[sdiy] Pressing buttons

[The Old Crow]

On Sun, 12 May 2002, >>>marjan<<< wrote:

> I'm in need of nice little cct, there are 4 buttons SPST, I need 4
> outputs, when you press button#1 output#1 goes to logic 1 (high) all
> other outputs to 0, press button#2 output #2 goes high all others to 0
> and so on. Also output must stay high until active, not just a pulse.  
> And it should be CMOS.  I'd make it but my mind spits just sorta
> priority codec with f-fs solution, with lotsa gates. There should be
> something more elegant and simple, even inside a cmos chip?

  Here is something I've used since 1984 when I needed a 6-button 
front-end for a process controller:

http://www.oldcrows.net/~oldcrow/6buttons.gif

  Now, the latch (373) would need to be replaced with a F/F to debounce 
better, but I didn't need that at the time.  Also, note the outputs are 
active low, but by using the remaining inverters of U2 or the !Q outputs 
of F/Fs then the active-high state is accessible.  I got good debounce out 
of this when U2A was a 74LS14 and hung a 1uF cap to ground on the U2A 
input.


> And no PIC solutions please :)

  Aw, darn it.. ;)  Can show you how to scan a 5-octave keyboard using 1 
PIC and nothing else. :)

Crow
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