DEV Community: Dhananjay kumar Seth

LabBench grew to 9 tools, and I shipped a second product

Dhananjay kumar Seth — Sat, 25 Jul 2026 21:29:49 +0000

A few weeks ago I turned four free browser-based engineering demos into LabBench — a suite people could actually use, not just look at. This week it grew again, and I also launched something completely separate.

Two new LabBench tools (7 → 9)

EV Battery Management Simulator — a 4-cell pack model with real coulomb-counted state of charge, an OCV + IR-drop voltage curve, a lumped thermal model that trips a simulated BMS above 70°C, and passive cell balancing you can actually watch converge over time.

Smart Energy Meter Dashboard — toggle household appliances and watch real vs. apparent power, power factor, a source-impedance voltage sag model, breaker-trip current logic, and live kWh billing, all computed from scratch.

Both reuse the same Supabase-auth + Pro-gated save/load pattern as the rest of the suite.

Try them: https://labbench-hub.vercel.app/

A second, separate product: PrepBench

Not ECE this time — general aptitude-exam prep (Percentage, Profit & Loss, Time & Work, Speed-Distance-Time, Number System, Probability, Squares & Cubes, Geometry, Data Interpretation). Free interactive reference + practice drills with instant feedback and a live streak/accuracy tracker, no sign-up needed, plus a paid cheat-sheet PDF bundle.

Try it: https://prepbench.vercel.app/

Both are open source under a source-available license — code's on GitHub if you want to see how any of it works: https://github.com/Dhananjay-ku-seth

Feedback welcome, especially on the battery thermal model — first time modeling BMS behavior client-side.

I Turned My Free Browser Tools Into a Paid Product (Here's Exactly How)

Dhananjay kumar Seth — Thu, 16 Jul 2026 17:26:57 +0000

A few weeks ago I wrote about four interactive engineering tools I built — a real-time FFT lab, a PID control simulator, a logic circuit sandbox, and a comms simulator. They were free, and they still are. This post is about what happened after: turning that into LabBench, a real (small, but real) product with paying customers.

The tools stayed free. The brand didn't exist yet.
Four separate demo names don't add up to anything memorable. So I built one landing page — labbench-hub.vercel.app — presenting all five tools (added a fifth: a Verilog-style timing-diagram viewer) as one suite, with a shared visual identity layered on top of each tool's own accent color.

The licensing mistake everyone makes (including me, at first)
None of the repos had a LICENSE file. That sounds harmless — it's actually the opposite of open. Under the Berne Convention, no license means "all rights reserved" by default. The fix wasn't slapping on MIT, though — that would let anyone fork the free tools, build the exact paid features I'm planning, and ship them first. I wrote a short custom source-available license: free to use the deployed demos, free to read the code, not free to resell or rehost. Every LabBench repo carries it now.

The payments problem nobody warns you about (if you're not in the US/EU)
This was the real surprise. Every "add a tip jar" tutorial assumes Stripe works for you. I tried Ko-fi first — its bank-transfer country search returned zero results for India. Buy Me a Coffee's own FAQ confirmed payouts route through a personal Stripe account, and Stripe Connect payouts to individual Indian creators are heavily restricted without a registered business entity.

The fix: stop chasing Western platforms, go regionally native. Razorpay — an Indian payment processor — supports individual/freelancer accounts directly, with UPI, cards, and netbanking working once KYC clears. If you're building for a specific country's audience, check that country's creator payout path before you build anything around a platform everyone recommends by default.

Shipping actual paid products
Two PDFs, sold through Razorpay Payment Pages:

LabBench Study Companion — a hands-on guide to DSP, PID control, digital logic, and comms theory, each chapter pointing back to the matching interactive tool.
The Making of LabBench — the build-log version of this post, with real code snippets and the productization story in more depth.
Automating delivery without a real backend
None of the five tools have a server — they're static Vite apps on Vercel. Rather than stand up a whole backend just to email a PDF after payment, I added one file: /api/fulfill.js, a Vercel serverless function that:

Verifies the Razorpay webhook's HMAC-SHA256 signature (constant-time comparison, using Node's getRawBody since the signature is computed over the raw request body, not the re-parsed JSON)
Matches the payment description against a small product list
Emails the matching PDF via Gmail SMTP (an App Password, not OAuth — zero new accounts, since Resend/SendGrid both wanted a verified custom domain I don't have yet)
Vercel deploys anything under /api as a serverless function automatically, regardless of the frontend framework — no Next.js required. One file, no database, and it works.

What's next
A real Pro tier (saved sessions, exports, classroom mode) gated behind actual accounts — and the highest-ceiling option that needs zero new code: reaching out to colleges and coaching institutes directly for classroom licensing, since they have budgets individual students don't.

