what the heck is an optoisolator?

[pinkjimiphoton]

is it the same thing as a vactrol?

or am i missing something here? cuz they sell these puppies at radio shack..

and i was looking at this:

http://www.jameco.com/Jameco/workshop/diy/recipe9.html

you can even roll your own.

i realize that two diodes isn't gonna be the same as a diode and a resistor, but... is this close enough for rock n roll when ya need a vactrol and don't wanna spend 15 bux after shipping for a 7 buck part?

please educate my uninformed self! thanks!

[R.G.]

Optoisolator is the generic term for anything that couples signal but isolates from ground (and other common mode noise sources) using light.

Next less generally, it's a light emitter (generally LED, cause they're fast-ish) coupling a signal to one of:
- photo diode (which may be any diode exposed to light, including another LED)
- photo-(bipolar) transistor, where the light makes the base leak when light impinges on it
- photo-FET (like H11F1) and some logic photo-FET front ends

or, a light source and an LDR.

The light+LDR is what we usually want for effects. The LED to LED in that article couples a signal, but not the variable resistance you want most times for an effect. It's not really close enough for rock'n'roll in most cases.

[smallbearelec]

R . G. has explained the technical stuff better than I can. "Vactrol" was a trade name given to this kind of part by Vactec, a company in New Jersey that made them. Vactec was bought by Perkin-Elmer, and later by some private-equity operation that gave them a case of "MBA disease." The bean-counters decided that they could double the price, which got me pissed enough to find comparable parts:

http://www.smallbearelec.com/servlet/Detail?no=1255

The Macron parts have been vetted by Ed Rembold, who is known here. I find that I need to occasionally bang the drum about them, because Macron just isn't as well-known as the trade-name "Vactrol."

The stuff I carry by Silonex in this section:

http://www.smallbearelec.com/servlet/Categories?category=Opto+Devices%2C+Thermistors%3APhotocouplers

is also good and reasonably-priced, and I have inexpensive photocells if you want to roll-your-own.

http://www.smallbearelec.com/servlet/Categories?category=Opto+Devices%2C+Thermistors%3APhotocells

Regards
SD

[Mark Hammer]

Leave it to Forrest Mims to come up with something that wacky. 

One of the things about optoisolators that is often important in our little corner of the word is the time aspect.  Much like our retinas, photocells do not have instantaneous reset.  They, and the photochemical reaction they depend on to do what they do, take time to recover from exposure to light.  So, apart from just the resistance range of a photocell (which is not UNimportant at all), one of the aspects that the LDR brings along with it is the time it takes to both respond to light from the LED and drop its resistance, as well as go back to the maximum resistance when that illumination has ended.  In general, te recovery time is always longer than the "on" time, for any given photocell, though different brands and part numbers will have different recovery times.

In his various projects, back in the golden age, Craig Anderton used to turn to the Clairex CLM6000, because it had a fast-enough turn-on time, but had a long-ish recovery time.  That recovery time made it ideal for circuits intended to respond to "impulses", like compressors, limiters, noise gates, and auto-wahs.  Depending on whom you talked to, the long-ish recovery time was either helpful or a hindrance to LFO-based circuits, since the sluggish recovery meant that you couldn't expect the same sort of change in resistance at fast LFO rates that you could at slow ones.  The LFO would be saying "Go back to mac resistance!", and the LDR would be saying "Just a sec, I'm still getting my pants on!".

As you are no doubt aware, FETs can be used as voltage-controlled resistors (see the Orange Squeezer, the EHX Zipper, and a great many noise gates).  So, in some respects, the photoFETs that RG mentions ought to be a reasonable substitute for an LED+LDR unit.  Ought to be.  Where they certainly meet and better what LDRs do is that they respond immediately (well, "immediately" from our bandwidth perspective).  However,where LDRs do not have any practical headroom issues when it comes to audio, FETs will distort if asked to handle too much signal.  Moreover, the immediate responsiveness can be a nuisance in some contexts.  For instance, if you have an envelope-controlled anything, you want that unit to respond to the overall average signal envelope, and NOT to momentary ripple.  Where an LDR will go "Huh?  What was that you were saying?" and  effectively "ignore" the ripple, the photoFET will snap to it and respond with "Yes SIR, sir!", necessitating a more sophisticated envelope follower/rectifier circuit to get around that undesired sensitivity and responsiveness.

The punch line is that you always want the parameters of the control element, whether it is an LDR, a transistor of some kind, or an OTA, to match the functional requirements of the circuit in question.  If you want an LFO-based modulation device to automatically adjust the sweep width to faster rates, then a sluggish response rate is a good thing.  If you want to maintain independent control of rate and sweep width, then you'll want something with a faster recovery rate, and maybe even a photoFET, for that matter.

[pinkjimiphoton]

thanks guys.
between the three of you, i think i get it.

so... fine for everything but what we use 'em for.

[PRR]

> please educate

What they said, and.....

Forrest has used a neat trick. Photo-diodes are not SO expensive, but not dirt-cheap, and are not laying-around every workshop. Photo-diodes are just plain diodes in clear cases. What is THE most common clear-case diode? The LED.

