Re: phase reversal: can an LM393 do this?

I was tinkering with a circuit that I put together about a year ago. It
listens to the tick tock of a clock using an 8 ohm transistor radio type
speaker as a microphone. This circuit uses a single 5V supply. I have
an LM741 with the + input biased at half of Vcc (2.5V... yeah I know
already it's a sucky op-amp for that, but it's what I had on hand). The
speaker is connected to both inputs differentially thru 3.3uF caps.
Gain is set to 1000 using a 100K feedback and 100R input resistor on
the - input. The output is fed thru a 47K resistor to an LM393
comparator's + input. The minus input is biased to about 1/2 Vcc but
just a wee bit smaller than the value coming from the 741 so that its
output idles high when no sound is being heard. The comparators output
is positively fed back thru a .33uF cap and series 100K resistor. This
makes the comparators output snap low and stay there for about 120mS
after the sharp attack of the tick (or tock as appropriate) decays below
the idle offset voltage between the comparators inputs. This all works
well and fine, but.

Looking at the scope, with loud ticks, a mS or three after the output of
the comparator snaps low, it instantly snaps high for a few mS and then
back low again. The PIC chip trying to measure the spacing between
ticks and tocks, doesn't like this. It seems that this happens when the
output of the 741 exceeds a certain point. Apparently the 393 doesn't
like its inputs coming too close to Vcc? Does this sound likely? The
reason I suspect this is that I was able to cure the problem by placing
some cool blue LEDs on the output of the 741 (before the 47K resistor).
One LED between the output and Vcc, and one between the output and
ground. The LEDs have a typical Vf of 3.3V, but both light dimly with
the 2.5V applied. When the ticks (or tocks) occur, the LEDs flash
brightly while keeping the output of the 741 between ~1.8V and ~3.3V
above ground. No more funny "phase reversals" on the output of the 393
after that. The 393 didn't seem to care how near the voltage went to
ground.

Sorry so long, but I want to be clear. :-)


Well- you're not all that clear, leaving the reader to guess at more than few of the interconnections. And you can skip the ":-)" ploy used by the NG hucksters, con artists, trolls, and posers. No one will be asking you to give a presentation any time soon. The LM393 comparators do not phase invert when a single input goes above the Vcc rail- even as high as 36V- as long as the other input remains within the CMR range, which is -0.3V to Vcc-2V worst case. So you have something else going on- probably noise and errant feedback issues. Lastly, there is no need for a 120ms one shot drive of the PIC input. All you need is maybe a few 10's of microseconds and use the PIC to lock out responding for the 120ms. Your analog timing slew rates will then be 10,000x faster which vastly simplifies the comparator monostable layout and design. And there's no need for a 741 either, when at most a single 2N3904 CE is all that's required.

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Relevant Pages

  • Re: phase reversal: can an LM393 do this?
    ... listens to the tick tock of a clock using an 8 ohm transistor radio type ... makes the comparators output snap low and stay there for about 120mS ... ticks and tocks, doesn't like this. ... some cool blue LEDs on the output of the 741. ...
    (sci.electronics.design)
  • Re: phase reversal: can an LM393 do this?
    ... listens to the tick tock of a clock using an 8 ohm transistor radio type ... makes the comparators output snap low and stay there for about 120mS ... ticks and tocks, doesn't like this. ... some cool blue LEDs on the output of the 741. ...
    (sci.electronics.design)
  • phase reversal: can an LM393 do this?
    ... listens to the tick tock of a clock using an 8 ohm transistor radio type ... makes the comparators output snap low and stay there for about 120mS ... ticks and tocks, doesn't like this. ... some cool blue LEDs on the output of the 741. ...
    (sci.electronics.design)