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Bigglez
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Joined: Mon Oct 15, 2007 7:39 pm
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Post by Bigglez » Wed Feb 20, 2008 3:31 pm

Greetings (No Name Supplied),
sparkle wrote:
mikerd wrote:...I still occassionally get a phantom dim output light. But it is sufficiently different from an 'on' that I can explain it to my students...
One more consideration is the quality of the switch contacts themselves, a separate issue from bounce.
.....
Yet, the same contacts may clean up, and work well for a while thereafter, if subjected to a low voltage load of tens to hundreds of milliamps, the so-called wetting current (or sealing current) you can look up in Wikipedia and elsewhere.
The problem of mechanical switches not working well
in low voltage low energy "digital" or "logic" circuits is
an old problem, dating back at least a few decades.

The solution adopted by many switch companies is to
construct wiping contacts that provide self-cleaning
without the need for deliberate contact arcing.

I think the OP has several issues with their design
beyond mechanical switch selection.

Here's a couple of references for anyone interested
in wiping action and switch de-bounce.

Comments Welcome!

sparkle
Posts: 17
Joined: Thu Sep 20, 2007 7:43 pm
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Post by sparkle » Wed Feb 20, 2008 5:13 pm

Bigglez wrote:...The solution adopted by many switch companies is to
construct wiping contacts that provide self-cleaning
without the need for deliberate contact arcing...
And the relatively good performance of modern debounced keyboards worldwide (at least up to the first massive coffee or toner spill) shows how good a well selected non-hermetic, wiping contact switch manufacturer can do, in a practically zero current application. Not invulnerable, but pretty good.
Bigglez wrote:...I think the OP has several issues with their design
beyond mechanical switch selection...
The OP did say there are sometimes bad levels coming right out of the level asserting switches, if I am reading the explanations of flaky measured voltage levels and dim LEDs correctly. The OP wrote of budget limitations. So perhaps some simple experiments with the undebounced switches on hand, might be a good starting point. Starting with the ability to assert a one and a zero, and have both DVM, simple logic, and LEDs always agree that there is sufficient noise margin. Then incrementally inserting more combinatorial logic up to the complexity a full adder. My hope would be that certain flaky switches are identified (after careful observation with simple logic) and cleaned or replaced, the rest work, and simple classes then can be held without needing to explain around the unknown dim intermittent LED indication. If however as you now believe, and as the OP previously indicated, that as logic complexity builds the platform continues to go unstable even after switches are thus exonerated, there might indeed be more work ahead, some of which might be difficult without photos, full schematic, and perhaps instrumentation. Oh, and mentioning the decoupling word too.

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