Vactrol LED findings

[armdnrdy]

I've been working on a few projects that have custom "photomods" or, commonly known under their Perkin Elmer/Excelitas trade name, "vactrols."

I've come across a bit of info that I thought I'd pass along.

Last year I purchased a lot of NOS VTL5C3s. A few of the vactrols had broken LED leads.
I carefully "cut" away the surrounding potting material to salvage the LDRs.

In this model vactrol, I found 3mm red LEDs with a 5mm CDS cell.

I'm working on a Mutron Bi-phase clone project which has remained pretty close to the original boards.
This build includes a "work alike" of the 3-dual LDR Hamamatsu photomod.

There are a few images online of the original custom Hamamatsu HTVP653 photomod with the metal "cover" removed.





The LED appears to be a 5mm, white diffused LED.
One of the images was uploaded by Obsoletetechnology out of the U.K.
I contacted Obsoletetechnology and inquired if he had "fired" up the LED while the cover was removed....if so...what color was the LED.

This was his reply:
I remember that the led color was actually red, although the led package as you noticed is white diffused.
Hope that helps,
Obsoletetechnology

So...where am I going with all of this?
I've read through many threads on LED color and spectral response.
It seems that the general consensus is..green and yellow LEDs have the best response for CDS cells.

But wait! Why does the VTL model, and the Hamamatsu photomod use red LEDs?

Maybe it's not about optimum spectral response...maybe using an LED with a less than stellar response creates certain characteristics. Perhaps in the rise/fall time...or to attain a certain resistance parameter from the cell.

One thing that I've noticed about LDRs is that they have a similar rise/fall time, with the rise time being higher than the fall time.
This is just the opposite in vactrols. The rise time is less than the fall time.

Any thoughts?

[Kipper4]

For now I'll just leave this hear Larry until I get time to formulate a reply with even more questions than answers......


http://www.diystompboxes.com/smfforum/index.php?topic=107530.msg977216#msg977216

[armdnrdy]

For now I'll just leave this hear Larry until I get time to formulate a reply with even more questions than answers......


http://www.diystompboxes.com/smfforum/index.php?topic=107530.msg977216#msg977216

I read through that thread when it was first posted...I even commented on it! 

I've been messing with DIY vactrols and matching factory vactrols for a bit now.
The best way to see if the resistance swing suits your needs is to connect them to the LFO of the circuit that they are intended for.
That's the best way to match them for the circuit as well.
http://www.diystompboxes.com/smfforum/index.php?topic=106920.msg968890#msg968890

[PRR]

> optimum spectral response...

Red traditionally has so much better efficiency that "spectral response" hardly matters.

Red has low and consistent voltage. Same numbers on the oldest spec-sheets for LED-LDRs. 1.6V is dandy for TTL control. The 2.1V or 3.3V of other colors can be marginal, especially since the 16mA spec is at the upper end of what TTL can do.

> rise time being higher than the fall time. This is just the opposite in vactrols.

Not sure what you mean. The LED rise/fall is zero for our purposes. The LDR is what it is. LDR rise/fall specs can be taken at different points, and may "look" different if you don't read all the fine print. The physics is that light hammers the electrons quickly, dark allows recombination at a much slower rate. (Not "necessarily" slower; but if recombination exceeds light excitation there is hardly any photo-effect to exploit, so nobody uses those recipes.)

[Kipper4]

Sorry Larry I wasn't being a smart Arse.
I came across response 5 here and thought it might be relevant.

http://www.diystompboxes.com/smfforum/index.php?topic=105586.msg951542#msg951542

I've been messing with lots of vactrol builds too.
I'll read through your article thanks.
Rich

[armdnrdy]

Rich,

I didn't think you were being a "smart arse" 

I was agreeing with Duck Arse's use of an oscillator (LFO) in his test rig.

Paul,

The rise/fall times I was referring to are the LDR's relatively small ms swing between rise and fall.
Vactrols such as the VTL5C4 have a much higher swing.
6ms turn on to 63% final Ron
Turn off to 100K max 1.5 seconds.

Here is a link to Westhost Sound (Elliot Sound Products)
http://sound.westhost.com/project145.htm

About halfway down the page, Rod posted an image of the aforementioned VTL5C4, with it's long 1.5 second turn off time.
The vactrol is cut open. When powered, Rod claims to see no light and believes after measurements that the LED is an infrared.

Now what I was wondering is...if the type or color of LED has anything to do with the turn on/turn off time of the complete vactrol package.
Or...is it about the makeup of the CDS material.

I find it odd that after reviewing LDR data sheets by Perkin Elmer, Clairex, Silonex, Hamamatsu, and a great deal of lesser known brands, that I cannot find similar responses to those of Vactrols.

[digi2t]

I remember the Mutron Flanger LED's were red. I just can't recall exactly if they were clear or diffused, I'll have to go through my pictures to see if I have any shots of them. I have a vague memory of them being clear.

