LDRs as pots?

[tbryanh]

Also, the resistor pairs that will replace P1 can be tweaked too.

All this tweaking is not good for mass production, but we are not doing mass production.

[Tiago]

Hi everybody,
At first I was sceptic but now I'm really curious about this...
Has anyone made oscilloscope/spectrum spectrum analyzer measurements?

[Gregg]

I am thinking that the matching may not need to be real precise. If they are close, it is probably good enough.

If you are Mesa Boogie and you want all of your Triaxis's to be interchangable with each other, then precise matching might be needed. But for the typical one-of-a-kind amps we make, precise matching is probably not be needed.

Well, I currently need them for DIY purposes but if I can find them in bulk at reasonable price maybe people will start thinking about limited production as well.

Somebody mentioned earlier in the thread about a manufacturer offering such 4 pin LDRs in 1000+ quantities. I think this quantity is big even for a group buy but maybe they can be custom ordered with some chinese manufacturer in smaller /sample/ quantities or even the same quantitiy at reasonable prices?

[tbryanh]

Here is another little mod to Gregg's circuit.

[tbryanh]

2) The LED datasheets I've looked at allow a 3:1 range in brightness for a given current. Unless the P-E LEDs are matched, you're going to have to match your own pairs.

I am thinking the 3:1 range is probably a result of comparing LEDs between different manufactures. Each manufactuer has its own process. This is probably what causes such a wide range.

The chances are that the Vacrols from Perkin Elmer will be fairly matched to each other, especially those from the same lot (wafer) They are made with the same process.

[Don Moose]

I am thinking the 3:1 range is probably a result of comparing LEDs between different manufactures.

Sadly, no. I found this on individual datasheets for LEDs by the likes of HP, Sharp and at least one other. To put it another way, the HP parts I looked at had a 3:1 range for min/max at given current(s). So did the Sharp LEDs, and so did at least one other mfr.

Now a 3:1 range for min/max doesn't sound so bad if you're buying enough quantity to get them binned (matched to some subrange of that range). If you're buying hobbyist/boutique quantities ...

Where I've been spending some time is trying to visualize a clean, repeatable way to EVENLY illuminate two photoresistors from one light source, all in a compact package. Then I only have to worry about matching the photoresistors.

On the other hand, I spent all that energy worrying about an oscillating control amp, and that turned out to be nothing.

Anyway - hope this helps!

[tbryanh]

Where I've been spending some time is trying to visualize a clean, repeatable way to EVENLY illuminate two photoresistors from one light source, all in a compact package.

I can't find the article, but I remember somebody showing on the internet how to connect an LED to a single photoresistor, and I think this would work for connecting an LED to two photoresistors.

They used a glass shaft that was specially made to conduct light. I think it is available through Edmund Scientific, and it is probably also available at many other places.

The glass shaft was the right diameter so that it fit into the face or the round photoresistor. This provided good alignment.

The LED was connected to the other end of the glass shaft by drilling a hole in the center of the end and sticking the LED in it.

It apparently worked real good.

The setup could possibly be modified to make LDR Pots.

Instead of drilling a hole for the LED at one of the ends of the glass shaft, the hole can be drilled on the side of the shaft in the middle of the length. Then a photoresistor would be connected to each end of the shaft.

Hopefully there would be enough light radiating out of the sides of the LED to illuminate the photoresistors, and hopefully the light would radiate evenly on both sides.

I am not actually sure if the shaft was glass or not. It could have been some type of plastic too, but it was a shaft specially made to conduct light.

There also might be something in the field of Fiber Optics that could be used instead of the glass shaft, but chances are the glass shaft would work.

Ideally there would be some way of dividing the light from the end of the LED into two equal beams and then direct the beams down to each end of the shaft.
I guess you could use some sort of tiny mirrors to do this, but it sounds like the machining and placement of the mirrors into the tiny hole for the LED would have to be ultra precise. This does not sound within our reach, but the simple approach just described might work well.

[tbryanh]

Now a 3:1 range for min/max doesn't sound so bad if you're buying enough quantity to get them binned (matched to some subrange of that range). If you're buying hobbyist/boutique quantities ...

The bottom right of each page that shows the Typical Perfomance Curves for Output Resistance vs. Input Current for each of the vactrols in the VTL5C Series of Vactrols says“At 1.0 mA and below, units may have substantially higher resistance than shown in the typical curves.”

This does indicate that characteristics vary widely between vactrols with the same part number, but the sentence that follows says “Consult factory if closely controlled characteristics are required at low input currents.”

This indicates that the factory will sort through the vactrols and match them for you. The question now is: What is the minimum quantity you must buy to get matched vactrols and what is the price per matched vactrol?

It sounds like one of us needs to contact the factory. I will volunteer to do that.

http://optoelectronics.perkinelmer.c...toisolator.pdf

[tbryanh]

It might be best to ask the factory for matched pairs of vactrols instead of matched batches of vactrols. Its probably easier to sort out matched pairs than to sort out a handfull of vactrols that are matched to each other.

[tbryanh]

I just sent the following email to Perkins Elmer with the subject heading "Need Matched Vactrols"

I noticed the datasheets for your VTL5C Series of Vactrols says to
contact you if I need closely controlled characteristics at low input
currents.

I am a member of a community that builds boutique vacuum tube guitar
amplifiers, and we are interested in using your vactrols to make
programmable potentiometers.

We need matched pairs of vactrols. The characteristics can vary
amoung pairs, but the vactols in each pair must match.

What is the minimum quantity we must purchase in order for the
vactrols to be matched, and what is the price per matched pair?

Thanks

[Enzo]

Just curious, how many of these things do you estimate there will be a demand for? Who would be the customer for them?

[tbryanh]

Just curious, how many of these things do you estimate there will be a demand for? Who would be the customer for them?

I'm going to order 500 for you. What is your credit card number?

[Gregg]

This is what a google search returned for the original part number:

http://www.eleson.net.cn/eleson_Product_1357357.html

Chinese anyone?

[Enzo]

tbr, I wasn't being flippant, though maybe I often am. I seriously wondered how many of these you think there will be a market for, and who would find them interesting commercially. I realize the few individuals here in the discussion will find them of interest, but as a product in the marketplace, who might be the intended target?

[tbryanh]

The whole interest in this thread is probably inspired by Mesa Boogie's Triaxis Preamplifier.

Just check out the Triaxis for an idea what LDR Pots can do.

Thanks