboard and jumper wires (UNO R3 Starter Kit)

Before you start

The LCD has 16 connections in a row. If yours has no pins, solder 16 pins from the 40-pin male header in the kit first, so it plugs into the breadboard. Ask an adult to help if you're new to soldering.

Make it real: the slot is only about 5 mm wide, so the coin has to fall through it on its edge. Fold a strip of card into a narrow channel that ends right above the slot, and put a cup underneath to catch the coins.

Wiring

Sensor modules from different makers can label and order their pins differently. Always follow the labels printed on your module.

The sketch

#include <LiquidCrystal.h>

LiquidCrystal lcd(12, 11, 5, 4, 3, 2);   // RS, E, D4, D5, D6, D7

const int GATE_PIN = 7;
const int BUZZER_PIN = 8;

int gateRest;                 // the reading with nothing in the slot
bool blocked = false;
unsigned long lastCoin = 0;
long coins = 0;

void showCount() {
  lcd.setCursor(0, 1);
  lcd.print("Coins: ");
  lcd.print(coins);
  lcd.print("       ");
}

void setup() {
  pinMode(GATE_PIN, INPUT);
  pinMode(BUZZER_PIN, OUTPUT);
  lcd.begin(16, 2);
  lcd.print("Coin counter");
  delay(500);
  gateRest = digitalRead(GATE_PIN);       // the slot must be empty now
  showCount();
}

void loop() {
  bool nowBlocked = digitalRead(GATE_PIN) != gateRest;

  // count at the moment the beam is broken, at most once every 80 ms
  if (nowBlocked && !blocked && millis() - lastCoin > 80) {
    coins++;
    lastCoin = millis();
    showCount();
    digitalWrite(BUZZER_PIN, HIGH);
    delay(30);
    digitalWrite(BUZZER_PIN, LOW);
  }
  blocked = nowBlocked;
}

How it works

The photo-interrupter has a small infrared LED on one side of the slot and a light sensor on the other. While the slot is empty the sensor sees the light; a coin passing through blocks it for a moment, and the module's output changes. The sketch learns the "empty" reading at start-up, so it works with modules that switch to HIGH and those that switch to LOW.

A falling coin passes the slot in a few thousandths of a second, so the sketch checks the gate all the time and never waits, except for the 30 ms beep after a coin. It counts only at the moment the beam goes from free to blocked (an edge), and ignores anything within 80 ms of the last coin. A coin that wobbles in the slot can't be counted twice that way.

Try this next

• Add a reset button that sets the count back to zero.
• Save the count in EEPROM so the money box remembers it without power.
• Measure how long each coin blocks the beam: bigger coins block it longer, which is a first step towards telling coins apart.

Troubleshooting

• The screen lights up but shows only blocks or nothing: turn the potentiometer slowly from one end to the other.
• Coins are not counted: make sure each coin really passes through the slot, edge first, and that the slot was empty at start-up.
• One coin counts several times: raise the 80 ms to 150.
• It counts without coins: strong sunlight can reach the sensor. Shade the slot.