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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