Adventures in Topre-land

[Daniel Beardsmore]

Why a non-conductive material? That won't stop capacitance, by definition. After all, the screen on a phone has a sheet glass dielectric. Why do you need the rubber? The keycap isn't conductive, nor is the slider — what does the rubber do that the keycap cannot?

I am not sure what your point is, honestly. Perhaps you're suggesting that it's something to cut down on the extent to which the operator's finger registers capacitance and throws off the actuation position. Would that have any relationship to the foil in foam and foil?

If you do know how it all works, you should update the wiki and explain all this stuff for the record. I never did understand electronics ;-)

(TIL: now I now what makes PCBs green: solder mask!)

[woody]

Mr. AD5258, a 64bit (!!!) digital potentiometer.

64-position digital potentiometer, controlled by I2C bus.

So why rubber is such a chic switch? As far as I understand the spring duty is just to vary the electric field, and of course to insulate the field we need to use a material such as rubber (the field would be altered by other materials). My guess is also that it would be practically impossible to change the spring (say with a stiffer one) because the slightest material variation causes a variation in the field altering the actuation point.

The rubber cups are hardly any isolation, the conical spring is all it takes to increase the capacitance to a threshold level. Most likely the threshold level is variable, with decisions based on delta, not absolute values. Take a look at basic capacitive sensing.

[matt3o]

not "insulating", sorry. What I mean is that you can't use a material that interferes with the electric field.

guys, you are concentrating on a detail by the way and losing the whole picture. The rubber dome is used to hold the spring. A conductive material would alter the field and I don't think you could read reliable values.

[woody]

not "insulating", sorry. What I mean is that you can't use a material that interferes with the electric field.

guys, you are concentrating on a detail by the way and losing the whole picture. The rubber dome is used to hold the spring. A conductive material would alter the field and I don't think you could read reliable values.

Indeed, the rubber dome is holding/guiding the conical spring and giving the overall tactility. I doubt it's conductivity matters as it is too far away, and many factors change the measured capacitance, including humidity and EMI. That's why the measured value has to be tracked and the thresholds dynamically adjusted.

[Muirium]

64 position = a whole lot more sense!

By the way, I think we've just wound up talking three different conversations at once. The usual, then.

[matt3o]

it seems I can use a teensy to communicate with the digital potentiometer ( http://www.pjrc.com/teensy/td_libs_Wire.html ). It seems pretty easy actually...

I'll give it a shot, maybe I'm closer to hacking that keyboard than I thought

[Icarium]

Did those Embotec boards ever come out? Can't seem to find anything on them.