The dual Vactrol I made for the Foxx Guitar Synth (Mutron III derivative) I used a bright white clear LED. I tried a bunch of different colors, but the white one gave me the best wonk. Maybe because the light spectrum is hitting the cell quicker/harder? I don't know. I just know what my ears hear.

[armdnrdy]

Hey Dino,

I don't have any pictures of the LED. I only have the drawing you made of the light tube assembly in which you stated red LED.

I was looking through different gut shots of effects that use an LED/LDR combo.
Almost all of them use red LEDs.

But....as you said, let your ears decide!

In the case of the Mutron Bi-Phase photomods...I want to get as much information before I "build" the actual photomods.
Constructing them isn't quite as easy as an LED, LDR, and a bit of shrink tubing! 

[samhay]

If you look at the datasheet for the Silonex LDRs (link below), you see they offer then with a range of peak wavelengths from:
515 nm (green) - 615 nm (red) in CdS-based LDRs.
690 nm (far-red) in CdSe-based LDRs
You can also find PbS and InSb LDRs with peak response in the mid-infrared.

The light emitted from an LED is often fairly broad:



The optimal LED colour for many of the LDRs appears to be yellow/amber. However, these tend to have pretty poor efficiency, and as Paul noted you can probably get away with using an efficient red LED in most cases as the tail of the response extends down into the green/yellow region and the extra light output trumps all else.

LDR datasheet: http://www.farnell.com/datasheets/12656.pdf

[digi2t]

Maybe that's why the bright white LED I used worked so well? According to this chart, it covers a pretty broad spectrum of the rainbow.



Looking at the charts now, maybe warm white would have been even better?



Kinda like using the shotgun approach to hit a small target.

[italianguy63]

When I did mine... I had a white, a green, and a red.  They all worked.  I ended up using the green.  MC

[smallbearelec]

Constructing them isn't quite as easy as an LED, LDR, and a bit of shrink tubing! 

These headers:

http://smallbear-electronics.mybigcommerce.com/headers-and-covers/

might help.

[armdnrdy]

Constructing them isn't quite as easy as an LED, LDR, and a bit of shrink tubing! 

These headers:

http://smallbear-electronics.mybigcommerce.com/headers-and-covers/

might help.

Thanks Steve.
I don't think that I can fit "three little Indians" around a "5mm campfire" with the proper distance and fit the cover on this header.

I made provisions to "fab" something close to the original 10 pin opto.



I worked up a small, round PCB to fit the components. A 16mm diameter capacitor "can" to cover the lot. The whole thing will be finished off with black PVC shrink tubing. The type that's on electro capacitors.

[armdnrdy]

Here's a pretty good article about LDRs:

http://www.resistorguide.com/photoresistor/

[Kipper4]

Thanks Larry for the article.
Maybe one of the guys with a 3d printer could make a 2 piece clip together solution for the housing for in the future.

[armdnrdy]

Thanks Larry for the article.
Maybe one of the guys with a 3d printer could make a 2 piece clip together solution for the housing for in the future.

You're welcome Rich,

If I want to follow the original photomods design...I think it best to go with the capacitor housing.
If you look at the image of the original, you'll see that it's made of metal...probably aluminum.
I would imagine that the inside is bare metal...not painted.
There will be a lot of reflection from the single LED around all sides of the cylinder.
If 3D printers could print metal...then I'd jump on the idea!

I have seen individuals on other sites 3D print tubes with caps to fit single LED/LDRs.
I wish those were commercially available.

I still have to make something with set spacing that I can change out the LED and LDR easily, but is light proof.
I need such a rig for testing different LEDs and matching LDRs. I recently acquired a lot of dual, center tapped LDRs which test pretty good for phaser use.

[midwayfair]

It seems that the general consensus is..green and yellow LEDs have the best response for CDS cells.

This is a poor consensus for people to arrive at, because photocells cells don't have a general, specific "best response." samhay already pointed out that you can buy photocells that respond to different peak wavelengths, and their response to those wavelengths can produce different results. I've also mentioned elsewhere that the testing we DIYers do with photocells typically amounts to little more than gut feeling about what might sound better, because we almost never do proper A/B testing.

A white LED (which digi2t brought up) covers basically the entire spectrum: It is basically three LEDs mixed together, which gives you white light.

The LED appears to be a 5mm, white diffused LED.

Some history would have benefited you here! When was the pedal produced? It was well before the early 90s, which is when the first blue LEDs were invented, much less commercially available, and a Nobel prize was given in 2014 for inventing them. A big reason blue LEDs were sought-after? Because they let you make white light! No blue, no white, either. At the time the original pedal was produced, I would be surprised if anything except red would have been considered for use in a commercial product, but I'm not sure when yellow and green started to become available for cheap. I know you already solved this mystery, but I thought some historical context might help put the matter to rest.

