[sdiy] Help with lm339

[Ian Fritz]

At 02:36 PM 4/1/06, J C wrote:
>I think I am missing something easy.  I have put a rough sketch here:
>http://home.cfl.rr.com/entropy/comp.jpg
>What I have is a sustain voltage selected by the pot, buffered through
>the TL072 and then presented to the comparitor to be compared to the
>envelope being generated.  What I want to do is to add a slight
>voltage to the + input so that the comparitor will trip as the
>envelope decays and approaches the sustain level.

Very easy.  Create a voltage one diode drop (.65V) above the sustain level 
by connecting one end of the diode to the output of the opamp and the other 
end through a resistor (22k, say) to Vcc.  Then put a voltage divider 
across the diode to get whatever voltage difference you want at the 
comparitor input.  For example, if you want a .1V difference,  then make 
the divider from a 1k and a 4.7k resistor:
(1/(1+4.7))*.65V = .11V.

Hope this helps.

   Ian