[sdiy] gates and clock inputs to a microprocessor

[Jason Proctor]

>  > on the Arduino module i used MC34072 comparators with the threshold set to
>>  1.2v or thereabouts, on digital inputs and outputs. works fine. the opamps
>any reason for this particular opamp over say the lm741/747?

it's a rail-safe opamp i trust. other options would be lt1013 
(overkill) or lm358 (cheap though Ken reports some mfring issues).

>  > are powered from a unipolar supply so there's no negative voltage
>>  possibility.
>makes sense. What about protecting the input pins from negative voltage?

the unipolar supply on the opamps means that the output can't go 
negative. in comparator mode, the output is +5v or 0v. i use this 
mechanism in both directions. of course if you're driving gear that 
requires 10v, like an ARP or something, then you might want to power 
the output comparators from +15 then divide the output down to +10, 
like Ken does.

>  > someone correct me if i'm wrong here, but the pullups only come into play
>>  when there's nothing connected to the input. it's to prevent the pin from
>I was wondering about that... I did see a lot of designs that used
>100K pulldowns though..

100k is a lot compared to the 1k output impedance of (eg) an MOTM module :-)

>for me I am only outputtin unipolar gates but they should probably be
>scaled to 0-12V ompamps should be fine for this app.

no i'm talking about the Arduino's *analogue* I/O here. the digital 
"gate" I/O is done via the comparators discussed above and earlier in 
the thread.

hth