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I've been trying to get my hands on some vactrols (opto-variable resistor) for a long time, unsuccessful. Not even Digi-Key has them.

Some examples: VTL5C2, VTL5C3, NSL32

I can't see them as "too expensive to manufacture" considering you can improvise one using an LED+LDR+tape, but it's in no way as "linear" or robust as the actual components.

Does anyone know the story behind it?

Edit: So there are places where you can get them, but at least where I live it's something almost unheard of. They look useful in many ways (at least to me,) yet I've never seen them anyhere except some old audio equipment.

Edit2: What could be an alternative to a vactrol if I require a significant resistance change controlled by voltage? A MOSFET as a resistor is not an option for me because of the very low resistance range. Is there such a device/equivalent circuit that allows all this with the simplicity of the vactrol? This is a hypothetical question, based on many compressor units I have seen. Itseems hard to achieve such functionality in such a simple way. The closest I've seen are OTA based for closed loop circuit, but they aren't nearly as simple as vactrols.

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  • \$\begingroup\$ New: store.synthrotek.com/Vactrol \$\endgroup\$
    – JRE
    Commented Dec 23, 2016 at 20:36
  • \$\begingroup\$ Also new: thonk.co.uk/shop/vactrols-vtl \$\endgroup\$
    – JRE
    Commented Dec 23, 2016 at 20:37
  • \$\begingroup\$ Maplin has them: maplin.co.uk/p/vtl5c3-vactrol-n72eh \$\endgroup\$
    – JRE
    Commented Dec 23, 2016 at 20:41
  • \$\begingroup\$ Ok so they are still being manufactured, let me correct.... \$\endgroup\$
    – ndelucca
    Commented Dec 23, 2016 at 21:05
  • \$\begingroup\$ That's a much better question. \$\endgroup\$
    – JRE
    Commented Dec 23, 2016 at 21:19

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I've never used a Vactrol ( or even heard of it till now) nor would I want to. The same goes for LDR's.

Photo Diodes are so accurate often 0.5mA/mW +/-1% unlike LDR's +/-10~50%. When diodes are put in OptoCouplers are very stable unlike phototransistors which have wide ranges in hFE.

But the real reason are the penalties for using the raw materials used inside like Cadmium, which have been banned in most healthy countries around the world for ground water contamination reasons after disposal.

( but hey , the chinese will sell them to you for disposal in your country)

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  • \$\begingroup\$ I imagine there are a few wingnuts who can't articulate it they disagree. \$\endgroup\$ Commented Dec 23, 2016 at 22:07
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    \$\begingroup\$ atsdr.cdc.gov/csem/csem.asp?csem=6&po=7 CADMIUM safety The use of CdS and CdSe photoresistors is severely restricted in Europe due to the RoHS ban on cadmium. silonex.com/datasheets/specs/images/pdf/102899.pdf \$\endgroup\$ Commented Dec 23, 2016 at 22:15
  • \$\begingroup\$ Cadmium sulfide is a problem, but I don't think that's why they're not used. Since you haven't used a vactrol, you don't know what hateful little creatures they are. They are awful. The vactrol is never a good solution. \$\endgroup\$
    – vofa
    Commented Dec 24, 2016 at 1:52
  • \$\begingroup\$ Yes, from the same low class of quality components as LDR's still taught by academics. \$\endgroup\$ Commented Dec 24, 2016 at 2:03
  • \$\begingroup\$ They can now make LDRs with no cadmium but I have not used them .There are optos that should work that use variable resistance fets like the H11F1 but I have not tried this .A normal mosfet can do this when the DS voltage is very low . \$\endgroup\$
    – Autistic
    Commented Dec 24, 2016 at 6:25
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Vactrols contain cadmium, so it is not RoHS compliant.

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They are not stocked because nobody is using them in large quantities. The kind of designs that can be (kind of) achieved with vactrols can also be done with op amps and LM13700s, and will yield much better results.

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  • \$\begingroup\$ The opto provides galvanic isolation which may be useful .If you wanted to say make a variable bandwith tuned circuit tho opamp will struggle as F goes up .So despite the opamp being better there are still some uses for the opto. \$\endgroup\$
    – Autistic
    Commented Dec 24, 2016 at 6:29
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Vactrol/Vactec designs have a delay to respond time of about 100 Hz. Yes, that delay timing is not warranted in audio DBX type circuits.

Advantages: The light resistance variability makes them ideal for "light emitted tracking." As such, an LED or small 4 mm lamp at 1.5 volts is perfect for overall audio leveling and were also used by M/A comm's video cyphers in 1985.

Disadvantages: I mentioned "DBX" audio corrections. Now, what designers are trying to correct are sudden out bursts of kicker drums and snare drum levels and trying to control those attack levels. In theory, this is a nice try. Even a recording studio understands that it's that tone/frequency cannot be masked.

  • Corrections of drums or voice levels are best removed/replaced by "phase shifting" for which vactrols are just too slow to respond correctly.
  • Once again, the combination of lamps, LEDs and sound corrections are not what this design is engineered to handle.
  • There is another design using light sensitive transistors - but, the results are the same.

What was the photo cell resister designed for, you might ask.

Ready for this?:

Counting bottles being made at breweries. Yup, that's it.

  • Count the bottle numbers for boxing them, and automated packaging equipment. In other words, "breaking the light beam counters."

Why use them in video cyphers? Simple. In the 1980's the video sync was missing. In order to replace the video sync, the video needed a steady reference voltage so that the sync can be re-attached correctly. After all, digital timings cannot see the actual video itself.

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They are not used because other, easier to use alternatives exist -- namely, translinear VCAs and multipliers. These don't require specialized drive to cancel out nonlinearities (in fact, you can get VCAs with linear-in-dB control inputs nowadays, which makes life quite easy), nor do they run afoul of RoHS regulations as they are quite standard bipolar or CMOS ICs, unlike the Vactrol which requires both a CdS photocell and specialized manufacturing processes.

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