Key bounce problem

"N Cook" schreef in bericht news:f30oan$1fl$ snipped-for-privacy@inews.gazeta.pl...

Saw similar panel in a garage covered with a ordinary clear plastic bag that was replaced daily.

Is it sure that the grime causes the problem? Most of the time worn out switches cause this kind of bounce. (But of course, dirty contacts may behave like worn out ones.)

Besides, bounce can be intercepted to a wide extent in firmware. But that's in the hands of the manufacturer unless you are prepared to do a reverse engineering job.

Did you contact the manufacturer about the problem? (Not a salesman. He will advice to buy a new panel or even a new machine.)

If al other options fail, you can cut the wires from the panel to the 6520 and place your your interface in between. Guess you need only one micro and maybe some discretes. You can expand with an input line display and a backspace switch.

petrus bitbyter

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I tried caps to Vcc or 0V but if on all 5 column lines say then they cross interfere. One cap of about 1uF on one row or column intercepting the worst one made a difference, but not consistent. Sometimes it would suppress bounce registers, sometimes fail to input any parameter and sometimes allow a bounce entry. The worst aspect of this kit is that to change parameters you have to enter a "password" and key sequence totally blind. There is nothing like the 1 asterisk echoed to one password character on a pc, maybe the wrong character but at least you know how many characters. I would have arranged the firmware , so for each character entered, then 1 to 7 segments of a 7-segment display lit as an echo. I've tried cleaning out each click switch well, quite a lot of fine crud in the most used ones , with the perforated top cover membrane. Replaced the cover membrane with some thicker plastic sheet, glued to the welled structure. Another one of these I've put some graphite+silicone grease on the contacts under the click domes and replaced the membrane with thicker to see if that reduces the short duration on/off contacts causing the bounce. I will have to research vacuum forming clear soft plastic sheet to dimples for covering the keypad.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

I had a go at making a small scale (as its only calculator size keypad) vacuum forming machine. From a de-solder vacuum pump , 2 oz tobacco tin, polythene tube, rubber sheet and a hot air gun. From what i remember of radiant heated sheet, vacuum forming machines, they could do with an adjunct of hand assisted hot air because of cold spots or tight radii. Tried doming out in the button positions some expanded ali sheet as a mould , did a superb job of giving a diamond patterned surface of the plain sheet and blisters formed but were not large enough or deep enough. Will have to try step-repeat drilling out some metal sheet with larger holes than the original front panel and some backing of expanded ali as an internal stop face.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

"N Cook" schreef in bericht news:f36ph2$9io$ snipped-for-privacy@inews.gazeta.pl...

Apparently, you're reinventing the mechanical part of the switches :)

If you want to try capacitors in the matrix, you'd better place them across the switches. Nevertheless it may help against short, fast bounces. I do'n expect very much effect as your bounces are apparently slow ones.

What's the age of the panel? There must have been times it did not bounce or at least not that bad.

The more I read about it, the more I'm convinced that the switches are worn out. Contacts (metal or carbon) may be damaged, domes or sheets are weakened (or even perforated like yours) and so on. So I think you'll have to replace the switches now or in the near future. Of course, once you do so you will take every measure you can to extend the lifetime of the renewed panel.

petrus bitbyter

On Fri, 25 May 2007 07:11:47 +0100, "N Cook" put finger to keyboard and composed:

I have a bunch of click domes of all different shapes and sizes that were sampled to me many years ago.

If you can supply dimensions and a photo, I will send them to you for the cost of postage from Australia.

- Franc Zabkar

domes

Thanks for that but there is a spare, used, keypad that its possible to rob the little used domes from. It is a dome problem for if one key produces 2 or more entries per touch and/or release then swapping that dome to another well transfers the problem to that well. The long term solution would be to extend the 10 wires for the 5x5 matrix to an external covered keypad that uses off-the-shelf switches that can easily be replaced. A dab of graphited silicone grease certainly improves the bad domes and moving them to the little if ever used key positions will solve the problem for now. I may try SMD caps on each click-switch as there is only space for SMDs on the back of the original pads. The trouble is there are 6 of these 10 to 15 yearold testers costing about

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on