Sunday, March 14, 2021

Circuit simulation

 I've been playing with designing simple digital circuits this weekend. Since my breadboards are not with me at the moment, I decided to look for circuit simulators.

Here's a nice comparison of several:

Before I found that comparison site, I tried out and even threw them money for a month's worth. And I can say that I've gotten that much worth of enjoyment out of my tinkering this weekend, so money well-spent. But I knew that I didn't want to keep shelling out every month (I don't do digital design that much), and there was no way to export the circuits in a way that I could save them. So I kept looking.

Here's my CircuitLab home:

I haven't tried all the choices in the "top ten" list, but I did try the "CircuitJS1" simulator maintained by Paul Falstad and Iain Sharp. See It isn't quite as nice as CircuitLab, but it's hard to argue with free, especially given my infrequency of use.

CircuitJS1 doesn't host users' designs. In fact, they don't integrate well with any form of storage. You can save your design to a local file, but the simulator doesn't do a good job of remembering file names. It presents you with a link containing a file name of the form "circuit-YYYYMMDD-HHMM.circuitjs.txt". You can save the linked contents with your own file name, but the next time you go to save, it obviously won't remember that name since it was a browser operation. All of this will make it a little inconvenient and perhaps error-prone to manage different projects. If I were doing a lot of hardware work, I would probably choose something else. But for occasional fiddling, this is fine.

Here's a simple state machine that checks even/odd parity of an input bit stream:

If I want to make anything public, I'll make them as github projects.

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