🚀 go-pugleaf

hobbit@topaz.rutgers.edu (*Hobbit*) writes:
>What's the correct way to drive TTL from an op-amp?  I'm proposing to do some
>kind of threshold detection by sending an op-amp output into a Schmitt; the
>op-amp seems unable to pull *low* enough to make the Schmitt input see
>a low level.  I suppose I could go out and buy one of those fancy op-amps
>that drives all the way to the - rail, but I'd rather have a simple fix I
>can do from my junkbox.
>
Given the constraints, assuming an inversion is ok, put an NPN transistor
in between.  Ground emitter.  R from opamp to base.  R from base to
gnd.  R from collector to +5.  collector to TTL input.  Should be able
to do this with less than 400 microamps from opamp output, at somewhere
around the midpoint of its output voltage range.  If you have a small
enhancement mode mosfet in your junkbox, you could save a couple resistors,
but be careful not to exceed its gate voltage maximums.

Or use CMOS logic (old 4000 series, or 74HC -- not 74HCT series), which
has low input current and Vil threshold spec'd at 1/3 of Vcc.