So for a straight on/off (digital) situation, where you need isolation from ground-loops or high voltage, this is a sweet trick.

For analog.... there's a long history and few real uses.

The first solar-cell rejects were crappy, but they sold them to experimenters with plans for a Morse-Code system which would transmit across a dark room.

Later projects used a microphone and power amp driving a flashlight bulb, and photo-transistor to an earphone. The slow-heat/cool tungsten lamp smeared-off the highs. Fidelity was a joke.

LEDs fixed the speed problem. (Small photo-diodes were always fast.) Now you find noise problems due to the very low current available from photo-diodes. I experimented with this for a while and never got "professional" audio out. Also you had to re-calibrate every diode (a production headache).

I did look into several complications. However at that point other options look better. To break a Volt of ground-loop, active balancing is a dime cheaper and no calibration. To break many-Volts (such as long lines across power feeds) there's always transformers, costly but dead-on reliable.

Photo-diodes don't make good photo-resistors. (One way they are a linear current source; the other way they are a LOGarithmic voltage source.)

OH!! There is one moderately wide-spread use. There is a system for lecture-halls and such to assist hearing-impaired users. The user gets a "pocket radio" with a red lens which must be exposed (not in pocket). On the wall of the hall is a huge panel of IR LEDs, hooked back to the PA system. It works, and doesn't go through walls (to an adjacent lecture hall) like radio does. The quality is not terrific, but adequate for communication (folks deaf enough to use this will have high tolerance for background hiss).
______________________

> the photoFETs ... respond immediately (well, "immediately" from our bandwidth perspective).

Maybe. Some of them are slow. It is a eensy-teensy "solar cell" which must charge-up the gate capacitance and a bleeder resistor. I remember numbers like 1mS, which is mid-audio.

But the other problem (even for the small/fast ones) is distortion versus threshold voltage. Because the gate has to charge-up, the incentive is for very low threshold voltage. But that means poor signal handling. While I see the most common LED-FET chip suggested for audio, I think it barely-works in some applications.

[pinkjimiphoton]

gotcha.

thanks for the explanation!!

i had been wondering if i could swap out a led/ldr combo for one of these, but it appears it would be a waste of time.

[Mark Hammer]

> please educate

What they said, and.....

Forrest has used a neat trick. Photo-diodes are not SO expensive, but not dirt-cheap, and are not laying-around every workshop. Photo-diodes are just plain diodes in clear cases. What is THE most common clear-case diode? The LED.

So for a straight on/off (digital) situation, where you need isolation from ground-loops or high voltage, this is a sweet trick.

For analog.... there's a long history and few real uses.

The first solar-cell rejects were crappy, but they sold them to experimenters with plans for a Morse-Code system which would transmit across a dark room.

Later projects used a microphone and power amp driving a flashlight bulb, and photo-transistor to an earphone. The slow-heat/cool tungsten lamp smeared-off the highs. Fidelity was a joke.

LEDs fixed the speed problem. (Small photo-diodes were always fast.) Now you find noise problems due to the very low current available from photo-diodes. I experimented with this for a while and never got "professional" audio out. Also you had to re-calibrate every diode (a production headache).

I did look into several complications. However at that point other options look better. To break a Volt of ground-loop, active balancing is a dime cheaper and no calibration. To break many-Volts (such as long lines across power feeds) there's always transformers, costly but dead-on reliable.

Photo-diodes don't make good photo-resistors. (One way they are a linear current source; the other way they are a LOGarithmic voltage source.)

OH!! There is one moderately wide-spread use. There is a system for lecture-halls and such to assist hearing-impaired users. The user gets a "pocket radio" with a red lens which must be exposed (not in pocket). On the wall of the hall is a huge panel of IR LEDs, hooked back to the PA system. It works, and doesn't go through walls (to an adjacent lecture hall) like radio does. The quality is not terrific, but adequate for communication (folks deaf enough to use this will have high tolerance for background hiss).
______________________

> the photoFETs ... respond immediately (well, "immediately" from our bandwidth perspective).

Maybe. Some of them are slow. It is a eensy-teensy "solar cell" which must charge-up the gate capacitance and a bleeder resistor. I remember numbers like 1mS, which is mid-audio.

But the other problem (even for the small/fast ones) is distortion versus threshold voltage. Because the gate has to charge-up, the incentive is for very low threshold voltage. But that means poor signal handling. While I see the most common LED-FET chip suggested for audio, I think it barely-works in some applications.

But now that you and Forrest have me hallucina....er, thinking, can we dynamically vary the characteristics of the "receiving" diode in musically useful ways?  For example, couold we take a signal tap, and drive one LED to shine on some others, and make them clip differently?

[pinkjimiphoton]

i was kinda wondering if it could be used as a noise gate somehow or something...

that would be cool, to turn audio into light back into audio!!!

forest mims is a ...ummm... "trip", for sure!!!!!

[Paul Marossy]

Much like our retinas, photocells do not have instantaneous reset. 

Cool. I like the idea that my eyeballs are like a couple of photocells.