One more thing...

http://electronics.stackexchange.com/questions/30017/which-configuration-is=
-better-for-pulling-down-an-npn-transistors-base

That is about Pull-ups and Pull-downs used on BJT transistors to prevent un=
intentional switching.  I lack the reputation there to add a comment, but s=
ince we are already on the subject of PULL-UPS it seems fitting to talk abo=
ut here.

That article says "Values up to about 100K are probably OK in most cases...=
" =20

It's all "probably," isn't it?

Obvious questions after reading that Stack Exchange dialog are:

1. If 100K is probably OK, why is 150K probably NOT OK?  What about 220K?

2. Would that "up to 100K is probably OK" thinking apply to the B-E "pull-d=
own" for an NPN only, or would that resistance value be equally safe and ef=
fective as the B-E "pull-up" on a PNP?

3. Would it be prudent to keep the pull-up/pull-down the same or on order o=
f several times larger than the Base resistor?  (In an attempt to answer th=
is question, I would think the pull-up/pull-down size would not matter in r=
elation to the base resistor size, so long as the pull-up/pull-down is doin=
g its job; although if the size is too small there would be unnecessary cur=
rent flow through it, making your design less battery efficient.)

=20
In one of my designs currently being tested, I am using the following circu=
itry for reading +12V and -GND from a car:

https://cl.ly/jn0P/Image%202017-03-30%20at%2010.29.42%20AM.png

The NPN circuit inverts the car's +12V signal and feeds GND to the PIC's in=
put.  Likewise, the PNP circuit inverts the car's GND signal and feeds +3V =
to the PIC's input.  Decent PIC input protection, and no Zeners required.

Both the NPN and PNP circuits work reasonably well in my testing, although =
I am now rethinking the 220K pull-up on the PNP.  Maybe 220K is too high?  =
But what is too high for that pull-up?  How can I effectively test, on a br=
eadboard, if those transistor pull-ups are doing their job to prevent unint=
entional switching of the transistor? =20

I suppose I could use a higher value like 100K on the NPN's pull-down (inst=
ead of the existing 1K), but what advantage would that bring?  The current =
through the Base resistor would be unchanged:

Current Flow using 1K:

https://cl.ly/jnMB/Image%202017-03-30%20at%2010.34.07%20AM.png

Current Flow using 100K:

https://cl.ly/jmyO/Image%202017-03-30%20at%2010.35.14%20AM.png


Thoughts?


Thanks,

James W.


--=20
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