[sdiy] interlocking switches

[Bob Weigel]

Oh yeah and I meant to mention the pic solution.  (I sent private 
figuring someone would say the same thing :-) )  It's a super easy 
program and a cheap chip that will save you a lot of time unless you are 
dead set on doing it discretely of course.  Boot up, clear all LED's.  
Just dedicate a line to each one with a 470 ohm or whatever resistor 
gives you the right brightness in line and drive them directly with a 
microchip pic series seems to work fine.  Then write a loop that just 
polls each switch line and some even have weak internal pullups so for 
an app where it wouldn't kill someone if it glitched you can just use 
those.  Then branch to a routine that will turn on the LED and give an 
apprpriate debounce routing call then return.  Or..you may want to use 
the pic to do other stuff as well while you are there. PICs can be 
beautiful.  I wish I could have figured out why the RV project I was 
using one in did this weird thing where data gets corrupted somehow on 
the chip even with a really robust power supply.  ARgghh!  That's an app 
where you dont' use the weak pullups, and we didn't.  When the jack goes 
off the wrong direction and starts stripping gears.. nobody gets real 
happy.  -Bob

Scott Gravenhorst wrote:

>I don't have a circuit, but I think it can be done with CMOS flipflops (D or JK
>wired like D) and a one-shot or two.  Each switch should be debounced.  The
>debouncers each feed a multi input OR gate.  The OR output starts a one shot
>intended to time out prior to when you could release the switch but after the
>debouncer's output has stabilized.  The one-shot clocks all of the flipflops
>simultaneously.  Each debouncer output also feeds the D input of it's flipflop.
>
>What I have described will have the flaw that it's possible to set more than one
>flip flop at a time.  However, it's very simple.  If I were building this to use
>myself, that's how I would do it.  For someone else, maybe not.
>
>This could also be done with a PIC very easily and with more/better features
>such as not allowing more than one output to be set on.
>
>"Roy J. Tellason" <rtellason at blazenet.net> wrote:
>  
>
>>I *know* I've seen this in some service manual or other,  but can't remember 
>>where...
>>
>>Say you have a row of buttons,  and you push one,  an LED lights up.  When you 
>>push another,  that light comes on and the first one goes out.  Sort of like 
>>the old mechanically-interlocked switches I remember from ages ago.
>>
>>Posted about this elsewhere,  and have of course the immediate suggestion to 
>>throw a processor at it,  which I'm not inclined to do,  and some more that 
>>discuss using lots of logic.
>>
>>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...
>>
>>
>>
>>
>>
>>
>>    
>>
>
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