Hi The Heath kit has 9 op-amps that are connected such that they are all inverting. It is intended to primarily model things that are linear systems but one can add such things as diodes to make limiting functions. They do this for one of the example programs to put a bouncing ball on an oscilloscope screen. Other nonlinear functions can be added. Although, one can model things with differentiators, these are generally not very stable and tend to require more dynamic range than integrators. Most linear systems can be described as a differential equation. If one integrates the equation a number of times, one can remove all the differentials and be left with a number of integrals and initial constants. These can be quickly scaled and represented by resistors and capacitors. This modeling method can solve quite complex problems. Things like aircraft controls to auto suspension systems can be modeled this way. I had a friend describe how it was used to make rail cars that wouldn't oscillate at the same frequency as the spacing of the track joints, over normal speed ranges. One can imagine what would happen to the load and car if it were increasing the oscillation on each track joint it crossed. I currently have a EC-1 but a while back, I needed to solve a control system problem with an oven. There were several factors that effected the temperature and I needed to make sure that the system could handle these changes without oscillating or over shooting. I found an analog modeling program called TutSim that I used to understand the system. This is just like using a real analog computer except it used discrete processing to simulate the analog world. There are potential problems doing this and one needs to understand the possible issues. If you have a spice program, you can build up the equivalent of a EC-1 in software and see how it works. It wasn't until the late 80's someplace that digital computers became fast enough to replace the simpler op-amps for solving these kinds of problems. In many cases, the analog machines still have better speed/accuracy tradeoffs. My biggest complaint with the EC-1 is that they needed more binding post for the common ( 0 Volts ). Dwight