gEDA-user: furnace controller I/O port again

[Mark Rages]

On 11/1/06, DJ Delorie <dj at delorie.com> wrote:
>
> I played with the op-amp idea, and actually got a working prototype on
> proto-board.
>
> http://www.delorie.com/pcb/io_port4.sch
>
> The "out" (line-driving) op amps come in a tiny dual-amp package, and
> the use of resistor dividers and ability to swap +/- inputs lets it
> drive the two MOSFETs directly.  The "in" (sensing) op amp uses a
> divider to bring 0-5v down to 0-3.3v, with the reference suitable for
> both 3.3v CMOS and 5v TTL levels (effectively 1.65v).  That op amp is
> available in a SOT-325 package, making it about as big as a
> transistor.
>
> The use of op-amps also nicely solves the voltage conversion issue,
> letting me run the R8Cs at 3.3v while still driving the thermostat
> lines at 5v.

Be sure to check how long it takes the op-amps to come out of
saturation if you intend to use this for high-speed signals.
Comparators (ones with totem-pole outputs) are usually better suited
to this application.

Regards,
Mark
markrages at gmail
-- 
You think that it is a secret, but it never has been one.
  - fortune cookie