Almost OT: Pushbutton switch latching

[Chuck Guzis]

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. 

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.

Cheers,
Chuck