Almost OT: Pushbutton switch latching

Roy J. Tellason rtellason at verizon.net
Sun Dec 9 17:05:22 CST 2007


On Sunday 09 December 2007 16:32, Chuck Guzis wrote:
> On 9 Dec 2007 at 14:45, der Mouse wrote:
> > For an N-stable circuit, this technique requires 2*(N-1) inputs on each
> > gate.  For a bistable, or even a tristable, this is tolerable; for an
> > 8-stable, it requires eight 14-input NANDs.  You could do it with
> > 8*14=112 diodes (hmm, might need another 8) and a relatively small
> > handful of resistors and transistors.  Doing it with just TTL would
> > be..somewhat inconvenient.
>
> Perhaps something with UJTs and diodes--a UJT would give you a stable
> state for each button and some steering logic would get you there.
> But UJTs seem to be pretty scarce nowadays.

You can make your own with a pair of cross-connected transistors.

> Better to use a single RS FF per button, with reset being driven by
> the OR of all of the other buttons.  You could certainly do the OR
> with diodes; for an 8-button setup that would be 8 RS flip-flops and
> 56 diodes (fewer if you want to make a tree of them).  That'd be 2
> 74279s  or 4043s with a pile of diodes.

A similar application that comes to mind is the "game controller" where each 
contestant has a push button and the first one in gets lit and locks the 
other ones out.  There are a bunch of circuits like that around the web. See 
the "4QD" site (in the uk?) for an example of both this and the 
how-to-make-your-own UJT particulars.

My initial reply on this went offlist,  and I still think it's the simplest.  
You take a '374 (well,  I have a pile of 'em here...  :-) and connect a push 
button with a pullup resistor to each input.  Then connect a diode to those 
points too,  with all of the diodes tied together.  A single transistor fed 
by those diodes will generate the clock...

My application for that was a multiple-battery charging setup.  I still have 
to finish up the design in figuring out how each select output will choose a 
different circuit (different sized batteries) and build the darn thing,  
saving me the effort of having to manually hook up all sorts of stuff and 
deal with it.

-- 
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James 
M Dakin




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