By Bryan Bergeron View Digital Edition

If you’ve been limiting your application of circuit theory and design to electronic devices, then it’s time to think outside of the box. If you have a working knowledge of Ohm’s Law, basic R-C and L-C filters, and operational amplifiers, then you can model systems ranging from the circulatory and renal systems to rocket propulsion and fluid dynamics.

Moreover, if you’re quick at breadboarding or — better yet — fluent in one of the many circuit simulation languages, then you can experiment with dozens of what-if scenarios within minutes.

As an example, let’s say you want to model the cardiovascular system. You could start from first principles, modeling the contractility of the heart muscle fibers and the like, or simply use electronic equivalents. For the heart, let’s use a function generator set to a rate of, say 72 pulses/minute.

For the aorta, use a relatively large value electrolytic capacitor fed through a resistor from the function generator; for the peripheral resistance, a potentiometer across the electrolytic capacitor. For monitoring the equivalent of blood pressure, let’s put an analog voltmeter across the electrolytic capacitor.

When we apply power, the analog voltmeter registers a peak value that we’ll call systolic blood pressure. The minimum value registered on the voltmeter corresponds to diastolic blood pressure.

Now, let’s say we want to model systemic hypertension. We increase the value of the potentiometer, which signifies an increase in peripheral resistance. Now, the peak voltage across the capacitor — representing systolic blood pressure — is increased, as is the minimum voltage across the capacitor, representing diastolic pressure.

Of course, you’d have to pick the appropriate values of capacitance and resistance to model a particular system and disease state. Also, to get a better idea of what’s happening second-to-second, you’d need an oscilloscope.

This is just one example of how electronic circuits can be used to rapidly model the behavior of a variety of systems. In practice, few people actually do modeling with discrete components, and instead opt to use a circuit modeling and simulation application.

My latest favorite free circuit simulator is “Quite Universal Circuit Simulator” or QUCS from SourceForge (**https://sourceforge.net/projects/qucs**). It’s probably too advanced for someone new to circuit simulation, but the graphical user interface is both intuitive and easy to use.

If you want to learn more about modeling systems with electronic circuits, search the Web for “electrical equivalent model of <domain>” — where <domain> is the system or process you’re interested in modeling. For example, when I searched for “electrical equivalent model of steel corrosion,” I found circuit diagrams, academic papers, and other references that detailed the design and use of specific circuit diagrams. Most models were based on a simple voltage source and groups of interconnected RC filters.

Clearly, whatever your areas of interest, you can start modeling with electronic circuits today. **NV**