[sdiy] Help with lm339

[J C]

Exactly - the envelope is decaying from ~10v and when it reaches the
sustain voltage plus a little (that's the part I need), I want the
comparitor to flip.
On 4/1/06, Bob Weigel <sounddoctorin at imt.net> wrote:
> Oops...I read the front end and thought you were still asking about
> that..NEver mind.  So the remaining confusion is...is the envelope going
> from zero to a positive voltage?  So for example if the sustain is at
> 1V, you want the comparator to flop to positive output when and ONLY
> when the EG signal reaches..say ...1.05V or like that? -Bob
>
> 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.  I do not need
> >hysteresis per se because I am only looking at this when the envelope
> >is decaying to the sustain level.  The sustain voltage is also being
> >used by another part of the circuit and so the injected voltage can
> >not change that - this circuit is to only add state LEDs to the EG.
> >Does this make any better sense?
> >On 4/1/06, harrybissell <harrybissell at prodigy.net> wrote:
> >
> >
> >>without seeing your schematic, its tough to give exact advice...
> >>but the usual solution to your problem is to add hysteresis
> >>to the circuit... you need a series resistor in the positive input
> >>and a large value resistor from the output to the positive input.
> >>
> >>Hysteresis moves the positive voltage a little bit... so that when the
> >>trip point is reached the signal is suddently moved WELL into the
> >>region and there is no oscillation.  Use a small amount, or you will
> >>create a latch situation.
> >>
> >>I did an envelop gen years ago where I chose your proposed solution...
> >>it will work but it is a little complex.
> >>
> >>H^) harry
> >>
> >>J C wrote:
> >>
> >>
> >>>I am trying to use a lm339 comparitor to determine when an ADSR signal
> >>>reaches the sustain level.  What I want to do is add a small voltage
> >>>to the sustain level voltage so that the comparator fires just above
> >>>when the envelope reaches it (if I don't do this, it just waffles
> >>>around when the two voltages are equal ie:when the envelope is
> >>>sustaining).  Is there a simple way to do this (without adding a
> >>>summing opamp)?
> >>>
> >>>
> >
> >
> >
> >
>