Arduino Vending Machine Interface

#include <LiquidCrystal.h>

LiquidCrystal lcd(12, 11, 10, 9, 8, 7);

const int btnDollar = 2;
const int btnQuarter = 3;
const int btnDime = 4;
const int btnNickel = 5;
const int btnSnickers = 6;
const int btnMMs = 7;
const int btnDoritos = 8;
const int btnGum = 9;
const int btnChange = 10;

float balance = 0.00;

void setup() {
  pinMode(btnDollar, INPUT);
  pinMode(btnQuarter, INPUT);
  pinMode(btnDime, INPUT);
  pinMode(btnNickel, INPUT);
  pinMode(btnSnickers, INPUT);
  pinMode(btnMMs, INPUT);
  pinMode(btnDoritos, INPUT);
  pinMode(btnGum, INPUT);
  pinMode(btnChange, INPUT);
  
  lcd.begin(16, 2);
  lcd.print("Vending Machine");
  delay(2000);
  updateDisplay();
  
  Serial.begin(9600);
}

void loop() {
  if (digitalRead(btnDollar) == HIGH) {
    balance += 1.00;
    updateDisplay();
    delay(300);
  }
  if (digitalRead(btnQuarter) == HIGH) {
    balance += 0.25;
    updateDisplay();
    delay(300);
  }
  if (digitalRead(btnDime) == HIGH) {
    balance += 0.10;
    updateDisplay();
    delay(300);
  }
  if (digitalRead(btnNickel) == HIGH) {
    balance += 0.05;
    updateDisplay();
    delay(300);
  }
  
  if (digitalRead(btnSnickers) == HIGH) {
    dispenseItem("Snickers", 1.25);
  }
  if (digitalRead(btnMMs) == HIGH) {
    dispenseItem("M&Ms", 1.00);
  }
  if (digitalRead(btnDoritos) == HIGH) {
    dispenseItem("Doritos", 1.50);
  }
  if (digitalRead(btnGum) == HIGH) {
    dispenseItem("Gum", 0.75);
  }
  
  if (digitalRead(btnChange) == HIGH) {
    returnChange();
  }
}

void updateDisplay() {
  lcd.clear();
  lcd.print("Balance: $");
  lcd.print(balance, 2);
}

void dispenseItem(String item, float price) {
  if (balance >= price) {
    lcd.clear();
    lcd.print("Dispensing...");
    lcd.setCursor(0, 1);
    lcd.print(item);
    Serial.println("Dispensed: " + item);
    
    balance -= price;
    delay(2000);
    
    if (balance > 0) {
      lcd.clear();
      lcd.print("Change: $");
      lcd.print(balance, 2);
      delay(2000);
    }
    balance = 0;
    updateDisplay();
  } else {
    lcd.clear();
    lcd.print("Insufficient $");
    delay(1500);
    updateDisplay();
  }
  delay(300);
}

void returnChange() {
  if (balance > 0) {
    lcd.clear();
    lcd.print("Returning $");
    lcd.print(balance, 2);
    Serial.print("Change returned: $");
    Serial.println(balance, 2);
    balance = 0;
    delay(2000);
    updateDisplay();
  }
  delay(300);
}

This uses a parallel LCD. For I2C, use LiquidCrystal_I2C library and adjust initialization.