CV for vactrol rhythmic patterns?

[zloth]

Hi,
Sure everyone is fed up with novices asking about this, but should I use an r/2r ladder before the LED or would a simple set of resistors in series work?
Thanks

[JovianPyx]

[zloth]

Thank you for your reply
Perhaps if I explain as best I can:
As I understand it, a vactrol is basically an enclosure containing an LED & an LDR, the brightness of the LED controls the LDR which can be used in place of a potentiometer.
I want to be able to vary the brightness of LED in a rhythmic way to control pitch, filter cut off, anything that could be done with a pot. Basically a simple type of CV-driven sequencer.
I have used an r/2r with various timing sources (4093 LFOs, 4060 ripple counter, shift register) for creating a CV to control the pitch of a 4046, and assumed it could be used similarly to vary the brightness of the LED inside a vactrol.
On the dlbelectronics Lunetta page there is a schematic for a WSG filter modified "I got rid of the coarse and fine pots for cutoff and instead added 2 CV inputs via Vactrols for controlling cutoff with other modules... I used some hand made vactrols, 4N25 opto-couplers as well as a couple VTL5C3's. It would be a good idea to add a pot as an adjustable voltage divider before going to the Vactrol's LED. This way you would have more control over the level of CV getting to the LED." This got me thinking (never a good thing) that a CV from a DAC could be used, but wanted to ask more experienced Lunettists before investing in anything.
Hope that clarifies my situation at least a little

[PHOBoS]

hmm, never thought about using an R2R to control a vactrol, could be interesting. It might work if you choose the
resisors small enough, but don't make them too small or it will draw too much current from whatever you want to
drive it with. There is a better method though which is using an opamp as a current source. There are several ways
of doing it, with or without extra transistors, but you can easily connect an R2R to the input of it. If you search for
"opamp current source" you should be able to find something useful.

[zloth]

Thanks
I normally use a 3 or 4 bit DAC (50k/100k) with a pot between that and the 4046 to control amplitude and therefore tonal range, so I thought that might work. To be honest, it was the only way I could think of doing it with my limited knowledge/experience.
Whilst on the subject of DACs, has anyone tried using outputs from a 4026/4033/40110 with other clocks into a DAC to create unusual "polyrhythms" or would that not work from a technical point of view?

[gasboss775]

Another possibility is using a weighted resistor network. The idea is the most significant bit goes through the lowest value resistor, which we shall call R, then the next less significant bit goes to a resistor of 2R, then 4R, 8R, etc and theses are summed together usually at a virtual earth point of an opamp based inverting amplifier.

I tried this passive version on my Everycircuit simulator app & it appeared to work quite well, the switches represent the logic inputs ( I made them 0 & 12 Volts as that's what I usually use with CMOS circuits ) the 12 volt source and 47K resistor on the right provide a bias to overcome the forward voltage of the LED.

You would probably need to experiment with the values of the resistors to suite your specific vactrol as there is quite a large difference between different parts, same goes if you brew your own.

40K can be made with an 18K & a 22K in series, 80K can be made from 33K & 47K in series.

Here it is:

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[gasboss775]

[zloth]

Another possibility is using a weighted resistor network. The idea is the most significant bit goes through the lowest value resistor, which we shall call R, then the next less significant bit goes to a resistor of 2R, then 4R, 8R, etc and theses are summed together usually at a virtual earth point of an opamp based inverting amplifier.

Thanks!
Funnily enough I'd seen about those only a couple of days ago but couldn't remember the technical name.

I tried this passive version on my Everycircuit simulator app & it appeared to work quite well, the switches represent the logic inputs ( I made them 0 & 12 Volts as that's what I usually use with CMOS circuits ) the 12 volt source and 47K resistor on the right provide a bias to overcome the forward voltage of the LED.

I use a 9v clip or 5v from the Arduino.

You would probably need to experiment with the values of the resistors to suite your specific vactrol as there is quite a large difference between different parts, same goes if you brew your own.

40K can be made with an 18K & a 22K in series, 80K can be made from 33K & 47K in series.

How about this for a 3-Bit solution? Resistor network, star connection, 8 pin 7 resistor (R + 2R + 4R), 4 different values @ 0.09€ each, going into the 4 different sections of an LM324 (0.24€).

[zloth]

[zloth]

[gasboss775]

[gasboss775]

This is just to show you what I'm talking about:

8 ISOLATED RESISTORS

As I said a bit pricey.

[PHOBoS]

[Grumble]

Maybe a little over the top, but why use a DAC and a Vectrol in a Lunetta, when with the same efforts one could use a Digital Controlled Potentiometer.
I mean you have a processor (Arduino?) to control the R2R-DAC...
The advantage of using a DCP compared to a Vactrol is that they are VERY linear, may have different impedances (1k, 10k, 50k, 100k), different steps (32 - 1024), there are type to be used with a single supply (like 5 volt) or dual supply (up to 33 volt span)
A drawback might be that the footprint is usually SMD...

In my modular I use on one module four 256 step digital potmeters, two for creating some kind of VCA, one for a triggered S/H and one for supplying a triggered random voltage.


There are a numerous types to choose from:
http://www.analog.com/en/products/digital-to-analog-converters/digital-potentiometers.html
http://ww1.microchip.com/downloads/en/DeviceDoc/22017c.pdf

[zloth]

[zloth]

[zloth]

[gasboss775]

I've never been very interested in the programming side of my electronics hobby. I do like experimenting with digital hardware though. Occasionally there are situations where it would be a lot more convenient to just program a microcontroller. One day I'll give it a try.

[gasboss775]

[Grumble]

[zloth]

I could do anything I can with a Lunetta on SynthEdit in a fraction of the time with far less difficullty, but sometimes that isn't the point, just like I could order a takeaway curry but also I enjoy growing my own beans, potatoes and veg and making one from scratch

[gasboss775]

[Grumble]

Joy! Thats what diy is all about!

[RingMad]

[Grumble]

I use an Arduino using SPI to control the position of the runner of the DCP.
If you are interested I could write a piece about this module.
(remember: my module is almost 5 volt only!)