[sdiy] Funky flip-flops

[Karl Ekdahl]

All in all i counted on 28 TTLs for three boards,
about 8 on the first one and the rest on the second
one. Anyhow i desoldered some of them yesterday, to
make room for CMOS instead. I noticed that i did have
40xx replacements for most parts, and a few LS
replacements will be made as well. Roughly counting
power dissipation i'm going from 230mW to 12mW.....

Oh btw, is it safe to mix 74LSxx with 40xx? I'm still
going to run it on 5v in that case of course....

Karl


 --- harrybissell <harrybissell at prodigy.net> skrev: 
> 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
> 
=== message truncated ===