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Manchester (England England)
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R2 is not needed. Ther is no need to have a seriese resistor when using a TTL device as an input.
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No R2 isn't needed, It is good insurance though for issues on either side of the resistor and If the 328 port connected to that resistor were set opposite of the 'HC14 the resistor would limit current with no ill effects on a 'normal' circuit.

Bob
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If the 328 port connected to that resistor were set opposite of the 'HC14 the resistor would limit current with no ill effects on a 'normal' circuit.
I deplore this modern trend for trying to idiot proof things. If you are going to be that stupid you should not be messing with electronics in the first place.
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I deplore this modern trend for trying to idiot proof things. If you are going to be that stupid you should not be messing with electronics in the first place.

Well said!
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the land of sun+snow
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I deplore this modern trend for trying to idiot proof things. If you are going to be that stupid you should not be messing with electronics in the first place.
deplore - ha, now there's a very strong word. Since we're always telling stories of the days
of yore ....

Back in the olden days, I was making some pcbs for use with OOPics [a deplorably weak device,
after all]. All of my boards had small-valued series-Rs on the I/O lines. Then, oopic started
selling boards with the cpu surface-mounted, and there was no protection on the I/O lines
whatsoever. Well, almost every day on the forum, some poor sack would come on crying he
had blown up his oopic board by overvolting one of the pins, etc. The market of course, was
aimed at beginners. And of course, the smt cpu could not be replaced, so the board was
toast [at $70 a pop in those days]. Well, over time, not one person ever blew up even one
of my boards with the I/O pin protection - and I still swear by such measures.

At least in the Arduino world, we have a lot of boards with DIP28 chips that are easily and
cheaply replaced when blown, even if there's still scant pin protection.

That all being said, the series-R probably really isn't needed in the OP's original ckt.
[waiting for thunderbolts now, :-)].

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