Almost OT: Pushbutton switch latching

[Chuck Guzis]

On 9 Dec 2007 at 18:05, Roy J. Tellason wrote:

> 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...

You can probably leave out the transistor if you don't need to invert 
the diode "OR".  i.e. if you need to clock on the falling edge.  You 
could also fool with pulldowns, N.C. pushbottons, etc.

I was looking for a "valid state guaranteed" circuit where the output 
terms would feed back into the input to guarantee that only one 
output would ever be valid--even on powerup.  You can do it, but the 
gate count starts to climb rapidly as the number of states increases, 
as der Mouse has illustrated.

There are diode arrays in SIPs and DIPs if you don't like the look of 
discretes.

Cheers,
Chuck