Re: Momentary Switch Circuit

Terry Pinnell wrote: 
ehsjr <ehsjr@xxxxxxxxxxxxxxxx> wrote (to the OP):


I'm curious. Why didn't you try this?

+Vcc ---o   o---+---|<---- Vout
        \      |   Zener
         o     |
         |     |
       [C1]   [R1] 220K
         |     |
Gnd ------+-----+

Vzener is slightly below Vcc - eg 4.7 volts if Vcc is
5 V, 11 volts if Vcc is 12 volts etc.
Ed


Ed: That looked such an attractively simple circuit that I tried it
myself yesterday. I may have missed something but aren't there a
couple of downsides to it? 

Yes, absolutely.

I don't think the OP specified what sort of
pulse he wanted, but my starting assumptions were a clean, +ve, full
supply signal. 

He's replacing (or using in place of) a momentary switch.
Both the toggle and the momentary are subjecty to switch
bounce. Since whatever circuit he's driving will work
on a momentary, then switch bounce from a toggle is not
a factor.

But your observation is correct. The circuit will not
provide a clean, +ve, full signal supply. It is a definite
downside to the circuit for general use - it is good only
for use where switch bounce is irrelevant.


Your circuit:

1) Delivers only a low amplitude spike, so needs amplifying to get a
'full' pulse. 

That's wierd.  It should deliver close to Vcc. Your
Vcc is 14 volts and the zener is 12. It (the zener)
should conduct until Vcap drops to 12, meaning that
the pulse amplitude has to be at least 12.  Maybe
you are scoping at the input to the zener? The
duration depends on the load, and to a lesser extent,
on the 220K resistor, but primarily on the zener.
It will be a spike, but it should be a spike whose
amplitude is over the zener voltage. RC is ~ 9.5 mS
(way too short) down to ~1/3 vcc - but it is collapsing
only 2 volts or about 14% (instead of ~ 63%) when the zener
shuts it off. You need a bigger cap! The circuit "looks at"
just the top of the cap discharge curve, which is steep.

We disagree on spike amplitude, but that doesn't change
the fact that the output is another downside, as you
indicated.  You get what amounts to a spike instead of a
nice robust square wave.

Using say a simple NPN stage that results in a -ve going 
pulse, which may then need inverting. (BTW, such amplification would
presumably be simplified if the headroom voltage is rather larger than
your examples?)


Yes - you need that with a low impedance load, or a long
duration pulse, or if the pulse must be square. And as long
as you stuff enough drive current into the base, it drives
to Vcc minus the transistor Vce. So you can have plenty of
headroom between the zener and Vcc and still get square wave
output.

I got way over 40 seconds with a 120(?) ohm relay driven
that way through a darlington with Hfe > 1000 in a delay
off circuit I made.  Had to add a resitor in parallel with
the cap to get it down to the ~40 second target in that
circuit.  I don't remember all the values, but I can find
them if they are of interest.

The headroom was to make the pulse voltage unambiguous.
RC is ~ 63% discharged - the circuit is "looking"
at the top ~6% of discharge (eg from 5.0 down to 4.7).
I didn't want the the circuit that is being driven
to "see" an ambiguous voltage that was not clearly
either a high or a low.  By using the top of the
discharge curve, the voltage will be at a high
until the zener cuts it off.  It has a wicked slope,
but all of it will be above the zener V.  Also, I picked
common zener voltages.  I run into lots of 11 and 4.7v
zeners, but rarely a 4.3.


2) Transmits switch noise. 

Absolutely. As mentioned earlier, I don't think
that's a factor for the op, but it certainly is
in many applications.


I used a 14V supply and a nominal 12V 1W zener, with this circuit:
http://www.terrypin.dial.pipex.com/Images/Momentary-Ed1.gif

Here are a few screenshots of the results I actually saw: 

Thanks for these - they're great!  I have a question, inline
below.


Relatively 'clean' switch
http://www.terrypin.dial.pipex.com/Images/Toggle-Ed-1.gif

Noisy switch
http://www.terrypin.dial.pipex.com/Images/Toggle-Ed-3.gif

Noisy switch, detail
http://www.terrypin.dial.pipex.com/Images/Toggle-Ed-5.gif

Noisy switch, detail, after NPN stage
http://www.terrypin.dial.pipex.com/Images/Toggle-Ed-8.gif


I'm puzzled by the Vout curves.  It should "fall off the cliff"
at 12V when the zener stops conducting, but the traces all show
Vout curving down below 12 volts.

I think the output (without the noise, which I can't draw)
should look like this:

+14     |\
+12     | \
        |  |
        |  |
        |  |
0   ----    ----

Can you determine why there is that curve down to 0
volts?  I wonder if your zener doesn't "zen"  :-)  ?


BTW, I'm puzzled by the roughly triangular shape of the output. I
tried one simulation and got a squarish result:

http://www.terrypin.dial.pipex.com/Images/Momentary-Ed-SIM1.gif
Maybe it was just the choice of zener?



Regarding the simulation - my guess is that they treat
the switch as noiseless, no bounce.  But they don't
show the Vout "falling off the cliff" pattern - unless
the yellow is supposed to be Vout.  The green looks
more like a capacitor discharge curve than the yellow,
but the green doesn't fall off the cliff. And if the
yellow is supposed to be Vout, the trailing edge is
correct and everything before it is wrong.  So I'm
clueless.

Ed
.

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