#vactrol

The ICs you see in most "true" analogue circuits? Bundles of opamps.

The knobs on the front? Potentiometers, i.e. resistors whose resistance varies with knob position.

LEDs? Diodes that emit light. Clue's right there in the name.

But of course modular wouldn't be modular without the magic of voltage control. Signals controlling other signals. Invisible hands to turn the knobs for you.

And this presents a problem in my simplistic view of the field: with the exception of potentiometers, all these components do very specific jobs, over ranges set in stone based on their physical properties, to be listed ad nauseum on the inscrutable pages of online component stores. And the potentiometers? They need real hands to turn them. Hands neither invisible nor automatic. Hands of which I only possess two. What do you want me to do, use motors or something? What do you think this is, mechanical engineering?

No! We demand ethereal hands. Electronic hands. We demand control over more than two parameters at once, without resorting to such crass concepts as physical motion, and without unleashing our biblically accurate forms upon the delicate psyches of our listeners.

Well, reader, in a carefully constructed act of lying by omission, I have hitherto neglected to mention our saving grace: the LDR, or light-dependent resistor. A magical resistor whose precise resistance varies depending on its degree of illumination.

So. We have LEDs, that emit an amount of light proportional to the current running through them. And we have LDRs, whose resistance varies depending on the amount of light falling on them. Surely it can't be that simple? Surely the answer to our voltage-control conundrum can't lie in simply mashing these two together and calling it a day?

Presenting, dear gentlefolk, the mighty vactrol.

Source

(The actual filter being built there is not very interesting, it's just a passive, non-resonant LPF, but it's voltage controlled by a homemade vactrol.)

Yup. It really is that simple. So next time you are about to embark upon the study of an unfamiliar field, unsure of whether your feeble smooth brain will be capable of grasping the arcane magicks within, remember the humble vactrol.

Of course, there are also transistors. But those simply aren't as entertaining.