in sim/visuals/board.ts [268:272]
updateState() {
const opacity = this.pin.mode & PinFlags.Digital ? (this.pin.value > 0 ? 1 : 0)
: 0.1 + Math.max(0, Math.min(1023, this.pin.value)) / 1023 * 0.8;
this.ledElement.setAttribute("opacity", opacity.toString())
}