[sdiy] Minimum parts count switch de-bouncer.

[Harry Bissell Jr]

 One point NOT being made in many of these
proposed circuits... is that the capacitor
can draw unlimited current when the switch closes.

Bigger the cap value, the higher the current.

Enough current to eventually degrade of destroy the
switch in many cases...

Kind of the the moral and legal equivalent of
discharging the capacitor with a screwdriver
(hi Bob ;^)

In fact, the screwdriver will be more capable of
handling the current than the poor switch contact.

There should be a small resistor in series with the
switch, to limit that peak current. Just assume the
cap
is a short, and Ohms' Law will tell you the peak
current.

No resistor will also rock the power supply rails,
bigger the cap, bigger the problem.

You may want SOME substantial current to keep the
switch contact clean, but not enough to wear it out.

H^) harry

--- Tim Parkhurst <tim.parkhurst at gmail.com> wrote:

> On 4/12/06, Dave Kendall <davekendall at ntlworld.com>
> wrote:
> > Hi guys.
> >
> > Very grateful for all the help - going to have to
> decide now...... :-)
> >
> > Looks like the simplest, lowest parts count
> circuit, using stuff I've
> > already got, is this circuit;
> >
> >               
> I-------------------------I>o------------o out
> >                I                     schmitt
> trigger
> > +15V            I
> >    I     sw    I   I------I Rx I-------I
> >    I    ____   I   I                   I
> >    I----o  o-------I                   I
> >                    I      +            I
> >                    I------II Cx II-----I----GND
> >
> > <snip>
> > </snip>
> > where Rx = 10k, and Cx = typically 1uF. I'm not
> sure, but from what I
> > understand, the pulse duration in ms is R in
> Kohms, x C in uF = 10ms.
> > Am not sure if the output pulse will be negative
> or positive going.
> >
> > I guess the +15V and GND connections could be
> reversed (making sure the
> > electro is reversed too) to change this?
> >
> > I'm soldering away right now, so we'll see soon...
> >
> > cheers,
> >
> > Dave
> >
> *********************
> Hi Dave,
> 
> I would say that you're on the right track, but the
> solution you have
> above will produce a signal that goes from +V to GND
> when the switch
> is pressed (assuming that the schmitt trigger is a
> 40106, which is an
> inverter). I don't remember the original
> application, but if that's
> what you need then you're ready to go. Otherwise, if
> you need a
> positive going signal when the switch is pressed,
> just connect the R
> and C to +V and make the switch connect to GND when
> it is pressed. Oh,
> and reverse the electrolytic. Speaking of which, you
> don't really need
> an electrolytic cap in there. You can easily get
> away with a cheap
> ceramic disc cap for this.
> 
> Tim (cheap but not ceramic) Servo
> --
> "Imagination is more important than knowledge." -
> Albert Einstein
> 
>