Logic.ly – Digital logic simulator for teaching logic gates and digital circuits

Logic.ly – Digital logic simulator for teaching logic gates and digital circuits
logic.ly

If you find it fun to play with digital logic, you should first look at combinational logic, and then add some registers and get clocked logic with memory / registers. Once you've got that down, I'd recommend two next steps:

1. Look at some simple circuits that combine analog and digital. For example, [1] a digital 4-bit counter made up of four half-adders. (It's called a half adder because it only adds two bits A+B, producing a value 0, 1, or 2. In contrast, a full adder adds three bits A+B+C, producing a value 0, 1, 2, or 3.) The four output bits of the 4-bit counter are connected to a simple op-amp and resistor based digital-to-analog converter (DAC). This helps you understand that we have lots of ways of representing signals, and in engineering practice we combine these modalities to useful effect.

2. Instead of merely using software to model circuits, understand that circuits are a great way to model all sorts of real-world problems. You may have heard of "analog computers" [2] which predate digital ones, and were used for modeling all sorts of engineering problems. You may now use a circuit simulator to model mechanical systems, thermal systems, or even the spread of COVID-19. I've done the latter here [3], using a few capacitors and current sources to implement the differential equations of an epidemic. Because you can define current sources algebraically (i.e. the current can be proportional to other currents and voltages in the circuit), you can easily take advantage of the simulator's underlying ability to simulate arbitrary systems of differential equations. I've explained the COVID-19 model here [4] as previously discussed on HN.

[1] https://www.circuitlab.com/editor/53xa3r/

[2] https://en.wikipedia.org/wiki/Analog_computer

[3] https://www.circuitlab.com/editor/zubfhu8p3q3v/

[4] https://www.circuitlab.com/blog/2020/05/28/surprising-covid-...

Writing software to do digital logic simulation is quite simple. Everything is event-driven: if a signal "A" changes from 0->1 at time t_0, and this signal is an input to some gate "X", then at time t_0 + t_p (propagation delay) the output of that gate will update with the new value, as defined by that particular gate/register/whatever. So events can originate from (1) external signals (such as buttons/switches), (2) clocks, (3) the output of an earlier gate updating its output after a propagation delay. You can then just propagate changes throughout the digital circuit, looking at what's connected to the signal that just changed state.

It gets a bit more tricky once you combine analog and digital simulations simultaneously. :)

Show HN thread from a couple of years ago: https://news.ycombinator.com/item?id=17508151

I played with it while reading the excellent book Code: Hidden Language of computers by Charles Petzold.