[sdiy] interlocking switches

[Tom Arnold]

On Sun, Aug 14, 2005 at 03:19:41PM -0400, Roy J. Tellason wrote:
> Can any of you guys point me toward a circuit like this?  I know there were a 
> fair number of commercial keyboards out there that used such stuff,  rows of 
> momentary-contact switches with LEDs and some toggled on/off,  while others 
> "interlocked" like I'm talking about here...

The logic behind this, while simple, is high parts count.  I've done it with
PLAs in the past if I did it these days I'd probably use small AVR ( or PIC
if you want ). DPDT break before make pushbuttons to an octal latch, when
you press a button the "release" clears the latch, the "make" sets the
output and resets the latch via a short 1-shot timer ( so it has time to
return to the "released" position without the latch resetting.  If I
remember I had an inverter in there because latch-clear was high.  Come to
think of it, that coulda been made more simply ( the one-shot could just
clear the latch ) and use momentary pushbuttons.  Liberally sprinkle diodes
as needed.

Actually, come to think of it, I dont think it would be really high parts
count.  I may have used a PLA because I was playing with them at the time.
I'm looking at the datasheet for the 54ABT373 right this second.  If you put
8 pushbuttons on it, tie all 8 to +V, tie all 8 via diodes to OE# and a
one-shot tied to LE, and tie all the buttons to the inputs.  You press a
button, Output is disabled, the oneshot fires and latches the selection. You
release and output is enabled.  Thinking out loud... but it seems simple
enough. betcha a latch in DIP is more expensive then a cheap PIC these
days...

-- 
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