Vactrol LED findings

[digi2t]

From what I've seen, I've repaired a couple of Foxx Guitar Synths, one of which appeared to have the same vactrol as the one pictured in this Mutron gutshot;



What was cool was that I could actually slip the dual LDR portion out of the plastic tube part to observe if the LED was working. It was indeed.... red. It wasn't very bright though, not bright like the LED's we use today. The Mutron Flanger LED's weren't very bright either.

Insofar as the vactrol part number on the Mutron III schematic for units above serial #02050;



If that number refers to a particular vactrol that looks like this;



I have two of these...



If you want me to run some tests (not the type where I have to saw it open to see what color the LED is!), let me know what the parameters of your test are, and I can run them.

[Mark Hammer]

Not much I can add to this discussion, except to note that what we see is not necessarily a good measure of the spectral precision and spread of any given LED, and that photocells are also the sort of device that falls within a range, but tends not to be terribly precise.

[anotherjim]

I'd assume RED for that frosty/clear encapsulation -  until I see what really comes out. They don't need a red tint in the goop to be red.

Another LED spec I don't see much mentioned is viewing angle.
Quote from.
http://www.theledlight.com/technical1.html

"LED viewing angle is a function of the LED chip type and the epoxy lens that distributes the light. The highest luminous intensity (mcd rating) does not equate to the highest visibility. The light output from an LED chip is very directional. A higher light output is achieved by concentrating the light in a tight beam. Generally, the higher the mcd rating, the narrower the viewing angle.

The shape of the encapsulation acts as a lens magnifying the light from the LED chip. Additionally, the tint of the encapsulation affects the LED's visibility. If the encapsulation is diffused, the light emitted by the chip is more dispersed throughout the encapsulation. If the encapsulation is non-diffused or water clear, the light is more intense, but has a narrower viewing angle. Non-diffused and water clear LEDs have identical viewing angles; the only difference is, water clear encapsulations do not have a tint to indicate color when the LED is not illuminated. "

Have to say I've not had any worries using standard red (ones that are tinted red). They have a wide angle and I don't see problems matching LDR output to LED control current.

[R.G.]

Good point Mark.  Variation matters.

The early uses of photocells and LEDs used red and sometimes infrared because there weren't any other colors available, or not available at less than kingly prices. LEDs were a lower power upgrade from incandescent lamps in the first uses of photocells for variable resistance.

LEDs were pretty poor light sources back in the 70s. I remember being pleasantly surprised at LEDs that were bright enough to be used in sunlight conditions. Really high brightness LEDs became available in the late 80s and 90s.

LDRs are not only variable, their resistance is history sensitive. They both light-adapt and dark-adapt. Under strong lighting, they get low resistance, then slowly get a bit lower. Turn the light off, the resistance drops at the nominal speed of the LDR, then gets even larger as it dark-adapts.  This was a problem with the single-pot adaptation of the speed control I did for the Neovibe. The dual/centertapped LDR worked fine, but if it was turned to very low speeds, it could sometimes dark-adapt itself into the LFO stopping as the resistances got too big.

I envisioned an LDR tester where you'd stick an LDR into a black plastic foam tube at one end, and stick an LED into the other end. Then you'd drive the LED with a rectangular wave of varying duty cycle at a slow rate, maybe 1-0.1Hz and watch the resistance changes on a scope. That would get you the speed response for non-adapted use, and probably also the resistance change as long as you defined one LED as the reference LED for such tests, in the absence of a good, wide spectrum light intensity meter.

[Tony Forestiere]

>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.

Bing! Consider me having learned something new today.
Thanks!

[armdnrdy]

Thanks for all of the replies.

After looking through several LDR & Vactrol data sheets, it is apparent that the ratings do not easily correlate. It's apples and oranges.

LDRs are measured by applying light, generally 10 LUX. (equal to a very dark day)

Vactrols are measured by applying current to the LED which produces light on the CDS cell.
The current applied is generally 1-40ma.

Not being privy to a Vactrols LED specs, it would be difficult to test LED/LDR combos to try to get the same results as a factory Vactrol.

Unfortunately, it seems like the best DIY solution is...what we've been doing...
Test different LED/LDR combos in circuit to find the combination that produces the best results, and
is more pleasing to our ears.

[digi2t]

Unfortunately, it seems like the best DIY solution is...what we've been doing...
Test different LED/LDR combos in circuit to find the combination that produces the best results, and
is more pleasing to our ears.

Would it be more fun any other way? 

[armdnrdy]

I had another thought.

I always tried to choose an LDR that roughly "matched" the resistance of the Vactrol that I was trying to emulate.

I now see that this approach is wrong.

At 10 LUX measurement for the LDR resistance...this is misleading. Once again...apples and oranges!

It seems that if you choose an LDR with a higher "ON" resistance value, when you apply light from an LED, the resistance will be lower than the 10LUX spec.

Also, since the LDR is "hit" with a brighter light from the LED, the On response time should be quicker than the 10 LUX rating.
With that being said, since the LED lighted LDR is at a lower resistance than the 10 LUX rating, the "fall" time to 100K (as in the Vactrol ratings) should be longer in ms.

Thoughts?

[electrosonic]

I made this a couple of years ago. It uses an op amp set up as a comparator to force current through the LDR until is matches a set resistance. It uses a bridge configuration - sort of like Johh Hollis's Rockface.

It also has a window comparator so I know the resistance of the LDR is really balanced (well within a few percent).

I am working on a new version - using a arduino nano to automate it a bit and time the rise and fall times from 1k to 100k. I have assembled it and need to start working on the programming.

I have a bag of assorted vactrols I need to sort through - so this should speed things up a bit.

Andrew.

[armdnrdy]

Nice Andrew!

Gotta love the DIY test equipment!

I found this circuit some time back.

http://www.edn.com/design/analog/4368390/Simple-circuit-measures-optocoupler-s-response-time-item-2

[rankot]

I have just had in mind to ask this question: "How to measure rise/fall time for vactrols"

I have bought few vactrols from various sources (mostly Macron substitutes) and few sold as VTL5C1 from Aliexpress seller. My measurements of resistance show that all of them differ from VTL5Cx specifications a lot, so I wanted to measure response times, just to make sure if they are worth anything. Those I bought as VTL5C1 measure 125 ohm on 10mA, 450 ohm on 1mA and 1k1 at 0.1mA, so it seems they are much closer to VTL5C4/2 or VTL5C10. Their dark resistance is far behind my DMM's capabilities (200M).

I will try to measure using the circuit above.