Velxio is an open-source Arduino emulator that runs entirely in the browser. The project allows users to write Arduino sketches, compile them using arduino-cli, and simulate microcontroller hardware using real CPU emulation. The emulator currently supports: • Arduino Uno (ATmega328p) • Arduino Nano (ATmega328p) • Arduino Mega (ATmega2560) • Raspberry Pi Pico (RP2040) Key features: • AVR8 CPU emulation using avr8js • RP2040 emulation using rp2040js • 48+ interactive electronic components • Monaco-based Arduino code editor • Serial monitor with UART emulation • Hardware peripheral simulation (GPIO, SPI, I2C, ADC, PWM) • Arduino library manager • Self-hosting via Docker The goal of the project is to provide a fully open-source environment for experimenting with Arduino circuits and understanding how microcontroller emulation works internally. Live demo: https://velxio.dev Source code: https://github.com/davidmonterocrespo24/velxio