[sdiy] Funky flip-flops

[Karl Ekdahl]

I looked for shorts and outputs going directly to VCC
or GND but didn't find anything. I thought that if any
of the two boards i'm making (it will be 3 boards
eventually) is having some kind of short-circuit or
likewise, it should draw a tremendous amount of power
so i measuerd the mA usage of each board. Apparently
the 3 x 2 digit LED displays draw 30mA together, the
first board around 220mA and the second 210mA, does
this sounds reasonable? If so, i guess this baby will
draw around 1A when i'm done, thus i can totally
forget of making this one battery operated as my first
thought was. Maybe i should just drop this and do it
in PIC, though then i won't get rid of any 74 chips.

Karl


 --- harrybissell <harrybissell at prodigy.net> skrev: 
> 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
> 
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