H11F1 optocoupler woes

[Benny]

I just received a batch of H11F1's from DigiKey, and I'm having a little trouble with them.  As a simple test circuit, I connected the LED side to a battery with a 50k pot and a 4k7 resistor in series.  Then I connected an input jack to one end of the FET side, and an output jack and a 100k resistor to the output side, making a voltage divider with the variable FET resistance on one side and the fixed 100k resistor on the other.  I expected the whole thing to work as a volume control as I turned the pot and changed the current through the LED, which varies the resistance of the FET.

With the pot all the way down (max LED current, min FET resistance) the output sounds fine.  As I turn up the pot, the output starts to sound crackly and distorted.  The output definitely gets quieter as I turn the pot, but when it's all the way up it has a nasty mis-biased transistor sound to it.

Can anyone explain this?  I tried several devices with the same result.  The same circuit using a Vactrol works fine.  Isn't this essentially what the H11F1 is supposed to be good at?

Benny

[toneman]

Try this---

practicing my ASCII Schematacting  

Code Select Expand



                                  SIGNAL_IN
   **change this \/ to 680ohm**       |
     _           4K7                  |      ___
        | +       ___                 o----o|___|----- GND
  |----||--------|___|-----|          |
  |     |                  |          |      100K
  |       9V             + |        |-+   **ADD this**
 GND                       V H11F1->|
                         _ -        |-+
                   ___     |          |
                 -|___|-----          o----o
                    A                 |     SIGNAL_OUT
                    | 50K             |
                    |                .-.
                    |                | |
                   GND               | | 100K
                                     '-'
          BENNY's circuit?            |
                                     GND



whatchathink of my ASCIIizing???  :wink:  :wink:
with 4.7K, U don't have enought LED current.
did U get polarity of LED correct?  
for 9V, use 680ohm resistor for full brightness :idea:
the 100Ks are the loads 4 your input & output.
LMK your results
staytuned
tone

[Benny]

toneman,

Great job with the ASCII schematic!  I added the input resistor as you suggested but the horrible distortion is still there.  You are right that I could decrease the fixed resistor in series with the pot for more maximum LED current and lower minimum FET resistance.  But the problem I'm having is at minimum LED current, when the FET resistance is highest and the output signal should be at its lowest level.  Any more thoughts?  I'm starting to think this might not be a drop-in replacement for a LED/LDR.

Benny

[toneman]

Benny,
don't know what U mean by "distortion"(?)

U have 2 keep the signal level(amplitude) below, say 2VACpp.

What R U feeding it??
sine wave??   preamped humbucker guitar??

MayB U have some kind of DCoffset @ the input(?)
MayB add an input&output cap??  about a .1 microfarad(mfd).
afn
tone