[sdiy] Semi-ot: bistable switch

[Harry Bissell Jr]

You're right it should be easy.

The usual technique is to clock a D flip-flop
with the notQ output returned to the data input.

The trouble you are likely to be having is debouncing
the switch. When it makes or bleaks it will
shower a number of pulses. This makes the FF unstable.

The normal technique is to use a pushbutton with
normally open and normally closed contacts, and a pull
up (or down) resistor on each one. You connect the
common to ground (or positive) and feed the two
contacts to the inputs of an R-S flip-flop (could be
two nand or two nor gates depends on the polarity of
the switch.

When you push the button, the first set pulse will
make
the output of the RS true, and the first reset pulse
will make the output false, all other switch bounce
will be ignored.

Reset could go to the D flip-flop globally, to set all
outputs to the same state.

This is a really simple circuit with about a billion
variations.

Look at some of the guitar effect sites (like geofex,
etc) for some other ways to do this. Almost all guitar
pedals have push-push switching, often electronic. 

H^) harry




--- Karl Ekdahl <_nial_ at yahoo.com> wrote:

> 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.
> 
> thanks
> 
> Karl
>