[sdiy] Semi-ot: bistable switch

[WeAreAs1 at aol.com]

In a message dated 9/17/04 8:05:13 AM, _nial_ at yahoo.com writes:

<< Hi all. I've been banging my head for a day now trying

to do a simple bistable switch. one press and it's on,

another press and it's off and so on, state indicated

by a led. the actual push button will only be able to

deliver a +5v signal while depressed and the switch

has to be resetable. i've tried doing this with 7474's

(i've got way too many of those) but haven't had any

success. I need at least eight of these in a small

space. could anyone help me? this is newbie problem, i

know, but it's making my head cook.

 >>

I have done this kind of thing several times using a 4013 to do the flip-flop 
state change, and a 40106 Schmidt trigger inverter to do the switch contact 
debouncing.  It works very well, and has always been very reliable for me.  
Depending on certain factors, you could probably also use a 7474 and a 40106 to 
achieve the same effect.  The 4013 and 7474 are dual flip-flops, so you can get 
two of these switches out of each IC package.  The 40106 is a hex inverter, 
so each 40106 IC package can provide contact debouncing for up to six of these 
switches.

Please note that the typical 40106 switch debounce circuit uses a normally 
open switch, with the switch contacts closing to Ground when pressed (active 
"low").  If it's absolutely necessary that your switch have the opposite active 
polarity, that is -- using a normally open switch whose contacts close to the 
+5v supply when the switch is pressed, then you'd need to use two of the six 
inverters in the 40106 for each physical switch you need.  One inverter to 
invert the switch signal to active low status, and another to debounce that signal. 
 I always try to have my switches close to Ground in order to avoid needing 
this additional inverter.    BTW, the typical 40106 debounce circuit requires 
two resistors and one small capacitor.

If you can find schematics for the Oberheim OBxa, that unit has several of 
these debounced flip-flop switches in its left-hand controller section (used for 
octave switching and switching of pitch bend routing, etc.).  If you copy 
their circuit exactly, you'll get a very reliable circuit.  Their circuit uses 
the 4013 flip-flop, but you may be able to subsitute your 7474 instead.

Michael Bacich