[sdiy] Funky flip-flops

[harrybissell]

Whenever something happens with a scope it is a clue.
I'd say there is no way the scope should knock out your
power supply.  I'd suspect that you have the scope connected
to ground (third pin if you're USA - are you ?)... and when you
hook up the scope ground you are making an unwanted connection ?
Or maybe the scope is mistakenly a 50ohm input (rare but possible) ?

LEDs can eat amazing current... you can calculate from the series
resistor, the power supply voltage and figure a 1.5V drop in the
LED. Three seven segment displays could be more than 200mA
alone.

I'd look into that scope thing carefully...

H^) harry

Karl Ekdahl wrote:

> 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
> >
> >