Like Olin Lathrop said (and I missed the first time), if your are only
dealing with a signal in the audio range of 20Hz to 20kHz then you can
easily shift the DC level to 2.5V. Use a simple two resistor voltage
divider or a potentiometer to set the upper (2.7V) and lower (2.3V)
threshold voltages of the window. The LM339 is a very popular quad
comparator that should suffice.
--
Greg

Richard Sloan wrote:
>Might only detect the +0.2V positive swing, this make it much easier right?
>
>But on the input to the comparator how do you accurately put 0.2v on one
>of the inputs, also do comparators already have some imbalance on the
>inputs and how do you compensate?
>
>R.
>
>  >>  Two op amps or two comparators. What you're looking for is called a
> window
>  >>  comparator. The hardest part will be level shifting the signal into
> the 0
>  >>  to 5V range without actually affecting the signal over the whole
> frequency
>  >>  range. You may have to look into generating a negative voltage supply.
>  >>  --
>  >>  Greg
>
>  >>  Rick C. wrote:
>  >>  >I'll poke around and see if can a schematic. The one I have in mind
> uses
>  >>  >two opamps.
>  >>  >Rick
>  >>  >
>  >>  >Richard Sloan wrote:
>  >>  >
>  >>  > >  >>  A couple more things need to be known before anyone can really
>  >>  > give you an
>  >>  > >  >>  answer.
>  >>  > >  >>  1) Are we assuming p-p freq. to be within audio range?
>  >>  > >
>  >>  > > any freq, we are dealing with audio range freq however, so it should
>  >>  > work for 20-15k range
>  >>  > >
>  >>  > >  >>  2) How long does the LED have to sustain after receiving
>  >>  threshold?
>  >>  > >
>  >>  > > well could just blink for the instant it happens, might be nice to
>  >>  have
>  >>  > it stay on for 0.25 - 0.5s
>  >>  > >
>  >>  > >  >>  3) Are you planning to use a PIC to do the detecting? (overkill
>  >>  > really)
>  >>  > >
>  >>  > > no opamp / comparator
>  >>  > >
>  >>  > >  >>  4) Do you want to detect on > +.2 and/or -.2v?
>  >>  > >
>  >>  > > yes
>  >>  > >
>  >>  > >  >>  5) We need to know if you are wanting to mix analog with
> digital
>  >>  to
>  >>  > >  >>  achieve your goal, or straight analog citcuitry.
>  >>  > >
>  >>  > > hopefully its just analog
>  >>  > >
>  >>  > >  >>  There are many sample circuits in the National Linear Databooks
>  >>  and
>  >>  > >  >>  application notes that will probably provide what you want. A
>  >>  simple
>  >>  > >  >>  opamp will achieve your
>  >>  > >  >>  desired results. A Norton Amp such as the LM3900 will probably
>  >>  > work and
>  >>  > >  >>  will operate from 4v to 36v single ended supply. You'll have to
>  >>  dig
>  >>  > >  >>  around for a sample
>  >>  > >  >>  circuit in the databooks, but they're there. Otherwise
> someone on
>  >>  > the list
>  >>  > >  >>  might be patient enough to walk you through a working circuit.
>  >>  > >
>  >>  > > I have looked and seen non in national book.... i can look again!
>  >>  > >
>  >>  > > And I agree on patience as question after  question on here needs
>  >>  > answering and you could have a full time job just answering!
>  >>  > >
>  >>  > > Thanks!
>  >>  > >
>  >>  >
>  >>  >--
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>
>  >>  --
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>
>
>
>
>
>
>
>
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