Almost OT: Pushbutton switch latching

der Mouse mouse at Rodents.Montreal.QC.CA
Sun Dec 9 13:45:16 CST 2007


>> The only one that does anything is the one feeding the gate whose
>> output is low.
> Yeah -- I see that.  Release a button and the circuit goes into an
> unstable state.

Actually, _press_ it and that happens - it happens as soon as the input
driving the distinguished gate goes low.

> I wonder what the smallest number of active devices is for an
> n-stable circult with n greater than 2.

Well, you can do a tristable circuit with three _four_-input NANDs.
Cross-couple them, each output driving an input on each other gate;
then, each button drives an input on all but one of the gates.

For example, let the inputs be inA, inB, inC, and the outputs outA,
outB, outC.  Then set up

outA = NAND(outB,outC,inB,inC)
outB = NAND(outC,outA,inC,inA)
outC = NAND(outA,outB,inA,inB)

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.

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