[sdiy] touch-sensitive switches - how?

Joe Grisso jgrisso at det3.net
Wed May 26 07:28:53 CEST 2021

Hey All,

     I wanted to add a bit on the MCU-integrated solutions. You can
get LOTS of channels nowadays because MCUs have added a
mutual-capacitance sensing option in addition to the self-capacitance
sensing option. Instead of (n-1) or (n-2) pins available for
capacitive touch sensing, it can be a lot more, by configuring the
pins into a sensing matrix - this is actually how touchpads in laptops
and cell phones work. 4 tx and 4 rx lines can create a grid of 16
keys. If you're just looking for on/off functionality, this is super
easy and can be brought up on a board in less than a day with most
toolkits. Then you can get into sliders and touchpads as well, and
have some very wild user interface solutions.



On Tue, Apr 27, 2021 at 3:21 PM Brian Willoughby
<brianw at audiobanshee.com> wrote:
> There's not much magic in the microcontroller implementations. It's basically all the hardware of a discrete solution, but built-in and preconfigured. Generate a high frequency square wave, feed it into one plate of a capacitor, measure the frequency (or amplitude) and use changes in frequency (or amplitude) to detect a touch.
> Cons:
> A) Some MCU chips have a fixed frequency generator that fails miserably in the face of certain kinds of noise. I worked with a client who designed a product that only failed in one employee's home. Investigation revealed that this particular chip manufacturer had a poor design. Competitor MCU chips would adapt to noise, or used spread spectrum, or some technique that made the cap sense work everywhere, despite noise.
> B) Most MCU chips have only one cap sense input, or if they have more than one then Murphy's Law is that it will still be less than the number of cap sense channels you need.
> Pros:
> A) All of the basics are handled, so it saves you time setting up details that have already been solved.
> B) For those manufacturers that adapt to noise, it's especially helpful that your design doesn't have to bother with those details.
> If you need lots of channels (buttons), though, then a discrete design might be better because you can incorporate a multiplexer to expand to large numbers of cap sense inputs.
> Brian
> On Apr 27, 2021, at 11:50, Neil Harper wrote:
> > how are touch-sensitive switches like the ones on this done?:
> >
> > https://www.perfectcircuit.com/random-source-tkb-3-5mm-jacks.html
> >
> > being able to do something like this, especially if it's latching  (one
> > press on, one press off) would come in very handy. but i've never come
> > across it in any diy schematics.
> >
> > i see that a few microcontrollers like the teensy have capacitance touch
> > pins, but i'm wondering if things like sequencer are done all discrete?
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Joe Grisso
Detachment 3, Ltd.

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