Almost OT: Pushbutton switch latching

[Henk Gooijen]

> Date: Wed, 5 Dec 2007 11:53:11 -0800> From: ian_primus at yahoo.com> To: cctalk at classiccmp.org> Subject: Almost OT: Pushbutton switch latching> > This should be a much simpler problem than I'm making> it, but at the moment my brain doesn't want to come up> with a solution.> > Picture this - a number of switches, say, eight.> Momentary contact, normally open switches. And then> eight outputs, one for each switch, TTL level. I want> to be able to push one switch and bring it's> corresponding output high, and have it stay high even> when the switch is released. But, making this more> complicated, I only want to be able to allow one> switch to be active at a time. So, say switch four is> pressed, output four is high. Then, switch one is> pressed, and output four goes low, and output 1 goes> high. Timing is not critical.> > I know this should be a simple array of flip flops, in> some fashion, but at the moment, the solution eludes> me. I feel like there is probably even a single/two> chip solution to this.> > Now, I know that this is probably off-topic, but I'd> be using this in on-topic hardware...> > -Ian
 
What pops into mind, but would need a bit more thought,
8 D flipflops with common clock.
The 8 switches are connected to the 8 D inputs, but also
tied together with a small capacitor and connected to the
clock. When you press a switch, the D input is set to the
level of the pressed switch, while the other D inputs are
at the levels from the not-pressed switches.
Pressing any switch will produce a pulse on the clock
(with a small delay). The levels of the 8 D inputs is latched
in the 8 D flipflops.
 
I have build that circuit, but can't remember exactly how
the 8 switches connect combined to the single clock input ...
 
Henk