The unknown future of photoresistor, ruminations

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tk@halmi

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Joined
Jun 3, 2004
Messages
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Location
Oregon, USA
Sure, I have been away from GroupDIY for over ten years, but I have never left in heart. Rack gear was sold off, instruments, partial projects and parts boxed and hidden in various places to give room and time to family.

Returning I am finding that one of my favorite electronic component, the LDR, is in its death throws in Europe, and who knows what will happen next this side of the world. In a way I totally understand the concerns having been impacted by led in my home town’s water supply during my youth. On the other hand losing such a useful audio component is upsetting to the point of action.

I have been doing some research on ZnS which is a wide gap semiconductor similar to CdS, but non-toxic.
It is less ideal than Cadmium based materials due to the band gap being 60% larger. However, I think without adding a lot of complexity ZnS would be sufficiently photosensitive in the UV band given enough energy.  Mid last century there were no UV LEDs or other sources that could be easily modulated so we needed materials working in the human visible region. Later LEDs showed up with fast on/off times. Today high power UV LEDs are readily available tuned down to the wavelength needed.

Has anyone had similar thoughts or critique to flaws in my reasoning ?


 
One thing that might be an issue.
UV light can degrade things.

I would build a test device and take controlled measurements using some set time period and leaving the LED(s) on between the the measurements to speed up any degradation
 
Gus said:
UV light can degrade things.

Hi Gus! Yes, UV creates a new problem.
Rundown of the LDR sandwich:  the substrate should be glass or ceramic, ZnS is a mineral, electrodes should be metal (evaporated Zn?) so the issue will be the material of the final encasing that protects the exposed ZnS area from impurities blocking or doping into the photosensitive layer.  UV resistant epoxy exists,  but I am not familiar with its effectiveness.  A real passivation layer such  silicon oxide would be cost prohibitive. I wonder what UV LEDs are encased in?  The same stuff should work!



 
I would not call it death throes (unless you extend it to all TH components). Xvives and NSL32 are pretty available - in Germany at least.
Banzai, Musikding, UK electronics, etc.

Your idea sounds interesting though!
 
I have never been sure CdS should be banned. OK it is a compound containing Cadmium and unless it is liable to decompose into pure Cadmium it should not be a problem. Cadmium compounds are used to colour glass and Cadmium Telluride is used in solar panels.

Cheers

Ian
 
The joke is the AMOUNT of cadmium used. EU themselves estimate in the order of maximum 5 grams per year (!) total for the whole EU market under the exemption for audio attenuation. The cells I used were around 40-50ug (yes, micrograms) Cd per cell.

Tricky thing is the way the specification runs - emphasizing "mechanically separable parts" as the basis for the 0.1% Cd you can legally have.

And as the CdS in our cells need to be in an even layer distributed so as to be easily reachable by incoming light, it is possible (with lab equipment) to slice off a layer of material that has more than the allowed 0.1% Cd

Cadmium is a common impurity in e.g. aluminium, and at a quick estimate there is most probably more cadmium in one of the fancy rack handles on my units than there is in a T4b photocell assembly or five for that matter.

Interesting with the ZnS - but I think that that direction will be highly non-linear, thus not really for audio. The closest I have found so far is probably Indium Antimonide (InSb) - but simply haven't been able to get my hands on a single cell on that technology - I don't believe anybody produces them.

As for the photocouplers still available from xvive and such companies that all claim producing on original production line, try one day to measure their specifications and spread, comparing it to a handful of originals. You'll be shocked (I did). There are ways to use them, yes, but you need serious creativity to bypass the effects of the parameter tolerances. Don't expect them to drop-in replace anything from yesteryear.

The Coolaudio ones are probably the most consistent out there, but are hard to get hold on.

Business did a lot to try getting around this, but current consensus in the pro audio industry is "pretend you didn't know", "just lie about it" and "claim an obscure exemption and then claim incompetent". Yes, I talked to literally everyone at a couple of AES and Musikmesse shows, and this is the answers I got. A rather worrying large proportion of pro audio manufacturers simply continue as before, ignoring the problem away...

Talked to a guy from AES's technical committee that did not even know about the problem.

As a fun fact, one of the major manufacturers, Macron, at one point tried to corner the market by claiming (in front of the exemption evaluation committee) to have found a legal substitute material, and thus calling for an end to the exemption. Turned out it was based on falsified laboratory reports.. Imagine having the balls to try something like that.. :)

/rant

Jakob E.

btw, tk@halmi - great to see you here again, welcome back..!
 
