Switch Bounce RC circuit?

[valemike]

Which circuit below is good for debouncing ground bounce noise? THe top or bottom circuit? I've seen both but I would think the circuit on the bottom is better at shunting ground bounce noise to ground than the circuit on top of the page.

[imt]http://www.iit/edu/~valemic1/switchbounce.jpg[/img][/img]

[John Morley]

Hi,

Well, your link doesn't work so we can't check it out for you. I'm curious why you are looking for a hardware solution to debounce a switch input? You can do it just as easily in software, assuming you have the time (cycle overhead) to do it. Here is a link to a good description of both methods: http://www.ece.utep.edu/courses/web3376/concepts/debounce.html

Good Luck!
_________________
John Morley

[valemike]

Here is the picture...

[SherpaDoug]

Of those two the S2 circuit would be preferred. R4 does nothing for switch bounce, and connecting C2 directly across the switch could theoretically reduce the switch life. Select R3 based on switch size and available power. Preferably the current through the switch should be at least 1% of the switch rated current to avoid "dry contact" problems. But if the switch is rated for "dry contact" service this minimum current is basically zero.
If possible I would poll the switch, and confirm the reading a few milliseconds later.
_________________
The search for better is endless. Instead simply find very good and get the job done.

[asmallri]

There are a number of hardware and / or software combinations commonly used for debouncing switches. Where the switches are located remotely from the PIC (off board) then hardware debounce circuitry is often used to provide harware protection from possible over and under voltage transients.

In the upper example R4 will limit the input current into the PIC in the event of overvoltage condition occuring on the input either as a result of interference or possible phyically interference. I typically add a diode across R1 (cathode to +5V) in this configuration to provide this protection or, for very long runs, a transorb across C2 would be better. These circuits work best when fed into the schmidt trigger input of the PIC.

The lower example is flawed. If you consider the case when S2 is open the capacitor will charge to +5V via R2 and R3. When S2 is closed, the input to the PIC will be driven down to -5V while the capacitor is discharged through R2 / S2.
_________________
Regards, Andrew

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[asmallri]

[valemike]

I got the S2 circuit from the PICDEM-2 Plus demo board.

As for the top circuit, that's the one on my dad's previous designs (there is a an inductor and capacitor before it (not shown) to filter out EMI. That's also the circuit shown in Embedded Systems Programming back in the Summer 2004.

I would think that the resistor in the S2 circuit both limits current as well as acts as part of a low pass filter. Any switch bounce noise would go through the capacitor and not be seen as much since the resistor would provide an impedance.

Who knows! Almost everything that I thought I learned in circuit analysis class doesn't hold anymore.

[Guest]

[quote="valemike"]I got the S2 circuit from the PICDEM-2 Plus demo board.


CORRECTION:
I have it swapped around. The S1 circuit is from the PICDEM.

[asmallri]