Oh, and you might want to put a voltage follower op-amp circuit in between
the level shifting circuit and the window comparator to buffer the input
from the load.
--
Greg

Gregory D Wagner wrote:
>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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>>www.themindfactory.com
>>Richard Sloan
>>(613) 825-4758 phone
>>(613) 825-7758 fax
>>rsloan@themindfactory.com
>>
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