Switch Types

How about a single pole, double throw switch that grounds the set and reset inputs to a FF. That is a great debounce circuit that doesn't incur any RC delays. I've used it many times for switches you don't want to have bounce on.

Jon

Works great if you can use one.

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Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

This seems like a pretty simple debounce circuit with no delay. I don't think I've ever seen it used. It can be implemented in discrete components or done in an FPGA with a digital one shot.

+-------+ Switch | ____ | ____ +-------+ +------+ -->>---| _|_| |-----\\ \ | _ | | | Out Input | | || >----| _| |_ |O----| T Q |----+--->>-- +-------+ +--//___/ | | | | | | +-------+ +------+ | | | +---------------------------------------+

I'll need to remember this one.

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Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

A traditional key has instant mechanical feedback to the operator. Your proposals don't. Reed switches are not known for reliability & long life. You'd be better to use a traditional key fitted with magnetic sensing.

NT

I have used a software implementation of that. The monostable must be level triggered not edge triggered to avoid possibility of starting in wrong state.

piglet

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P.S. And I think the monostable should be non-retriggerable also. i.e. must complete an output pulse before being capable of triggering again.

piglet

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"Debounce_Switches.pdf" on the S.E.D/Schematics Page of my website. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
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     It's what you learn, after you know it all, that counts.

The original version I used had an AND gate following the XOR which makes it self synchronizing. If it is in the wrong state, it will trigger on a transition, then will retrigger on the end of the gate. I took out the gate thinking it was there to make the one shot not retriggerable, but it is otherwise needed. So back it goes. I also had an inversion in the wrong spot.

+-------+ Toggle Switch | ____ | ____ +-----+ -->>---| _|_| |-----\\ \ ___ +-------+ | _ | Out Input | | || >---| \ | _ |-----|_| Q|--+-->>-- +-------+ +--//___/ | & >---| _| |_ | | | | | +--|___/ | |O-+ +-----+ | | | +-------+ | | | | | | | +----------------------+ | | | +--------------------------------------------+

Done in software you just need the non-retriggerable one shot and take the triggering sample as the current state. A noise filter can be added in front if needed. Same thing in the digital domain when time sampled. Using discrete devices it is a bit harder to capture the present or previous value of a signal with glitches of undefined width. I guess that's why the simple RC and Schmitt trigger are so common. A little delay typically is not a problem.

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Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

I haven't found anything that would indicate there is any issue with a reed switch. The mechanical aspects of a key are from the mechanics of the key, not from the switch contacts... well, in a modern key anyway. I have an old railroad key and sounder my dad got when they took out the old equipment. It has a pair of hard cylinders as contacts and they are also the mechanical stop for the key. The feel is adjusted by a screw on a spring and a screw to limit the back travel and so the stroke. A modern key will have adjustable stops with spring metal contacts, no? So it has to be adjusted for feel just like a non-contact switch like Hall effect or reed switch.

Can you provide a source showing lack of longevity for mercury wetted reed switches, or any other kind?

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Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

Reed switch failures are a long known issue. Not surprising given the minimal pressure & lack of wiping. We can't use mercury wetted switches nowadays.

NT

Known by whom? All the references I've found point out that the precious or semi-precious metal contacts are in a hermetically sealed glass envelope and reliably switch low currents that are not sufficient to clean exposed contacts. "With some electrical loads, the operating life exceeds 50 years and one billion switching cycles."

Here is another, "The amount of movement of the reeds in the presence of a magnet is very small, and the flexing of the reeds is kept well within the elastic limits of their materials of construction. The result is a switch that potentially has an exceptionally long life. In most reed switches the glass housing is sealed and pressurised with an inert gas. The gas prevents oxidisation of the reeds and increases the break down voltage of the switch."

That same article may explain problems some people have, "To obtain their full benefits, reed switches need to be correctly applied. If they are overloaded or misapplied, they can be easily damaged or destroyed."

Reed switches are used in Aerospace and commercial aviation. How unreliable can they be?

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Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

Reed switches are used in Aerospace and commercial aviation. How unreliable

Pretty unreliable. I have a LED flashlight that is water proof to about 30 feet. It uses a reed switch so there is no thru hull joint. The flashlight lasted about two hours before one had to rap the flashlight to get it to light.

Dan

So your Chinese flashlight shows all reed switches are crap? Did you even read any of the references I quoted? I guess that's why airliners fall out of the sky every day.

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Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

ur proposals don't. Reed switches are not known for reliability & long life . You'd be better to use a traditional key fitted with magnetic sensing.

reed

key,

an old

nt.

anical

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sted

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reed

inimal pressure & lack of wiping. We can't use mercury wetted switches nowa days.

or

and

rs

ble

Inert gas does not prevent tiny arcing with consequent surface damage. Smal l elastic amounts of metal bending do not make switches reliable! It would be silly to suggest they did. Of course one must stay within their rather s mall specs on v, i and arc energy, and observe wetting current. But they st ill can and do fail, as does every switch on the planet. IRL reeds are not one of the best. To suggest an airplane falls out of the sky if a reedswitc h fails is also a little removed from reality.

Good luck with your project.

NT

I like that too. For a SPST you need something else. I usually do some RC time delay. ~10-100 ms.

George H.

The bit times being keyed are as short as 22 ms when keying at 55 WPM. Such a long debounce time, even 10 ms, would likely affect the "feel" of the key.

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Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

So if they fail at all, they must be "unreliable". I've provided sufficient evidence that used for signalling, reed switches are as if not more reliable than mechanical switches... which are the most often used choice in Morse code keys. It takes a *lot* of keying to put a million operations on a switch.

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Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

minimal pressure & lack of wiping. We can't use mercury wetted switches no wadays.

ous or

pe and

years

eir

liable

Small elastic amounts of metal bending do not make switches reliable! It wo uld be silly to suggest they did. Of course one must stay within their rath er small specs on v, i and arc energy, and observe wetting current. But the y still can and do fail, as does every switch on the planet. IRL reeds are not one of the best. To suggest an airplane falls out of the sky if a reeds witch fails is also a little removed from reality.

ent

ble

witch.

Your quote of someone overenthusiastically declaring that becuse they don't bend a lot they must be ultra-reliable is a total nonsequitur.

Those of us that have worked with the things within their specs, albeit man y years ago, know from experience that reliability is just not one of their strengths. And if you understand the basic requirements for a relatively r eliable switch, it is no surprise.

Really, good bye.

NT

You asked how unreliable can they be. I posted an answer to your question.

Now you bring in the reliability of aircraft. But are reed switches used in aircraft? I do not know the answer to that, but can say that they are not used in Submarine launched ICBM's.

Dan