Bitcoin Learning Sandbox - learn UTXOs, fees, mempool & blocks in your browser

Hi everyone,

I built a free Bitcoin Learning Sandbox for people who want to understand how Bitcoin actually works, without running a node, without testnet coins, and without risking real funds.

Link: https://cinexacode.fun/bitcoin-sandbox

Everything runs in the browser. Keys, UTXOs, mempool, and blocks are simulated for education. No mainnet connection.

What you can do

Wallet Generator - generate a mock keypair, see private key (hex), WIF, public key, Legacy (1...) and SegWit (bc1q...) addresses. Starter UTXOs are auto-funded so you can practice immediately.
UTXO model - balance = sum of unspent outputs. Each UTXO shows txid, vout, address, and amount in sats.
Transaction Builder - pick inputs, set receiver + amount, adjust fee rate (sat/vB). Preview raw JSON with vin/vout including change. Dust rule enforced (<546 sats rejected).
Mempool - broadcast a tx and watch queue position. Higher fee rate = higher priority. Fee slider (1-200 sat/vB) updates confirmation estimates live.
Block Explorer - mine the next block, confirm pending txs, increment block height, and follow events in the explorer feed.
Security Lessons - simulate address reuse warnings and private key exposure (exposed keys get swept on the next block).
Documentation tab - architecture overview and flow reference built into the UI.
Guided tutorial - first-visit walkthrough (12 steps) through each tab and button. Mobile-friendly bottom sheet on phones.

Typical learning flow

Code:

Generate Wallet → Build TX → Broadcast → Mempool Queue → Mine Block → Confirmed

Fee formula used in the simulator:

Code:

fee = fee_rate × vsize
change = inputs - amount - fee

Rough mempool behavior:

≥ 80 sat/vB — likely next block
≥ 15 sat/vB — ~2 blocks
≥ 1 sat/vB — ~4 blocks
Very low — sits at the back of the queue

Who is this for?

Newcomers who hear "wallet" and "UTXO" but have never seen them in action
Anyone preparing for deeper reading (Mastering Bitcoin, etc.) who wants hands-on practice first

Who is this NOT for?

Production wallet testing - keys are mock/educational only
Real signing or broadcasting - no network, no real coins
Advanced script/path-level accuracy - this is a teaching simulator, not bitcoind

Important: Never use sandbox keys with real Bitcoin. Treat all keys as disposable learning material.

Feedback welcome

This is early and meant to be useful, not perfect. Just for fun. I'd especially appreciate thoughts on:

Is the UTXO → fee → mempool → confirm flow clear enough?
What topics should the Security tab cover next? (RBF, CPFP, coin selection, etc.)
Any bugs on mobile or desktop?

Try it here: https://cinexacode.fun/bitcoin-sandbox

If you find it helpful, a reply or suggestion for improvement is appreciated.

Thanks,
Cinexa