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Vintage threads from the first ten years

opinions of optoisolators

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2/15/2000 5:41 PM
Ed Rembold	opinions of optoisolators As we all know, the Vintage LED/LDR opto's are hard to get, And pricey. R.G. has suggested Mouser# 512-H11F3 LED/Photo-Fet, I think this is great, because its cheap and available. My question is this- Does it have the Qualities which make for a good "Musical" part? Is it "smooth"? You guys know what I mean. Opinions Please. Ed R.

2/15/2000 11:49 PM
Ed Rembold	Jack musta been reading my mind I see Jack has a write-up at AMZ on this very thing, Answers my question too. Thanks Jack. Ed R.

2/17/2000 5:33 PM

I think there's an error on Jack's page. He lists VTL5C2 having rise and decay times of 2.5 ms and 35 ms. In fact those specs are for VTL5C1, the VTL5C2's correct specs are rise time of 3.5 ms and 500 ms. If you don't believe me check the page:

http://www.perkinelmer.com/Opto-110/vtl5c1.htm

So CLM-600, NSL-32 and VTL5C2 have identical specs (rise & decay times), but H11F3 is totally different thing.

- Mikko

2/17/2000 9:00 PM
Frank Clarke	For completness, the VTL5C3 has rise and decay times of 2.5 ms and 35 ms too. http://www.perkinelmer.com/Opto-110/vtl5c3.htm My Andertonish Compressor has a photocell epoxied into a drinking straw with an LED at the other end. It works well enough. I don't _hear_ a delay, but maybe it's just preserving the attack. CdS photocells are supposed to be generally slower than CdSe.

2/16/2000 6:18 PM

Re: opinions of optoisolators

Photocells have timing characteristics, and most commercial photocells (and LED/LDR combos) will have ratings in terms of the rise and fall time (where rise is response to light onset, and fall is recovery in light-absent condition).

The "smoothness" will be a function of what you want to use it for. For example, if an LDR or optoisolator is being used as part of an envelope detector/follower circuit, its "sluggishness" may be a desirable feature with respect to smoothing out the envelope votage. In effect, it can work like a lowpass filter on the envelope.

On the other hand, if it is being used in a gain control device, like the Anderton compressor, then it needs to have a fast rise time to handle transients. You can always stretch out the lag/fallback time by using a larger value cap in your envelope circuit, but you can't make it go faster than it can go.

If the LDR is being used as part of a phaser, the rate of change required is negligible, unless you want to crank up the sweep LFO into the audio range for ring-modulator type effects. Normally, though, asking an LDR to vary at 10hz is well within the capabilities of most LDR's and shouldn't show any variation across models at that rate.

Finally, bear in mind that the "smoothness" is also a function of the light source driving the LDR. Diodes respond much faster than incandescent filaments. No doubt this is a small part of what yields the kind of sweep produced by a Univibe, over and above the LFO and LDR characteristics.

To sum up:
1) You can make any light-dependent device behave more "sluggishly" with extra componentry, although one always appreciates the reduction in parts count.
2) You can't make it be more responsive than it can be.
3) Responsiveness is an advantage or disadvantage, depending upon application.

2/16/2000 9:13 PM

What about linearity with respect to voltage ? A photoresistor is pretty linear, that is, its resistance does not change significantly with applied voltage. But FETs are well-known for their non-linearity with large signals.

When I tried these photo-FETs with a phase shifter, the scope showed significant distortion even with guitar level signals. Maybe I was just unlucky, but has anyone been able to obtain an undistorted phase shift with these components ?

2/16/2000 11:53 PM

Guys, Thankyou for the added insight.
I'm working on some new stuff (what's Really new under the sun?) Anyways- I thought what I needed is fast rise, slow decay. After initial breadboard work I'm now, not so sure.
to me, it seemed as if the H11's just banged on and then banged off, but you guys know the truth, I really don't know what I'm doing- Anyways- I ordered Silonex and Vactrol,
several different part numbers- So we will see what happens.
Moocow, I liked Your observations of photo-fets, hummmm,
maybe in another application.
Ed R.

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