Replacing a JFET with an optoisolator

[Electron Tornado]

Just saw this thread:  http://www.diystompboxes.com/smfforum/index.php?PHPSESSID=6287780e15bf3727e3da24f6a1ec8db0&topic=105473.0

I've been wondering about replacing a JFET with an optoisolator in something like a phaser circuit. Disregarding enclosure space limits, is that a relatively simple thing to do, or is it more like putting a square peg in a round hole?

[Seljer]

Yep, couldn't be any simpler

Most of these phaser and filter circuits work around having some kind of "variable resistance". And an LDR is precisely that! Of course you'd probably have to tweak the LFO to drive the light source.

In the case of phasers go take a look at the the Commonsound Phaseur Fleur or the Mutron Biphase. You'll see that same string of recognizable opamp phase shift stages.

[R.G.]

The resistance side of it is very easy indeed. The LED side is completely different.

JFETs are driven by a voltage, and consume almost no current. An LED puts out light (and hence changes the LDR resistance) proportional to the current through it, and current only flows when the voltage across it exceeds the LED threshold voltage. So an LED really needs to be driven by a current source or something like that in optoisolator applications.

Output easy. Input completely different.