I use this a lot for quickly testing things out when I am not entirely sure what I need. It's incredibly powerful, and give you scopes so you can verify things work. It may be very old and look ancient by modern standards but it's much more usable for me. Also I don't need sonmething to simulate an Arduino, I need something to say test a op-amp summing amplifier.

the simulation blows up sometimes

it doesn't look ancient to me

I was trying to rig a feedback loop between a lamp and a photoresistor but it isn't working. Do you know if the photoresistor actually reads in light generated by other circuit components, or is it just supposed to simulate a variable resistor responding to light from outside the circuit?

I've contributed quite a lot to Paul's simulator, including doing the port so it runs natively in the browser.

The photoresistor is just controlled by the slider in the side-bar. It doesn't respond to lights in the circuit. There is an "optocoupler" under "active building blocks" that does connect optically.

I've seen a few requests to add feedback that occurs outside the electrical domain - light is one, but also for mechanical movement (motors that turn generators). But, I pesonally think it's hard to know how to model all these non-electrical interactions in a way that is visually clear and effective.

Thanks for your contributions. I played circuit.js obsessively during high school (as well as some of the other simulators). It was just way more intuitive than spice. I picked up Art of Electronics around that time.

Ten years later, just the other day I was using it to prototype some filter and resonator circuits.