Defining What I Really Want from This Simulator

The first step is to be clear about what I am actually looking for in this simulator.

There are plenty of drag-and-drop logic simulators out there; I have used some myself back in university. Even then, I had issues with their graphical interfaces; it drove me crazy to see wires crossing all over the place, and after carefully arranging them, they would get messed up again just by adding a new component or, sometimes, simply by closing and reopening the file.

So, the first requirement is:

1. It must be a pure code simulator. Being able to visualize the circuits somehow would be a bonus, but the core requirement is that I can implement and connect components using functions and Python code.

The second point that I think is important is that I want the simulation to be done at the lowest possible level: using universal logic gates. Many of the simulators I have seen simulate components based on their behavior, without using the sub-components they are actually made of. I want to build components from simpler ones, and then only simulate the simplest ones. Lots of bit-level operations!

2. It must simulate each component from the simplest logical elements. As a plus, it would be nice to have an optional way to simulate some components purely in Python for performance, but only if I choose to. This option will be explored if performance becomes an issue.

It is also important to be clear about what we do not need:

3. We do not need to simulate electrical signals in fine detail. We will only simulate 0s and 1s, and time will be discrete (stepped), not continuous.
4. There will be no intermediate states between 0 and 1, however it would be nice to simulate race conditions and glitches. This forces a better design.

Evaluating Existing Circuit Simulators

Before writing anything from scratch, I like to take a look at “the competition”. Even though writing a Python program to simulate circuits will surely teach me a lot, it is not the main goal of this project. The main goal is to understand how a computer works and to be able to build one myself with my own hands keys.

I am going to list some existing Python libraries and tools that I found. JavaScript alternatives are also included, as they can be useful for interactive online simulations. However, as I am already familiar with Python, and I now have Marimo Islands (Python in the browser) working in Cheese Bytes, I prefer to stick with Python for this project.

• PySpice: As described in the README: “PySpice is a Python module which interfaces Python to the Ngspice and Xyce circuit simulators.” Not what I am after; I do not want to install any extra programs.
• Online Logic Simulator (lodev): A JavaScript library that lets you interact with circuits. Does not seem suited to simulating a full computer, but it is fun to play in real time with the example circuits on their main page. The site also has a series of articles/tutorials explaining certain circuits.
• pyeda: At first it looked promising, but from what I can tell (and perhaps due to my ignorance), it is more about logical operations and simplification. Could be useful for designing efficient circuits, but that’s not my priority now. I just want something functional, speed is not a concern.
• pyhdl: Closer to what I want, but it is a small project (few stars on GitHub) and I do not like how it simulates components. I want to simulate NAND gates and build everything else from them — not simulate components directly in Python.
• myhdl: The most serious project I have found. It feels a bit complex at first glance and has many features I do not need, since I am not planning to take this to hardware (ASICs or FPGAs) for now. Like others, it does not simulate fundamental logic gates; it simulates higher-level components.
• CircuitLab: A modern web-based circuit simulator. It has a free (and incomplete) book teaching basic electronics and, I assume, how to use CircuitLab.
• CircuitVerse: Another interesting simulator. It also has a book/tutorial introducing circuit design. The code is on GitHub and circuits can be imported/exported in JSON format.
• logic.ly: Visually attractive and exactly the kind of simulations I am looking for. However, it is paid software, and it does not seem possible to program components or connect them using code.

After several hours of research, I decided to make my own implementation.
Deep down I knew this would happen… I would find some flaw in any library just to give myself the excuse to code my own. And honestly… it is going to be fun!