Make Analogue Circuit Elements

In our hands-on analogue computing simulation using OpenModelica, the basic functional elements were treated as black boxes, much as they were to the original analogue computer programmers. If you want to delve a bit more into how these circuit elements work, however, you can do so by wiring up a few electronic components on a piece of breadboard.

Potentiometers are trivial because you can buy them as single components. However, summers and integrators are more interesting. Both are built around an electronic circuit called an operational amplifier (op-amp) which is a high-gain amplifier that you can buy as an integrated circuit. A single op-amp is available as an eight-bit chip, while larger ICs containing multiple op-amps are available.

The diagram on page 75 shows typical circuits for a three input summer and a single input integrator. Note that, as with old analogue computers, both these circuits generate a negated output. Checking out the summer is achieved by applying different voltage sources to the inputs and observing the output using a test meter. To check the integrator, apply a voltage of, say, +0.5V to the input and you’ll see the output ramp down (i.e. to a negative voltage) over a couple of seconds. Remove the voltage and the output will remain constant, and change the voltage to -0.5V and it’ll ramp up again. Because we’re using a cheap op-amp, it’s susceptible to noise so the output of the integrator might increase or decrease slowly, even with no input. If you decide to try it out with real components, we suggest using a TL071 op-amp for which the pin numbers are shown on the circuit diagrams.