> Yes, that's it.  My question (posed to someone else privately) is:  Why do
> all the diagrams I see for this sort of thing typically use a BJT?  Is
there
> any reason that a BJT is preferred over a MOSFET?  Could one just as
easily
> use a MOSFET here?  I plan on having up to 3 devices being controlled by
> relay circuits like this, it seems that it would be easy to just get an IC
> that has several transistors on it and wire it up like that.

Each have their purpose.  The FET requires a voltage signal instead of a
current.  This also gives it nearly infinite power gain which can be very
handy in low power circuits.

At a first pass approximation, an on FET looks like a resistor, whereas a
saturate BJT looks like a fixed voltage drop.  One problem with FETs is that
the basic process produces FETs that want about 10V on the gate to fully
turn them on.  You can get "logic level FETs" that only require 5V, but
these cost more and/or have a higher on resistance.  On the whole, FETs are
more expensive than BJTs.

I suggested a generic "jelly bean" NPN transistor because these are dirt
cheap and quite effective for this purpose.  Just about any TO-92 small
signal NPN should work fine for drive currents up to a few 100mA.  I use
2N4401 for this sort of purpose, but there are lots of alternatives.


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