[sdiy] Funky flip-flops

[harrybissell]

If you have an output shorted to an output... the internal resistance
of the devices is in the circuit. It will get hot but might not draw the
huge
current you are describing...  just more than normal.

How many chips on each board ?

H^) harry

Karl Ekdahl wrote:

> 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
> >
> === message truncated ===