Almost OT: Pushbutton switch latching

[M H Stein]

------------Original Message:
Date: Sat, 08 Dec 2007 23:50:29 -0800
From: "Chuck Guzis" <cclist at sydex.com>
Subject: Re: Almost OT: Pushbutton switch latching

On 9 Dec 2007 at 0:15, der Mouse wrote:

> Nice idea, but it doesn't work.  Draw out three NANDs cross-coupled as
> you described.  Mark the logic states for a stable state with one
> output low.  Now consider what happens when each of the inputs goes
> low.  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.  I wonder what the smallest number of active devices 
is for an n-stable circult with n greater than 2.  

8 D flip=flops with each switch connected to a PRESET input and a 
NAND (or NOR) to edge-clock in a reset state looks to be far from the 
minimum.  How about some capacitively coupled logic with a few 
steering diodes?

Cheers,
Chuck

---------------Reply:

Well, with some diodes and caps, a single 8-bit latch (74564/574) oughta
do the job.

m