LDRs as pots?

[Gregg]

Hello,

I read somewhere that some of the dual cell Vactrols /the VTL5Cx series optoisolators/ can be used instead of mechanical potentiometers /by controling the LED brightness/. I can figure out how to do it with two separate LDRs. Wouldn't however the resistance of both halves of the dual LDR increase and decrease simultaneously?

[Tom Phillips]

Hello,

Wouldn't however the resistance of both halves of the dual LDR increase and decrease simultaneously?

Yes. There is only one light source inside so a dual element unit cant be used as a pot. If you are interested, there is an application note showing a dual unit in a circuit at http://optoelectronics.perkinelmer.c...on%20notes.pdf It's at the very end of the article.

Tom

[Gregg]

Thank you for the link but I don't see how it can be useful in a tube amp.

[Tom Phillips]

Right. I didn't mean to imply it would be useful in a tube amp. Just an informational link to give a clue about how the dual LDR part is applied in a circuit.
Cheers,
Tom

[Steve Conner]

I once helped design a DJ mixer that used a pair of dual vactrols in its crossfader. Each vactrol acted as a dual gang variable resistor to attenuate one stereo signal, and the crossfader was wired up to dim the LED in one vactrol while it brightened the other. I think that might be the kind of application you were thinking of.

If you want to use vactrols as a replica of a pot, you are absolutely right that you need two separate units and some kind of clever control circuit that dims one while brightening the other. In a pretty complex non-linear way, if you want it to act like a real pot, keeping its total resistance constant while moving its "tap". I guess you could use two dual units, and use the unused section of each for feedback to a control circuit.

I heard rumours that the Mesa Boogie Triaxis used LDR "Pots" like these to give it its programmability, though I don't know how they did the linearisation. Randall Smith probably patented it so you can go and look it up and figure out a way round it. (Or just rip it off if you're feeling brave.)

[Gregg]

Triaxis used LDR "Pots" like these to give it its programmability

I heard that too. I'm looking for something like this because I'm planning to build a programmable all tube preamp. Unfortunately the current production digital pots specs are not very suitable for applications where large voltage swing is present.

[Rick Erickson]

How about motorized faders or pots?. All you need is a circuit to control their position, and it's cool to watch them move by themselves.

RE

[Gregg]

This is interesting but how about when switching between different presets?

[Rick Erickson]

That's the fun of design - you have to sort through the options and find the best solution for yourself. Sometimes you even have to spend time and money on things that don't work. It's all part of the process.

RE

[Gregg]

I think I'll stick to the LDRs. Maybe create a logarytmic table in Excel, take say 20 positions /from -10 to +10 for example or from 1 to 20/, then set the values manually via regular pots to ADC and memorize them.

[Enzo]

To replace a volume control, could you not:

From the signal source - plate cap perhaps - run a series resistor to the following stage - grid perhaps - and also from that point an LDR to ground. The two form a voltage divider, and the variable resistance of the LDR makes the two work more or less as a pot would. Maybe?

[Gregg]

Do you mean something like Mesa style volume control? Using two Vactrols for Gain, Volume and Treble seems to me like a better idea. Bass, Mid and Presence can go with single LDR.

[carl gigun]

Enzo's suggestion of one LDR and one resistor as a voltage divider between stages is exactly how old tube compressors like the LA-2A accomplished their gain control. In their case they had the dry signal drive the light so it would automatically "turn the volume down" when a hot signal came along.

I guess storing presets is a separate problem that needs to be tackled whether you use one or two LDRs.

[Tobacco Soup]

This is a great thread! I had a similar idea about a programmable & MIDI controllable tube preamp of my own design. Storing parameters and their control/access can be accomplished with a microcontroller and EEPROM. What the data means, and what it does is the fun part. An 8-bit resolution can give you 256 increments, which is far more of an 'analog' interface than the ADA/Triaxis broad brushstrokes. Anyways, you've given me a lot of food for thought with the LDR idea, and I thank you! A lot of the other options I had considered weren't applicable to tube circuits, so I was mulling-over the possibility of controlling tubes with transistors and the like.

[Gregg]

The question /at least for me/ is how to control the LED current /brightness/? I did some search and it looks like LED drivers from different companies are available, some of them programmable, but are they suitable for a project like this? For example:

http://focus.ti.com/paramsearch/docs...ODE_STRY_PGE_T

http://para.maxim-ic.com/cache/en/results/4959.html


Any other ideas?