Title: Single pin capacitive touch sensing with an AVR
Post by: Tuomas on May 08, 2012, 05:24:37 pm
Post by: Tuomas on May 08, 2012, 05:24:37 pm
Hi there.
I've had a project idea on my mind for a while now that would probably need some capacitive touch buttons, but I've never used them before. About a month ago I started looking at some of the possibilities out there. After ruling out external controllers as an option, I noticed that Atmel provides a library for using a normal AVR ADC peripheral to do capacitive touch sensing, with only a single external component. The library is provided as a binary blob, however, and I couldn't find any source code for it. So I decided to try implementing it myself.
As far as I could figure it out, the idea is to take advantage of the sample&hold capacitor inside the ADC block.
I ended up making a dedicated PCB with an ATMega32u4 microcontroller for testing out the capacitive touch sensing (it's amazing having access to a PCB router at the university, transforming a PCB design into a physical board is simply a matter of some clicks of a mouse and waiting 30 minutes. Except for vias, they need to be painstakingly riveted one at a time...):
[attachment=0]
I've got a full writeup of the experimentation on my website, including my understanding of the theory of operation and code (and a video of the board in action, the three buttons controlling the three colors of an RGB LED):
http://tuomasnylund.fi/drupal6/content/ ... le-adc-pin (http://tuomasnylund.fi/drupal6/content/capacitive-touch-sensing-avr-and-single-adc-pin)
I'm sure this has been done before many times over, but I had fun trying to figure it out myself and it seems like a pretty elegant and reliable solution in my opinion. I'll probably end up using this in a project in the near future.
Thoughts, comments and corrections are always welcome.
I've had a project idea on my mind for a while now that would probably need some capacitive touch buttons, but I've never used them before. About a month ago I started looking at some of the possibilities out there. After ruling out external controllers as an option, I noticed that Atmel provides a library for using a normal AVR ADC peripheral to do capacitive touch sensing, with only a single external component. The library is provided as a binary blob, however, and I couldn't find any source code for it. So I decided to try implementing it myself.
As far as I could figure it out, the idea is to take advantage of the sample&hold capacitor inside the ADC block.
I ended up making a dedicated PCB with an ATMega32u4 microcontroller for testing out the capacitive touch sensing (it's amazing having access to a PCB router at the university, transforming a PCB design into a physical board is simply a matter of some clicks of a mouse and waiting 30 minutes. Except for vias, they need to be painstakingly riveted one at a time...):
[attachment=0]
I've got a full writeup of the experimentation on my website, including my understanding of the theory of operation and code (and a video of the board in action, the three buttons controlling the three colors of an RGB LED):
http://tuomasnylund.fi/drupal6/content/ ... le-adc-pin (http://tuomasnylund.fi/drupal6/content/capacitive-touch-sensing-avr-and-single-adc-pin)
I'm sure this has been done before many times over, but I had fun trying to figure it out myself and it seems like a pretty elegant and reliable solution in my opinion. I'll probably end up using this in a project in the near future.
Thoughts, comments and corrections are always welcome.
Title: Re: Single pin capacitive touch sensing with an AVR
Post by: ian on May 10, 2012, 10:42:51 am
Post by: ian on May 10, 2012, 10:42:51 am
Very cool, thanks for sharing and liberating the technique from the binary blob :)
I've heard before that most methods of cap sense touching are covered by patents, that may be why everyone is so fussy about it.
I've heard before that most methods of cap sense touching are covered by patents, that may be why everyone is so fussy about it.