Almost OT: Pushbutton switch latching

Roy J. Tellason rtellason at
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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