# Convert varying voltage to resistance

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.

• What specifically does "low-cost" translate to in your preferred unit of currency? Aug 15, 2016 at 8:15
• the vactrol seesm like a good solution to me, perhaps you can re-arrange the circuit to that the fact it doesn't go all the way to zero does not matter. Aug 15, 2016 at 8:19
• @Andyaka Fair question; I guess the parts for one pot replacement should cost no more than 2GBP, i.e. about the same as the pot itself. Aug 15, 2016 at 8:24
• @Jasen That's possible; my theoretical understanding is limited at the moment so I'd VERY happily accept an answer that suggests how to work around the problems with the vactrol approach. Aug 15, 2016 at 8:26
• Varactrols have a very slow response, and long term drift. If you can redesign the circuit to use resistors that vary over a smaller range, say 5:1, and using those in a Wheatstone bridge arrangement would allow voltage to be controlled down to zero, then low power high voltage incandescent lamps make good variable resistors. AC couple the signal, use a current source for control Aug 15, 2016 at 10:01

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.