Re: Momentary Switch Circuit

Terry Pinnell <terrypinDELETE@xxxxxxxxxxxxxxxxxxx> wrote:

>Terry Pinnell <terrypinDELETE@xxxxxxxxxxxxxxxxxxx> wrote:
>
>>ehsjr <ehsjr@xxxxxxxxxxxxxxxx> wrote:
>>
>>>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.
>>
>>I think I need a much *smaller* cap! If I make it 10nF instead of 1uf,
>>then I see Vout as an initial *very* brief spike.
>>
>>>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.
>>
>>You've lost me there. Can you draw that circuit please?
>>
>>>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" :-) ?
>
>We aren't possibly at cross-purposes here? We're talking about the
>circuit I illustrated above, with Vout taken from zener's anode, yes?
>
>>>>
>>>> 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.
>
>No, that's down to my choice of input to Circuitmaker's VCS. I
>simulated a noiseless switch, like the relatively clean one I used in
>my tests.)
>
>>>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
>>
>>I'm out for rest of day, but I'll get back on the case tonight. I
>>suspect that 'triangle' I saw for Vout might have been an artifact of
>>some sort, due to poorly chosen PC-based 'scope settings. All the
>>simulations I've tried show a square signal, amplitude Vcc-Vz, *apart*
>>from that initial spike which they now show, following the massive
>>reduction in the value of C.
>
>I re-assembled the breadboarded circuit yesterday with a different 12V
>zener. The waveforms now look OK to me. So the only remaining
>questions are:
>
>1) Amplitude; I still think it should be as seen here, Vcc-Vz, not
>close to Vcc?
>
>2) Simulated waveshape versus actual; must be something to do with
>unrealistic Circuitmaker models. I'll study further.

Forgot to include the links to latest screenshots
http://www.terrypin.dial.pipex.com/Images/Toggle-Ed-10.gif
http://www.terrypin.dial.pipex.com/Images/Toggle-Ed-11.gif

BTW, note that in these tests I'm typically toggling the switch on and
off again after only a sub-second pause. In practice it doesn't matter
how long it remains in the On position (providing you remember to
switch it off before requiring a subsequent action).

--
Terry Pinnell
Hobbyist, West Sussex, UK


--
Terry Pinnell
Hobbyist, West Sussex, UK
.

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