[sdiy] Funky flip-flops

[Karl Ekdahl]

So, i did add the 0.1uF decoupling caps and fired the
circuit, unfortuanley it behaved even more strange
than before so i tried plugging in my scope looking
for noise in the powerlines. Even more unfortunate is
the fact that every time i plug in my scope (an old
Tek scope) the fuse in my powersupply brakes.
Apparently, the 74xxx does draw a lot of current, but
is it really so that a couple of 15 74xx's + 3 LED
displays should blow a 500mA fuse? It's becoming clear
to me that i have NO idea whatsoever how much current
things actually draw. And by the way, yes, the 74xxx's
are free - i've got shitloads and i want to get rid of
them so i decided to build an overcomplicated
drummachine sequencer. I've also been thinking of
building a *huge* pocket calculator only using simple
logic 74xxxx's.

Karl

 --- harrybissell <harrybissell at prodigy.net> skrev: 
> Yes Jay...
> 
> I'd like to add... decoupling caps. TTL is
> desperately in need of good power supply
> decoupling.   I'd suggest a .1uF cap across EVERY
> single TTL chip.   I follow this rule even
> with 4000 series CMOS... which could use probably
> half that amount.
> 
> About the RC on the ... clock line ??   I would not
> use a filter on the clock line... TTL needs
> a good fast pulse to trigger well... although I have
> seen some times where a tiny tiny RC
> helps in stopping a bounce (and mistrigger).  Better
> to fix the real problem than try to mask
> it.
> 
> A modest proposal... switch to 4000 series CMOS.  It
> is way, way, way easier to use than TTL,
> uses a fraction of the power...
> 
> ...unless the TTL stuff was free, of course...
> 
> H^) harry
> 
> jays at aracnet.com wrote:
> 
> > Reset sounds funky.
> >
> > 1) Holding the reset lines low has the FF in a
> constant state of reset. They are active low and
> should be tied high (reset to +V) with a 1K-3K ish
> resistor. Or better yet put a cap in there between
> reset and ground (with the resistor in there) to
> form a RC that slowly charges up to V+ providing a
> power on reset. Also if you aren't using the resets
> individually you should just tie them altogether.
> >
> > If I were to guess I'd say there might be some
> interaction between the PS and the 10K reset
> resistors not providing a 'weak ground'.
> >
> > 2) If you tie stuff to ground don't use a
> resistor.
> >
> > 3) Put some sort of RC on the switches going to
> CP.
> >
> > 4) Put a pull up (1K-2K) on the ~set lines. It's
> active low to.
> >
> > Jay S.
> >
> > Karl Ekdahl <_nial_ at yahoo.com> wrote :
> >
> > > Hi all, i've got a strange problem. I have 8
> > > flip-flops (4 * 7474) which all share a common
> ~SET
> > > line and each have it's ~RESET tied to ground
> trough a
> > > 10k resistor. Each flip-flop is connected to
> itself
> > > via ~Q -> D and each is set/reset trough a
> push button
> > > connected to CP. All Qs are connected to one LED
> each.
> > > So, pushing the first button sets/resets the
> first
> > > flip-flop. Pushing the 8th button while the
> first
> > > flip-flop is set resets the FIRST one and sets
> the 8th
> > > one. Pushing the 8th resets the 8th flip-flop
> and then
> > > pressing the 1st sets BOTH flip-flops. All the
> > > flip-flops reacts like this with eachother in
> all
> > > kinds of strange patterns. The only way they
> possibly
> > > COULD interact with eachother should as i
> understand
> > > it be
> > >
> > > a) the comon SET
> > > b) trough the RESET resistors to ground (?)
> > > c) trough the 74245 that delivers the signals
> from the
> > > pushbuttons.
> > > d) powersupply noise (?)
> > >
> > > a) could probably be ruled out since they bot
> SET and
> > > RESET eachother. b) and d) is the same i guess
> and i
> > > have a pretty huge powersupply feeding only a
> couple
> > > of TTL chips, so that seems unlikley. I had
> > > powersupply problems earlier but fixed that. c)
> the
> > > 74245 seems to give perfect result while looking
> at it
> > > with my multimeter, so i don't think so...
> > >
> > > I'm stranded, anyone?
> > >
> > > btw. i might as well thank everyone for their
> help
> > > with various things the last couple of weeks,
> i'll put
> > > some pictures and stuff of what i'm doing
> somewhere in
> > > some time..
> > >
> > > Karl
> 
>