[sdiy] interlocking switches

[Steve Lenham]

There is a super-neat solution if you don't need more than four interlocking 
buttons. I'll try to describe it in words...

1. Connect the four switches to the four inputs of a 4-16 line decoder IC 
(e.g. 74154, 4514).

2. Take output 1 and feed it to a transistor in parallel with the switch 
attached to the least significant input, so that when the output goes active 
the input thinks the switch is depressed.

3. Do the same for output 2 with the next-most significant input, output 4 
with the next-most above that, and output 8 with the most significant input.

4. Take your four interlocking output signals from outputs 1, 2, 4 and 8 of 
the decoder.

Each time a button is pressed, one of the unused outputs is momentarily 
selected which turns off all the transistors. Then, the output corresponding 
to the pressed button turns on and feedback through the attached transistor 
holds it active until another button is pressed.

Elegant, eh? I wish I could take credit for inventing it...

Maybe there is a way to extend the principle to more buttons using 
cascadeable decoders.

Cheers,

Steve L.


----- Original Message ----- 
From: "Scott Gravenhorst" <music.maker at gte.net>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, August 14, 2005 8:48 PM
Subject: Re: [sdiy] interlocking switches


>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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