Convert varying voltage to resistance

Question

I'm just beginning with electronics, with the goal of building synth and audio effect modules. I'm currently building and tweaking projects from http://tagboardeffects.blogspot.co.uk.

One mod I'm very keen on is adding the option to use a 0-5v control voltage (CV) instead of manually turning a pot. On commercial modules this is achieved by plugging the CV source into a TRS socket on the unit, which overrides the pot.

I've been able to construct a simple version of this using an LED and photoresistor wrapped in electrical tape (a "vactrol") and hooked up to an expression pedal that provides the control voltage, but I'm running into the following problems with the photoresistors I can find online:

1. They only seem to be available in a small range of resistances. I'll need to replace quite a wide variety of pots -- typically combinations of 10k, 25k, 50k, 100k, 1M and 2M on a single unit. Some are linear, some log, though that's less critical.

2. They don't seem to go down to zero; typically I see something like "50k to 100k" instead. I'm not sure how (or whether) I can translate this into a replacement for (say) a 0 to 100k pot. That may be my lack of basic understanding showing through though.

I suppose it must be possible to do this, since many modern units include it, but maybe the vactrol scheme isn't the right one. I'd accept an answer that shows me how to adopt a vactrol solution to a particular circuit (see below) or one that suggests an entirely different approach. I'm specifically looking for a low-cost solution, ideally not costing more than the pot itself.

EDIT: For the sake of concreteness, if that helps, suppose I've built this circuit and want to replace the three pots with CV inputs.

Answer 1

The simplest way to do this may be digitally. If you get a low cost microcontroller (e.g. arduino, but there are many, many options), you can use the on-board ADC to sample the control voltages. Then, use either SPI or I2C to control commonly available digital potentiometers, which come in a wide variety of resistance values.

Searching for analog solutions to this problem, there are many posts on the subject - but none of them offers a clean, straightforward, universal solution. While analog solutions are certainly possible, my guess is in the long run the digital solution will be both easier and cheaper to implement. Digital potentiometers truly behave like physical pots, whereas any analog replacement will wind up with limitations (such as you noted above) that will mean you'll need to carefully consider how they interact with every circuit they are used in.