Re: Need help with op-amp/comparator circuit

From: John Popelish (jpopelish_at_rica.net)
Date: 01/15/05 

Date: Sat, 15 Jan 2005 15:00:32 -0500

Anthony Fremont wrote:
>
> "John Popelish" <jpopelish@rica.net> wrote in message
> news:41E94E93.92B2AFDB@rica.net...

> > It uses the whole signal. It adds the magnitude of the negative peaks
> > ot the magnitude if the positive peaks to produce DC that is equal to
> > the peak to peak voltage. But the output doesn't come out till both
> > peaks have occurred.
>
> Ok, that sounds good. I may implement it to gain some more use of the
> signal, but first I need to find something better to replace the 741.

Yes.

> The LM6132 looks like it might be a decent candidate for a jelly bean
> device. Do you recommend any others for generic low-voltage
> rail-to-rail audio processing?

Almost any rail to rail opamp will work much better than the 741.

The LM358 in not a rail to rail design, but can work with both inputs
and outputs from the negative rail to 1.5 volts below the positive
rail, and they are dirt common. This is the dual version of the LM324
quad. It also has the same input circuit used in your comparator. So
if you use this, you should change your reference divider from 2.5
volts to (5V - 1.5V)/2= 1.75 volts, to center the reference voltage in
the middle of both the input and output range.
http://cache.national.com/ds/LM/LM158.pdf

> In this particular case I don't want too
> much gain, just enough hear the ticking, and not all the ambient noise.

You can easily add a trim pot between your input and feed back
resistor on the opamp, with the wiper going to the - input to vary the
gain. That way you get the biggest possible range of the ratio of
feedback to input resistance for a given pot resistance.

> My current "microphone" is a discarded PC speaker (hey it's all I had
> "in stock" ;-).

Nothing wrong with that.
 
> John Jardine's little RC feedback circuit gives me the exact effect that
> I was looking for, but the pulse is still a bit longer than I want. I'm
> curious how the 47K resistor he suggested fits into the time constant
> calculations. His cap suggestion was a bit large so I lowered it to
> .1uF and that gives me ok results. NEWSFLASH: I'm now experiencing an
> oscillation problem with the 393. I'm fairly sure it was working fine
> at first (at least the scope looked right), but now it's self triggering
> and I can't seem to make it stop. :-(

You need to move the reference voltage on the other input of the
comparator ot a slightly different voltage compared to the center line
of the audio signal, so the comparator can lock up.

> The ultimate plan is to feed the cleaned up pulses into a PIC and time
> the distance between the beginning edges. The ratio of these two times
> will tell me how much out of beat the clock is. I will also be able to
> calculate beats per minute etc... 

-- 
John Popelish

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