Linear photo-resistance behavior is rare in nature. Talked to a material physicist from local uni a handful of years ago about a solution, he said that there are very few places to look for this if you "go by the book", that is, not counting out the possibility for inventions-by-accident.

But as far as I have been able to find out (and I'm badly hungry for these) there hasn't been anything even remotely usable made since the ban.

Jakob E.
 
Maybe missing something, but translation from https://www.vde.com/tic-de/news/2019-1/rohs-richtlinie-erweitert
------------
"Definition of "electrical and electronic equipment" according to RoHS Directive 2011/65 / EU Article 3 (1), (2)

(1) "Equipment which depends on the operation of electrical currents or electromagnetic fields for their proper operation and equipment for the generation, transmission and measurement of such currents and fields, for operation with alternating current of not more than 1 000 volts and for operation with direct current of at most 1 500 volts. "
(2) The word "dependent" is interpreted to mean that the device requires at least one of the intended functions - electrical currents or electromagnetic fields."
----------------
A voltage multiplier should get you above 1000VAC easily. There might be some indication/lamp/whatever that you claim essential for operating your device. Your remaining problem will be sourcing the desired LDR. YMMV.
 
Truth: nobody is investing in researching linear photoresistivity today. Only the audio market is hurting for this. Elsewhere they went digital and a photoransistor/photodiode works just fine. Maybe it will not be linear, but it is very difficult to predict. How much would you bet? There is pixie dust available to lower bandgap that cost a car for a gram ten years ago. Today you can order them off eBay for dollars. ZnS is also easily doped with copper, silver, manganese, etc. using cheap diffusion to modify charge/discharge time. LDRs are very low tech with today's standards, but the upfront research is rather time consuming.
 
tk@halmi said:
Truth: nobody is investing in researching linear photoresistivity today. Only the audio market is hurting for this.
And it's not even "consumer audio" devices, pro audio is a much smaller market. There's related use in some music synthesizers (Buchla clones as well as odd designs, but I don't think any of the big names ever used photoresistor cells, they don't change fast enough for most applications), but that's pretty small too.
ZnS is also easily doped with copper, silver, manganese, etc. using cheap diffusion to modify charge/discharge time. LDRs are very low tech with today's standards, but the upfront research is rather time consuming.
The attack/release times and memory effects of CdS cells was what made them so popular in compressors and such, and anything else would have to reproduce that. I imagine other light-sensitive substances would have their own timing effects and make it hard to reproduce CdS, but I don't really know enough to say.

CdS has a lot of memory effects, but it does seem it could be characterized and simulated digitally. I've heard of this being done with varying success such as the FMR RNC, but they haven't eliminated the demand for the real thing.
 
Maybe I’m gonna day something stupid, but here I go anyway:

In the past I’ve read here  about emulating different compression behaviours with a microcontroller.

What about trying to preserve the compression characteristics of the banned components by replicating the curves with a microcontroller?

If done well, it could be a way to get the compression style of both unobtainable or expensive parts for a fraction of the cost.
 
dirtyhanfri said:
Maybe I’m gonna day something stupid, but here I go anyway:

In the past I’ve read here  about emulating different compression behaviours with a microcontroller.

What about trying to preserve the compression characteristics of the banned components by replicating the curves with a microcontroller?

If done well, it could be a way to get the compression style of both unobtainable or expensive parts for a fraction of the cost.
Given adequate processing power and a software coder with good understanding of all the properties and discipline to rigorously execute an accurate mimic it should be possible. Wether it is merchantable is another question. 

JR
 
JohnRoberts said:
Wether it is merchantable is another question. 

JR
You've struck the heart of the matter. (Some) People want "Teh Real Thing" even if they can't tell it from a simulation in a blind test . This keeps a lot of companies in business, for both pro-audio and the audiophile markets.
 
you would have to emulate the EL panel  and the LDR to get an accurate repro in the digital world , or no?


i wonder how many Silonex NSL 5910's the Drip  guy has in stock,

so in theory, the actual number of LA2a compressors could be approaching a finite number unless there are 10 million Cd cells stashed in a cave somewhere (to get them ready for testing)  :D

we use to beat this topic, and i mean to death! so lots of good data floating around.

here is part of  a  UREI spec sheet  as related to screening cells, they were pretty picky back then, probably not so much nowadays , does UA still sell this compressor? wonder how many cells they have,
 

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here is a blurb on the EL panel,  from 2003

notice the 400 Hz for aircraft operation, used in fighter jets,

anybody know the status of being able to buy these?

 

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