I am not sure if the response below was meant for me or for
another poster.  I have neither a pull-up nor a pull-down on this
circuit but a +2.5-V reference voltage.  I then directly connect any
PIC outputs directly to the other LM324 input of the stage being
switched so that anything below 2.5 volts is seen as low and anything
above 2.5 volts is high and causes the output stage to slam over to
the opposite state.

        Another poster reminded us of the
slower slewing rate of the LM324.  This is quite true and must be
considered when designing this circuit.

        I would call it a cluge.  I have a bunch of LM324's on hand
and the switching rate is between 40 or 50 HZ and a few kilohertz.  I
will be surprised if it really works at the very high end, but this
was a quick and dirty solution designed as much to use up existing
parts as provide 0-12 volt logic levels from 0-5 volts.

        Voltage comparators such as the LM339 may also be good if not
better for this purpose except that their outputs are open collector
or maybe open drain which means that, when referenced to ground, they
will show a short to ground when low and nothing at all when high.

        I certainly wouldn't recommend using an audio op-amp if you
are designing that circuit and want the fastest switching speed
possible.  There are level converters designed as such and, if you can
get them and they are cost-effective, it is better to use them.


Martin McCormick WB5AGZ  Stillwater, OK
OSU Information Technology Division Network Operations Group

"Peter L. Peres" writes:
>You cannot use a pullup, you have to use a pulldown (1k should work). Do
>get the LM324 data sheet if you do not have it.
>
>Peter
>
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