Jim: I just went through a similar search for pedagogical electronic design. Google "EDA" (Electronic Design and Analysis). There are a number of FOSS (Free and Open-Source Software) EDA tools for both Linux and Windows. The venerable program "Spice" can do time-stepped simulation/emulation of electronic circuits, but I don't think that Spice will do this symbolically. However, I think that there are some other programs can also read the same Spice files and -- at least for *linear time-invariant circuits* -- produce and solve symbolic equations. Spice files have a trivial format, wherein each line describes a link in the schematic graph structure: C23 2 3 0.1 would be a *capacitor* going from graph node labelled "2" to graph node labelled "3" and have a capacitive value of 0.1 Farads. The graph node labelled "0" is "ground", by convention. The process of going from the schematic circuit to the mathematical matrix format is called "Nodal Analysis" or more likely "Modified Nodal Analysis". Each "link" becomes *four* entries in the matrix -- at four corners of a rectangle -- called a "stamp" (?!?). I believe that the mechanical engineers utilize a similar -- if not identical -- format for their matrices. If you want to play with *digital signal processing*, I can also recommend "GnuRadio" and "GnuRadio Companion" (GRC). GRC allows you to lay out a digital signal processing system in block diagram form and then emulate it in real time. You can play with GRC using just your microphone and speakers -- you don't need any actual radio hardware. At 05:09 AM 1/29/2018, James Propp wrote:

Anyone know of any good designs for an easy-to-make analogue computer based on springs, masses, and dashpots?

That is, we want a supply of easily-interconnectable components that we can combine in ways that correspond to a prescribed differential equation, so that the behavior of the system will be a solution of the equation.

As I recall, there's also a way to get hydraulic analogues of LRC networks (though I forget what the three sorts of components are called); is there a good design for a hydraulic computer?

Jim Propp