Capacitive Touch Tutorial from James Lewis - Bald Engineer

Hi James I like the subtitles

I'll watch it again and try to pick it apart. Don't get me wrong, the rest of it is great so I'm trying to be constructively critical of the only bit I thought wasn't as good!

Layout

I have no idea what "layout is a wash" means. I'm from the UK so it may be obvious to others. When you say "you can use more pins with mutual" it isn't clear if you mean this it takes up more pins for the same number of buttons or if it allows you to use more. (In reality you tend to use less pins for mutual as it's generally used for a matrix layout.)

Why are the guard bands on mutual stronger?

Pins

Showing mutual as using 2 pins per pad is a bit misleading without the explanation of using a matrix, which is a not really explained.

"Mutual has a potential advantage which I'll hit on in a just a second." It isn't clear what this was or when you got to it.

Pads

Is it correct to describe he mutual pads as interdigitated (i.e. interleaved as when the fingers of one hand are laced between those of the other)? That feels wrong. One pad is inside the other. They are often in a matrix. But I don't see how they're interdigitated. Actually, does interdigitated refer to the slider and scroll wheel designs? I've not used those but the description seems to fit.

"Interdigited pads make it a wash". Again, I have no idea what this means!

Matrix

If the test said single touch and multi-touch this would be clearer.

I suppose the problem I had with it is the fact that mutual capacitance tends to be linked with multiplexing. Whilst you can multiplex with self capacitance the ghosting issue means you probably shouldn't and won't. That tends to flip around many of the things like mutual using more pins than self. That meant a lot of it was technically correct but felt practically a bit confusing.

All in all though, a good video. I hope these comments were helpful.

Fred27  wrote:

 

although I did find the comparison table between self and mutual capacitance wasn't clear

In what way?

A really nice overview video, although I did find the comparison table between self and mutual capacitance wasn't clear - and I'm already familiar with this kit.

You asked viewers to say what they'd use it for. Well, I've just started a custom design for an NFC enabled lock as this is great for initial prototyping. The proximity / guard channel will wake the device up to scan for my NFC implant and also enable a keypad for another way to open the door. I was going to use the simpler capacitive touch features of the CC1310's Sensor Controller, but decided an auxiliary keypad would be useful too. My wife won't let me get the kids chipped. (Only kidding.)

The one thing I found odd about the CAPTIVATE-FR2633 development board is that it's set up to be used as a booster pack for another launchpad. For prototyping I would have found it far more useful if I could have used it as a launchpad and used a TRF7970A booster pack with it. Obviously this is possible with a load of jumper wires, but the pinout is all wrong to just plug them together.

I did like the high quality of the CapTIvate board. Just discovered that the ESP32 supports capacitive sense so will be trying that out.

Cool Idea.

Nice Touchy Feely overview of the MSP CapTIvate Kit.

I could see adding this to a remote Robot Control Unit where a pass key is needed to unlock the control panel and the user could program the Robot to perform various tasks.

jc2048  wrote:

 

Nice, enthusiastic tutorial.

An electric field forms between the plates. Different dielectric materials change how much charge can be stored in that field.

How does charge get stored in the field between the plates? Don't the electrons put up a bit of a fight if you try and take away their charge?

Yes, if you take away their charge, they become a 14-year old emo kid with no identity.

Saying "charge build up" is almost the same as saying "electron build up."

There's a subtle difference between electrons and electric charge that trip up many people. Most people understand that a conductor allows electric current to flow through it while an insulator does not. Dielectrics are insulators (at least, by definition.) Here's the thing to remember though, electrons move through insulators, and dielectrics, all the time. Electrons are not mated to a specific nucleus. They are swarming around in a cloud-like fashion. This movement effectively means electrons can move through an insulator or dielectric, but the net charge does not change.

In the presence of an electric field (aka voltage), electrons will move through a conductor carrying their charge with them. That is the basis of electric current. When they run into an insulator or dielectric, the charge gets stopped. The electron might keep moving as part of the cloud, but the electric charge does not move with it. The number of electrons, or negative charge, that can build up depends on the dielectric strength of the material. ("Dielectric" is how much of an electric field can the material transmit without conducting electric current.) It comes down to how many positive nuclei are available to attract negative charges? Electric fields will affect how those positive nuclei react, which is how we get capacitors.

I like to use charge because "electrons build up" make it sound like the electrons physically stopped moving. They didn't. They're still swarming around. However, the net number of electrons doesn't change.

Very good explanation of capacitive sensing.

DAB

jc2048  wrote:

How does charge get stored in the field between the plates? Don't the electrons put up a bit of a fight if you try and take away their charge?

I know, electrons are so negative!