Re: Tron: parts question

On Tue, 30 May 2000 07:28:04 -0700 (PDT), you wrote:

> So that helps, but I think it may be too specific. What general
>area should I be looking in? I'm guessing these are semiconducter
>devices, but there are about 15 areas w/i that section. (I'm sorry if
>I'm explaining this poorly, the site is radioshack.com.)

        The LM3900 should be under "linear I.C.'s", then "comparators"... but
of course, the Brainiacs at Radio Shack have managed to screw up their site
so that you can only see page 1 of any product line; trying to go to any
other page gives me a URL not found...

        However, according to my (low-tech, but reliable) Radio Shack catalog,
the LM3900 is available, but only as a special-order part (RSU 11391935)
and it costs $2.99 apiece. They do not seem to have the 4N28 optoisolator.

>Optoisolator; is that any relation to the OpAmp family?
>As always, forgive my ignorance, I really don't mean to be a pain, I'm
>just trying to learn. =)

        Not even slightly related. :) An optoisolator is, basically, an LED
and a phototransistor in the same package, like so:
(warning: bad ASCII art ahead)

              ---------------
          A__!__ ____!__C
             ! _|_ |/ !
        LED ! \ / -> _| ! PHOTOTRANSISTOR
             ! _V_ -> B| _ !
          K__!__| ||\___!__E
              ---------------

        The phototransistor will only conduct when its base (B) is stimulated
by light from the LED - so turning the LED on and off will control whatever
circuit is connected to the phototransistor. The most common use for
optoisolators is when you have one circuit that needs to control another,
but which needs to be electrically isolated from it for some reason.

In the case of the TRON power supply (in an attempt to bring this back on
topic :) ), you have U4's LED being driven by the output of the "Unreg. Aux
V" bridge rectifier (D101-D104), while the phototransistor is connected to
a circuit powered by the regulated +5V logic supply. It's a bit of a
screwy circuit (and sure as heck not how _I_ would've done it! :) ), but
the practical upshot of it all is that the circuit formed by the LM3900,
Q201, /et. al./ applies a RESET signal to the logic boards, keeping them
from starting until the UnregAuxV supply comes on. (UnregAuxV turns the
LED on, which allows the phototransistor to conduct, which starts a chain
of events through the "Pump Charger" and "One-Shot" circuits that
eventually causes the RESET signal to go away and allow TRON's logic boards
to start.)

        By the way - this whole circuit is not all that necessary. In fact, it
has been known to take a perfectly working MCR-series game and stop it in
its tracks by just deciding not to work one day. (Yes, I speak from
experience. :) ) Personally, I'd suggest that you not even _bother_
replacing U3 and U4 - just pull the bad ones out, then snip the purple wire
at J4-15 to disconnect the RESET signal entirely.

>>The only downside to Digi-Key is that you either have to order a
>>minimum of $25 worth of stuff, or they zap you with a $5 "handling
>>charge"...
>
> Yeah, that's why I hadn't done to them or Mouser (anyone else out
>there?) yet, I knew about the minimums. I have already learned that
>when you want a handful of resistors and caps, plus a few other pieces,
>$25 can be a really big number to acheive. =) Thanks for the help so

        True... but on the other hand, it never hurts to have spares on hand,
so if you're going to order them, I'd say you ought to order at least ten
of each anyway. :)
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solarfox@DON'TMESSWITHtexas.net (Gary Akins jr.)
http://lonestar.texas.net/~solarfox
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Received on Tue May 30 14:02:39 2000