The Arduino Sketch

Published December 12, 2025 © Apache-2.0

USDC payments system for drinking water vending machine

A web-intergrated system that accepts stablecoins USDC payments to trigger Arduino microcontroller.

IntermediateFull instructions provided24 hours120

USDC payments system for drinking water vending machine

Things used in this project

Hardware components

Arduino Leonardo

Seeed Studio Grove - Relay

USB-A to Mini-USB Cable

Digilent 60W PCIe 12V 5A Power Supply

Seeed Studio 12V DC Water Pump

Software apps and online services

https://usdckiosk.vercel.app/

Arduino IDE

Story

Front view

This kiosk system detects USDC payments on Base Mainnet Network, triggers an Arduino Leonardo to dispense exactly 250ml of water, and mints a "Shadow Receipt" NFT to the merchant's vault for on-chain loyalty tracking and future user rewards.

Key Features

Direct communication between the Chrome Browser and Arduino Leonardo.
Real-time monitoring of the Base blockchain for incoming transactions.
Mints an NFT receipt to the merchant address containing the payer's data. This creates an on-chain ledger for cashback programs or "Random Rewards" without cluttering the user's wallet.
React-based interface with QR code generation, payment timers, and fluid success animations.

Operation Flow

1. The app displays a "Pay 0.5 USDC" button. It waits for user interaction via the touchscreen or a physical button connected to the Arduino (sending BUTTON_4_PRESSED). In current version used infrared sensor "Flying Fish" to detect the presence of a paper cup in the tray.

2. Payment Phase

A QR Code is generated with the specific USDC transfer URI.

The app begins polling the blockchain for a transfer of 0.5 USDC to the configured MERCHANT_ADDRESS.

3. Verification

Once a valid transaction hash is detected on Base Network, the payment is confirmed.

4. Dispense

The app sends the RELAY_ON command to the Arduino via USB. The Arduino activates the water pump relay.

The app calls mintReceiptNFT. A "Shadow Receipt" is minted to the Merchant's address, recording the Payer's address for future loyalty rewards.

A success sound plays (alert.wav) and a "filling cup" animation displays on screen.

Components connectioon

Current connection

How it looks inside the water dispenser. Laptop just are in normal condition only a keyboard is removed and a display is flipped inside out. It is about how to reuse old hardware from rubbish to make something working out.

Back side

In the current version used Dell Vostro 15' Laptop it mounted inside a wooden cabinet serves as the host for the web applications.

Laptop is powered by it's original power supply.

The water pump is powered by a separate 12V supply. However, a noise filter is mandatory because the Arduino Leonardo is very sensitive to electromagnetic interference (EMI) and may freeze after the relay operates and switches the brushed motor. Additionally, the USB cable connecting the Arduino to the PC should be as short as possible and equipped with a ferrite bead.

Laptop+Arduino+Relay

OS Linux Lubuntu

The Arduino Sketch

Your Arduino must be programmed to handle the following serial protocol at 9600 Baud

For example

Current sketch is here https://github.com/KUNZIT/usdckiosk/blob/main/public/sketch.ino

(Upload it via Arduino IDE)

Important:

The dispensed water volume (250ml) is determined by the timer value within the Arduino sketch. Because flow rates vary depending on the power of the water pump, this value should be calibrated experimentally.

Browser Setup

Open the app in Google Chrome or Microsoft Edge.

Important: Web Serial requires a secure context (HTTPS) or localhost.

Click "Connect USB" in the top right corner to pair your Arduino Leonardo.

The mouse should be disconnected from the machine after confirmation because the microcontroller will literally operate the web application interface.