Try the tools free: https://labbench-hub.vercel.app/ More of my work: https://dhananjay-kumar-seth.vercel.app/

If you're building something similar from outside the US/EU, I'd genuinely like to hear what payment path worked for you.

I Built 4 Interactive Engineering Tools You Can Run in Your Browser

Dhananjay kumar Seth — Tue, 14 Jul 2026 20:06:01 +0000

Most engineering concepts live in textbooks and MATLAB scripts — you read about a Fourier transform, a PID loop, or a constellation diagram, but you rarely get to touch one. I'm a game developer by day and an Electronics & Communication Engineer by training, and I kept thinking: why can't these be things you just open in a browser and play with?

So I built four of them. No installs, no accounts — click a link and start turning knobs. Each one is written from scratch in React + TypeScript with the actual math implemented by hand (no DSP or simulation libraries). Here's what I made and the interesting problem behind each.

🌊 DSP Signal Lab — real-time FFT in the browser
▶ Live demo: https://dsp-signal-lab.vercel.app/ · Code: https://github.com/Dhananjay-ku-seth/dsp-signal-lab

A signal generator (sine / square / sawtooth / triangle) feeding a live 2048-point FFT spectrum analyzer, with injectable white noise and switchable digital filters (lowpass / highpass / bandpass / notch).

The fun part: the FFT is real, not a fake animation. It uses the Web Audio API's AnalyserNode, so when you pick a square wave you can watch the odd harmonics appear in the spectrum, then sweep a lowpass cutoff down and see them get attenuated in real time. Switch to microphone mode and whistle — the peak tracker finds your pitch.

🤖 PID Control Playground — tune a line-follower robot live
▶ Live demo: https://pid-control-playground.vercel.app/ · Code: https://github.com/Dhananjay-ku-seth/pid-control-playground

A top-down robot follows a winding track using a real PID loop on its cross-track error. You drag the Kp / Ki / Kd sliders and watch the effect immediately.

Under the hood it's a double-integrator plant with sub-stepped Euler integration and integral anti-windup — so it behaves like a real control system. Set the derivative gain to zero and the robot oscillates around the line forever; add it back and the wobble damps out. There's a ⚡ disturbance button to kick it off course and watch the loop recover, plus a live error-vs-time strip chart.

🔌 Logic Circuit Simulator — drag, wire, and watch signals flow
▶ Live demo: https://logic-circuit-sim.vercel.app/ · Code: https://github.com/Dhananjay-ku-seth/logic-circuit-sim

Place gates (AND / OR / NOT / NAND / NOR / XOR / XNOR), wire them port-to-port, toggle the inputs, and see the signals propagate live — with an auto-generated truth table.

The tricky bit was supporting sequential circuits, not just combinational ones. A naive left-to-right evaluator deadlocks on a feedback loop like an SR latch. I used an iterative relaxation solver (40 passes) that evaluates combinational logic instantly and converges feedback loops, so latches and flip-flops actually settle. Load the SR-latch example and toggle Set / Reset to see it work.

📡 Comms Simulator — modulation, constellations, and BER
▶ Live demo: https://comms-simulator-pi.vercel.app/ · Code: https://github.com/Dhananjay-ku-seth/comms-simulator

Three tabs covering a communication systems course:

Analog: AM/FM modulation in the time domain, with the AM envelope and over-modulation shown live.
Constellation: BPSK / QPSK / 16-QAM ideal points plus the received cloud over an AWGN channel, with a live symbol-error rate.
BER curve: a Monte-Carlo bit-error-rate simulation (60,000 bits per SNR point) plotted against the theoretical Q-function on a semilog axis.
Everything uses unit-energy symbols, Gray-coded PAM-4 mapping, Box–Muller AWGN, and Q(x) = ½·erfc(x/√2). The satisfying moment is watching the simulated BER curve hug the theoretical one — and seeing it floor out at ~10⁻⁵ because 60k bits can't resolve errors any rarer than that.

How they're built
Stack: React + TypeScript + Vite, deployed on Vercel.
No black boxes: every FFT, filter, control loop, logic solver, and modulation/demodulation routine is hand-written. The goal was to understand the math, not import it.
Rendering: plain and SVG — no charting libraries.
Design: a shared dark "lab" aesthetic so they feel like one toolkit.
Why bother?
Because interactive beats static every time. You can read that a PID controller overshoots without derivative gain — or you can drag a slider and watch it happen. I wanted tools a student (or a curious recruiter) could open and immediately get an intuition from.

If you find these useful or spot something to improve, I'd love to hear it — the repos are open.

More of my work: https://dhananjay-kumar-seth.vercel.app/ GitHub: https://github.com/Dhananjay-ku-seth · LinkedIn: https://www.linkedin.com/in/dhananjay-kumar-seth-4a5b31283/

Thanks for reading — go break one of the demos. 🔧