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Bounceless switches?

I was trying to measure the bounce time of a tactile switch so that I could design a debounce circuit based on a timer. But when I look at the switch output on a scope, I don't see *any*! The output is as clean as it gets and does not show bouncing down to 100 ns/div.

I am very used to scopes and I understand what can be hard to see and what can be easy to see. The sort of bouncing that a switch normally has should be very easy to see on an old fashioned triggered scope.

This switch has no electronics inside, it is just a mechanical switch. From the feel, it is a dome type with a dome switch with a snap action. But I would still expect to see bouncing.

One of the five on the board actually seems defective in that the bouncing is to a point that it is intermittent. But I really do thing it is defective. The other four are just fine.

Anyone seen this before?

Reply to
rickman
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Seen it.

The ones I'm familiar with are domes that are soldered over a pcb contact.

Probably something about the shape of the contacts and the pressure involved; I never saw any bounce.

G.

Reply to
ghelbig

Did you look at both edges ?

It can also depend on the load - For example, a switch with some capacitance, driving a 80C51 port, has a large current ratio between activate, and any bounce. So Circuit capacitance can slow the rise enough, that there is no apparent bounce in the voltage domain. (even tho there was in the physical-contact domain)

You can also get a 'bouncless' switch action, with a a SPCO Microswitch into a CMOS input. With this, the first contact make, charges the capacitance, and a later bounce-open, has no threshold-sized effect. When the Switch is releases, the other contact pulls the IP to the other supply, and again a bounce-open is held-over by the circuit capacitance.

-jg

Reply to
Jim Granville

Give it a thousand presses and then see how it looks.

Reply to
Neil

Yes, but only on some brand new switches of the small-piece-of-steel-taped- on-the-PCB-type. Bouncing seems to increase with age, probably due to corrosion and piling up of dirt. I wouldn't count on the switches to stay bounce-free in the future, though.

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Reply to
Ico

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You should look at the problem from the other direction. How fast can a finger operate the switch? I use 20mS for each edge.

Reply to
cbarn24050

Yes, I've noticed this too- even with certain designs of mechanical relays. It follows that one should not assume that debounce routines are working properly unless they actually see some bouncing. I generally use switch datasheet values for maximum bounce time.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

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