Carbon pads

[Gideon]

Hi, guys!  Earlier in the week, I got a fancy-pantsy digital multimeter, and I've been poking around in things ever since!  The first thing I opened up was an SNES controller that I plan on hacking.  First, I located the best place to put the ground connections, then I just started having fun seeing how it works.  Well, I have always been skeptical about the carbon-impregnated rubber pads that, when pressed down, supposedly carry the current from the common wire to the hot.  So, I turned the multimeter to continuity mode, and put the two metal pokers on the same carbon pad, making sure to put the cathode down first.  Well, it didn't take long to realize that the thing wasn't beeping!  I just thought it was interesting.  I suppose that the carbon pads are in the grey area of conductivity.  And, the multimeter only uses a teeny-tiny electric pulse in testing continuity.  Thus, no beep.  Am I right?

So, why are carbon-rubber pads used anyways?  Is it because they don't harm the exposed metal on the PCB?  Or, is it because of the infeasible precision that is required in using a non-squishy substance (In other words, the rubber has some give, creating a "window" of distance that it can be pressed while still making contact with both connectors)?

Ah, well.  I just thought it was interesting when the multimeter didn't beep.  Have a good week!

[LoRDDeVO]

Wow put the cathode end down first, aren't we the little electrical engineer   j/k  The carbon pads are to stop the signal between the two metal conductors. Like to snuff the singal from jumping from one conductor to the other

Just for Future reference it doesn't matter which is which when testing conductivity, it just checks if the signal can check from one spot to the other no matter which way you put them

[Gideon]

LOL.  Yes, I was pretty certain that the order didn't matter, but the instruction booklet (also the place where I got the word "cathode") told me otherwise.  Thanks for straightening that out, though!

The carbon pads are to stop the signal between the two metal conductors. Like to snuff the singal from jumping from one conductor to the other

That's what I thought!  Thanks for clarifying this also!  I should have stuck with my instinct:  carbon has 4 valence e-'s, and forms #-shaped insulating bonds... But, when reading certain tutorials, I'm told to wire to the normally open terminal.  This is confusing to me:  If, on a console controller, a closed circuit is the buttons' idle state, why would the wire go to the N.O. terminal?  "Normally closed" seems to be the right choice, and--to make matters worse--it seems to be obvious.

[Gideon]

Before you took appart your gamepad each button simply made a connection between something specific and Ground when pressed.

See?  This guy says so, too.  I really don't understand...

[bluGill]

Instead of the beep, try the ohm setting next time.   Carbon has a fairly high resistance, and likely isn't enough to cause your meter to beep, while it is enough to trigger a button push event.  

In the case of the playstation 2, the carbon is designed so that the harder you push the less resistance you get.  One good point about carbon.

Otherwise I agree, carbon isn't a good switch material.

[Gideon]

... it [carbon] is enough to trigger a button push event.

Okay, so carbon is running a connection between the common and hot wires.  Is this correct?  Please do well in convincing me, as I seem to have two intelligent gentlemen giving contradictory information.

[bluGill]

Turn your meter to the ohm setting and see what happens.   Logicly I don't see how carbon can stop a signal, but I'm not good enough with words to convince you of anything.   For that matter you can design buttons many ways, and not knowing exactly how your button is designed I don't want to make any statements that a latter look at the design would disprove.

Carbon is a conductor, but not a particularly good one.  

[Spaced Invader]

I've hacked many pads...If you hook it up normally closed your button will be on until you press it. So, the carbon pads DO conduct the current; through what dark occult magicks they occomplish this I don't know.  

[Gideon]

I've hacked many pads...If you hook it up normally closed your button will be on until you press it. So, the carbon pads DO conduct the current; through what dark occult magicks they occomplish this I don't know.  

Thank you.  That settles it for me.