*How to shutdown the system : simply reconnect the mouse ,exit the browser's full-screen mode , and then shutdown the system as usual.

mouse should be disconnected

Configuration (CONFIG Object)

You can modify the CONFIG object in PaymentApp.tsx to change pricing or addresses:

const CONFIG = {

MERCHANT_ADDRESS: "0x...", // Where money goes & where NFTs are stored

USDC_CONTRACT_ADDRESS: "0x...", // USDC on Base Mainnet

REQUIRED_AMOUNT: 0.5, // Cost of water

⚠️ Troubleshooting

"Web Serial API not supported": Ensure you are using Chrome/Edge. Firefox and Safari do not support this API yet.

Arduino not connecting: Check if another app (like Arduino IDE) is hogging the COM port. The browser needs exclusive access.

Payment not detected: Ensure the user is sending Base USDC, not native ETH, and the amount matches REQUIRED_AMOUNT exactly.

💧 How to Use our Water Dispenser

current version

Please follow these steps to get your drinking water:

1. Preparation

Open your crypto wallet (e.g., MetaMask, Coinbase Wallet) on the Base Mainnet. Ensure you have the following minimum amounts:

• 0.5 USDC (on Base network)

• Ethereum (ETH) for gas fees (on Base network)

2. Dispensing Process (Must be completed in 50 seconds)

• Put your paper cup into the dispenser tray.

• A QR code will immediately appear on the display.

• Open the scanning camera in your crypto wallet.

• Scan the QR code quickly.

• A transaction request will appear in your wallet.

• Crucially, confirm that you are transferring EXACTLY 0.5 USDC on the Base Network. (Payments for any other amount will not dispense water.)

3. Complete & Collect

• Wait for the on-screen message: "Here you are".

• Take your cup with water away from the dispenser tray.

⚠️ Important Note on Timing

• The application session duration is 50 seconds. Please be quick to scan the QR code and confirm the transaction in your wallet.

• In case the session expires, or if you experience a network delay and the water is not dispensed after your payment is confirmed, please contact us immediately for a refund.

Additionally this hardware setup may be used with several web application for example World ID app that allows verified humans to get water for free

World ID App

Code

Sketch arduino Leonardo

// Arduino Leonardo WebUSB Relay Control Sketch with Auto-Timer and Button Input
const int RELAY_PIN = 2; // Connect relay to digital pin 2
const int BUTTON_PIN = 4; // Connect physical button (input) to digital pin 4
unsigned long relayStartTime = 0;
bool relayTimerActive = false;
bool commandSentOnPress = false;
void setup() {
  Serial.begin(9600);
  // Setup RELAY pin
  pinMode(RELAY_PIN, OUTPUT);
  digitalWrite(RELAY_PIN, LOW); // Start with relay OFF
  // Setup onboard LED
  pinMode(LED_BUILTIN, OUTPUT);
  //  The button must be wired between pin 4 and GND.
  // The pin will read LOW when pressed.
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  Serial.println("Relay Controller Ready!");
}
void loop() {
  
  if (Serial.available()) {
    String command = Serial.readString();
    command.trim();
    
    if (command == "RELAY_ON") {
      digitalWrite(RELAY_PIN, HIGH);
      digitalWrite(LED_BUILTIN, HIGH);
      relayStartTime = millis(); // Start timer
      relayTimerActive = true;
      Serial.println("RELAY_ON_OK");
    }
  }
   // --- Relay Auto-Off Logic ---
  
  if (relayTimerActive && (millis() - relayStartTime >= 7000)) {
    digitalWrite(RELAY_PIN, LOW);
    digitalWrite(LED_BUILTIN, LOW);
    relayTimerActive = false;
    Serial.println("RELAY_AUTO_OFF");
  }
// --- Button Monitoring Logic (Arduino to App) ---
  int buttonState = digitalRead(BUTTON_PIN);

  if (buttonState == LOW) {
    if (!commandSentOnPress) {
      
      Serial.println("BUTTON_4_PRESSED");
      
      
      commandSentOnPress = true;
    }
  } else {
    
    commandSentOnPress = false;
  }
  
  // A small delay to keep the loop from spinning too fast
  delay(5);
}