Paul also pointed out that red is simply more efficient. A major consideration if you were to use a blue or white LED is how the LFO handles the additional current requirements of the LED. My observation has been that higher-current-draw LEDs result in a lopsided waveform. This can translate to different things in different effects. I built CultureJam's duo vibe with a blue LED because it put a single noticeable bump in the waveform at speeds that sounded like a warp in a record, for instance. I used blue in the Blue Warbler to mimic the lopsided response of a bulb but mainly for thematic reasons, and the lopsidedness was slightly minimized there anyway. The wrong color LED inside a vactrol can also mess up a circuit's behavior ... I just fixed someone's Nautilus (Mutron III clone) last week that the problem was almost entirely due to using the Macron vactrols in place of the VTL5C3 -- and the main difference between those appears to be the internal LED's Fv. (FWIW, just measuring the Fv, I also would expect those to be infrared -- the Fv is only about 1.5V on my multimeter, while discrete red LEDs are almost always 1.6V or higher.)

Another consideration is that higher current through an LED typically means more brightness -- not just from the perspective that humans see yellow and green better but actually more bright. You can adjust the brightness of an LED that is appropriate to your photocell's peak response by adjusting the current limiting resistor for the LED and vice versa. This is why samhay and PRR both mentioned efficiency.

The color of the LED has little, if anything, to do with the rise-fall time of the LDR. The rise and fall times are created through the chemical composition of the photocell itself. However, an LED that is not near the ideal waveform of the LDR will have to reach a higher brightness before it causes as much of a change in resistance, which can SEEM like a slower response. But we're dealing with a few dozen milliseconds, at most, for fall times and anything from milliseconds to over a second for a rise time.

Finally ... you're fussing over the housing for these, but ... it's dark inside an enclosure. What do you believe you are changing by putting the parts inside another housing? Or is it that you want an easier way to breadboard or tweak with the back off the pedal?

[armdnrdy]

Some history would have benefited you here! When was the pedal produced? It was well before the early 90s, which is when the first blue LEDs were invented, much less commercially available, and a Nobel prize was given in 2014 for inventing them. A big reason blue LEDs were sought-after? Because they let you make white light! No blue, no white, either. At the time the original pedal was produced, I would be surprised if anything except red would have been considered for use in a commercial product, but I'm not sure when yellow and green started to become available for cheap. I know you already solved this mystery, but I thought some historical context might help put the matter to rest.

This was all taken into consideration...I looked up when other color LEDs were commercially available...my brief search did not turn up anything definitive.
I am 53 years old, played with electronics briefly in the mid seventies, and recall other colors being available but....nowhere near as common as red.

Paul also pointed out that red is simply more efficient.

Yes...and I took this as a good reason to use red.
I moved on from there and inquired as to why LDRs that are available to the public have a resistance rise/fall time much quicker than commercially available vactrols.

Another consideration is that higher current through an LED typically means more brightness -- not just from the perspective that humans see yellow and green better but actually more bright. You can adjust the brightness of an LED that is appropriate to your photocell's peak response by adjusting the current limiting resistor for the LED and vice versa. This is why samhay and PRR both mentioned efficiency.

Understood. This is why most circuits have trimmers in place to make adjustments to the LED to compensate for the inconsistent nature of vactrols/CDS cells.

The color of the LED has little, if anything, to do with the rise-fall time of the LDR. The rise and fall times are created through the chemical composition of the photocell itself. However, an LED that is not near the ideal waveform of the LDR will have to reach a higher brightness before it causes as much of a change in resistance, which can SEEM like a slower response. But we're dealing with a few dozen milliseconds, at most, for fall times and anything from milliseconds to over a second for a rise time.

I had to ask....that is what I feel this site is for...to ask, learn, and hopefully help other people along the way.

Finally ... you're fussing over the housing for these, but ... it's dark inside an enclosure. What do you believe you are changing by putting the parts inside another housing? Or is it that you want an easier way to breadboard or tweak with the back off the pedal?

I am actually not changing...I am emulating the original.
Hamamatsu is a company that has specialized in opto products since 1953. I think I'll follow their lead!
It can be dark inside an enclosure...depending on the enclosure, the amount of empty 1/4" jacks and the amount of light hitting the enclosure. (think lighted stage...or outdoor event)
I've found that walking with an LDR connected to my meter...into a dark room... into a seemingly dark closet...will not produce the same resistance results as encasing the LDR into something lightproof.

The original photomod that Mutron contracted Hamamastu to construct remains a mystery.
We do not know what parameter LDRs were used in it's construction so...I don't think it's a good idea to slap the thing together...box it up and hope for the best! I'm a bit more of a stickler for detail than that.
There will be quite a bit of testing before I "fit" the finished photomods onto the Bi-Phase PCB.

This thread is about learning more about "Vactrols," their behavior, and what causes this behavior.

I think that one of the best things to come out of this thread is dispelling the "myth" that green and yellow LEDs are best for DIY vactrols. I was one who fell into that trap.

[PRR]

The LED specs, color, obviously have no effect on off-time. Dark is dark.

[samhay]

>The LED specs, color, obviously have no effect on off-time. Dark is dark.

Except that white LEDs may contain phosphors, which will glow for some time after the LED current is switched off.
The lifetime of these phosphors (how long they emit light after excitation) is typically on the order of some ms, so this may slow down the rise time a little.