Which optocoupler would work best...

[nirvanas silence]

I've been experimenting with some optocouplers but I'm having problems finding just the right one.  The ones I use now are too slow to respond so it's useless in my tremolo.  On top of that I want under 500 ohms light resistance and over 1M dark.  Any ideas on what fits the bill without breaking the bank?

[Mark Hammer]

How fast do you need them to respond?

[nirvanas silence]

A really fast stutter tremolo is what the first one's going in.  I don't know the actually frequency though.

[Chico]

I have had luck with Silonex NSL 32 R3 devices.  I built an Anterton tremolo with one and it turned out real nice.  

Unfortunately, I do not know how fast a "real fast stutter" is, so I cannot say if this particular opto will do the trick.  

You can check it out here:
http://www.silonex.com/datasheets/specs/104058.htm


If all else fails, you can always use FETs.

Good luck

[Mark Hammer]

Okay, that clears up a little bit more for me.  In the case of something like an envelope controlled filter or noise gate, you might want a super-fast attack/rise/on rate, but could tolerate a little sluggishness in terms of decay/off/settling/recovery rate.  In those contexts it can help to let you cram more than one note into a segment under the banner of one on-off cycle of the LDR.

When you have a constant request for on-off cycles, though, your "on" becomes a function of the last "off".  That is, if your LDR is still resettling back down to high off-resistance after the last sweep to minimum on-resistance, then you won't really notice how fast it is capable of initially changing in resistance because it hasn't reset completely before the next request for it to sweep.

This is the primary reason why LFO-driven LDR-based circuits tend to lose depth as speed/rate is increased.  Once you crank the speed up fast enough, the LDR never really gets enough time to resettle/recover so you're actually squeezing less coverage of the full resistance range as you make them work faster.

What you want, then, is something that has not only a fast attack time, but an equally fast decay/fall time.  If you want crisp edged square-wave transitions for max stutter effect, you may want to use a FET, or photoFET rather than LDR.  For example, an H11F1/f2/f3 tacked onto the end of an LFO and used the way the FET is used in an Orange Squeezer might be appropriate.

Alternatively, take into consideration how much the LDR *CAN* recover in the desired timeframe and figure out how to translate that into audible changes of the type you desire.  It may be that you need to make it swing back and forth from 500R to 1meg with the present design and component values, but is there a way that you can produce an acceptable effect with resistance changes from 500R to, say, 10k?  If so, then you won't need for the LDR to completely recover to max dark resistance.  As long as it can go part of the way you'll have your effect.

For example, if the LDR was installed as one leg of a resistive divider, either input-to-wiper or wiper-to-ground, is there a resistance value for the  opposite divider leg that would produce the degree of choppy on-offness you want?