velxio

Velxio lets you write, compile, and run microcontroller code in your browser without owning a single board. Pick an Arduino, ESP32, or Pico, write your C++ or Python, wire up components on screen, and it actually emulates the chip, not a cartoon of it. Under the hood it runs real CPU emulation through projects like avr8js, rp2040js, and QEMU, with a Monaco editor, more than 48 interactive components, a serial monitor, and Arduino library support. For makers and students, that means prototyping before the parts arrive.

You can use the hosted version at velxio.dev for free, which is how most people will start. Self-hosting is supported through Docker, but it is a multi-service app, a TypeScript frontend with a JavaScript and Python backend doing the emulation, so it is more involved than a single container. Call it moderate effort to run your own.

The license is AGPLv3, with a separate paid commercial license for anyone who wants to use the code inside a closed-source product. That is the key thing to understand: you are not paying to unlock features, you are paying to escape copyleft. The hosted app is free and full-featured. Solo makers, students, and educators are the obvious fit. It substitutes for the paid tiers of Wokwi, and on the desktop side competes with Proteus and the open-source SimulIDE and Renode.

The catch is maturity. This is a young project with a solo maintainer, so the bus factor is one and the long-term roadmap is unproven. The simulation is impressive today, but bet your curriculum or your product on it with that in mind.