[sdiy] touch-sensitive switches - how?

Brian Willoughby brianw at audiobanshee.com
Wed Apr 28 00:19:05 CEST 2021


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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