Build a Solar Pay-As-You-Go Payment System

Tables: devices (serial_number, customer_id, model, install_date, total_cost, total_paid, status, credit_days_remaining, last_token_used), payments (device_id, amount, paystack_ref, token, token_days, sent_at, applied_at).

var crypto = require('crypto');

// Generate a deterministic but unpredictable 12-digit unlock token
function generatePaygToken(deviceSerial, paymentRef, secretKey) {
  var hmac = crypto.createHmac('sha256', secretKey);
  hmac.update(deviceSerial + '|' + paymentRef);
  var hash = hmac.digest('hex');
  // Convert first 12 hex chars to decimal — device firmware validates same way
  var tokenNum = parseInt(hash.slice(0, 12), 16);
  return (tokenNum % 900000000000 + 100000000000).toString(); // 12-digit token
}

function calculateTokenDays(amountKobo) {
  // NGN 200/day of credit
  return Math.floor(amountKobo / 20000);
}

async function initializeTopup(customerPhone, deviceId, amount) {
  var device = await db.devices.findById(deviceId);
  var customer = await db.customers.findById(device.customer_id);
  var reference = 'PAYG-' + device.serial_number + '-' + Date.now();

  var res = await fetch('https://api.paystack.co/transaction/initialize', {
    method: 'POST',
    headers: { Authorization: 'Bearer ' + process.env.PAYSTACK_SECRET_KEY, 'Content-Type': 'application/json' },
    body: JSON.stringify({
      email: customer.email,
      amount,
      currency: 'NGN',
      reference,
      metadata: { device_id: deviceId, serial: device.serial_number, customer_phone: customerPhone },
    }),
  });
  return (await res.json()).data.authorization_url;
}

// Webhook: generate token and send via SMS
if (event.event === 'charge.success') {
  var meta = event.data.metadata;
  if (!meta.device_id) return res.sendStatus(200);

  var device = await db.devices.findById(meta.device_id);
  var tokenDays = calculateTokenDays(event.data.amount);
  var token = generatePaygToken(meta.serial, event.data.reference, process.env.PAYG_TOKEN_SECRET);

  await db.payments.create({
    device_id: meta.device_id,
    amount: event.data.amount,
    paystack_ref: event.data.reference,
    token,
    token_days: tokenDays,
    sent_at: new Date(),
  });

  // Update device credit
  var newTotal = device.total_paid + event.data.amount;
  var newCredit = device.credit_days_remaining + tokenDays;
  await db.devices.update(meta.device_id, { total_paid: newTotal, credit_days_remaining: newCredit });

  // Send token via SMS
  await sendSMS(meta.customer_phone, 'Solar token: ' + token + ' (' + tokenDays + ' days). Enter on device keypad. Balance: NGN ' + (newTotal / 100));

  // Check ownership milestone
  if (newTotal >= device.total_cost) {
    var ownershipToken = generatePaygToken(meta.serial, 'FOREVER', process.env.PAYG_TOKEN_SECRET);
    await db.devices.update(meta.device_id, { status: 'owned' });
    await sendSMS(meta.customer_phone, 'Congratulations! Your solar system is now fully paid. Final unlock code: ' + ownershipToken + '. Your system is yours forever.');
  }
}

// Cron: track credit expiry and flag inactive devices
async function checkDeviceCredit() {
  var devices = await db.devices.findAll({ status: 'active' });
  var today = new Date();

  for (var device of devices) {
    // Deduct one day of credit per day
    var newCredit = device.credit_days_remaining - 1;

    if (newCredit <= 0) {
      await db.devices.update(device.id, { credit_days_remaining: 0, status: 'disabled' });
      var customer = await db.customers.findById(device.customer_id);
      await sendSMS(customer.phone, 'Your solar credit has expired. Top up to restore power: yourapp.com/topup');
    } else {
      await db.devices.update(device.id, { credit_days_remaining: newCredit });
      if (newCredit <= 3) { // low credit warning
        var customer = await db.customers.findById(device.customer_id);
        await sendSMS(customer.phone, 'Low solar credit: ' + newCredit + ' days remaining. Top up: yourapp.com/topup');
      }
    }
  }
}

// Field agent: register a new customer and device
async function registerCustomerDevice(customerData, serialNumber, model, totalCostNGN) {
  var customer = await db.customers.create(customerData);
  var device = await db.devices.create({
    serial_number: serialNumber,
    customer_id: customer.id,
    model,
    install_date: new Date(),
    total_cost: totalCostNGN * 100, // store in kobo
    total_paid: 0,
    status: 'active',
    credit_days_remaining: 3, // 3-day free trial on installation
  });
  await sendSMS(customer.phone, 'Welcome! Your solar system is active for 3 free days. Top up at yourapp.com/topup using device ID: ' + device.id);
  return { customer, device };
}

See build a savings goal app with scheduled debits for card tokenization auto-debit — useful for customers who prefer automatic weekly PAYG topups.

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