gEDA-user: furnace controller I/O port again

[DJ Delorie <dj@xxxxxxxxxxx>]

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.


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