I've followed your description:


 +12V ----------------+---------+-------------+-------
       OPTO           |         /        470  |
    +--------+        |         \     +\/\/\/-+  _ /|
    |       _|_     |/C     100K/     |       +-| | |
  INPUT     /_\ --> |           \    -+--| |----|_| |
    |        |      |\E         |  |/C   1uF       \|
    +-\/\/\/-+        |         +--|
        1K            |         |  |\E  100
                      |         |    +-\/\/\/---+
                      |         |    |  10uF    |
                      |         |    +---| |----+
                      |         |       10K     |
                      |     +---+------\/\/\/---+
                      |     |                   |
                      |    ---                  |
                      |    ---10uF              |
                      |     |           10K     |
                      /     +----------\/\/\/---+
            INPUT POT \   |/E                   |
                  10K /<--|      +----PWM INPUT |
                      \   |\C    /              |
                      |     |    \ PWM POT      |
                      |     +--->/              |
                      |          \ 10K          |
 GND -----------------+----------+--------------+-----

Note the extra 10K from Q1E to GND. I think it is needed there (i.e.
it wasn't working properly without it)

This circuit sounds different, more buzzer-like, because of the
capacitors of course (specifically the 10uF from Q2E to GND; if I
short that one it goes back to the normal tone). I don't care much
about that except that it seems to also lower the volume (apparently,
especially since it kind of drowns out lower freqs).

I'll play with the values and see what I get :-) thanks for the ideas,
now zero volume is zero PWM and full volume is somewhere around 1.5-2V
PWM.



-- 
Hector Martin (hector@marcansoft.com)
Public Key: http://www.marcansoft.com/hector.asc

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