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Is it possible to DIY your own capacitative touchpad on a PCB? The last page of this mentions it:

http://www.ebv.com/fileadmin/products/Products/Fujitsu/FMA1127/FMA1127_PCB_Layout_Ap_Note_FINAL_021809.pdf

"The sensor channels are grouped as rows and columns. Row and column cells are arranged in an interleaving pattern. The size of the cells should be designed so that when a finger touch is detected, a few cells are covered by the finger. Proper overlapping of cells is important to obtain good finger resolution."

So it seems like you have to have two layers, one having metal vertically and the other horizontally. I'd like to do this as cheaply as possible so I'd like to try to implement this on a standard 2-layer piece of FR-4, probably 0.8mm thick (half the standard thickness).

Or, the matrix keypad method mentioned above seems like it'd work, if the "keys" were placed together to have little gap. My overall goal is to have smooth input instead of discrete keys.

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  • \$\begingroup\$ Another document here: pe-gmbh.com/ew-addon/ftp/icproduct/datasheets/… Pg 13, fig 4. This looks pretty reasonable to do. Any thoughts? \$\endgroup\$ – allanw Aug 10 '11 at 5:00
  • \$\begingroup\$ Sparkfun uses the MPR121 capacitive touch sensor fordiscrete pads, a basic 12-pad model is here and there's an Arduino shield here (with, paradoxically, 9 pads). The IC that they're using is the Freescale MPR121 12-input touchpad, so getting smooth input would be hard. How much resolution do you want? \$\endgroup\$ – Kevin Vermeer Aug 10 '11 at 16:25
  • \$\begingroup\$ Derp, finally got through to read your datasheet: 100 sensors per IC, and 2000 possible on one SPI port with 20 ICs (At $6 each) with a scan time of 200ms. Impressive! \$\endgroup\$ – Kevin Vermeer Aug 10 '11 at 16:41
  • \$\begingroup\$ Yeah, I'm not going to use their IC. I'm thinking more of a $4 PCB and a $2.50 PIC micro with 28 inputs. Resolution will be something like 8x8. My main goal is that I want something sort of smooth, instead of having discrete buttons. So it should perform well if people slide their finger across. \$\endgroup\$ – allanw Aug 11 '11 at 1:36
  • \$\begingroup\$ What $2.50 PIC micro has capacitive touch inputs? David's answer points out that Cypress PSOCs have them, but I'm not aware of any PICs with that option. It can't be done with standard digital inputs, if that's what you're thinking. \$\endgroup\$ – Kevin Vermeer Aug 11 '11 at 12:57
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I've done exactly that for this product: http://www.qscaudio.com/products/network/QSys/pagestation.php

Behind the lexan self-adhesive label is a 2 layer PCB using a Cypress Semiconductor PSoC1 chip. The chip is great, but their software is terrible. You'd probably be much better off using their PSoC3 series, or using one of many other MCU's that has cap-touch support.

Cypress has lots of app notes on how to do your PCB layout for cap-touch. We did descrete keys, but their app notes also cover sliders and track-pad type stuff.

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maybe something like this?

real rough sketchup diagram

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  • \$\begingroup\$ broken link.... \$\endgroup\$ – NickHalden Aug 10 '11 at 6:29
  • \$\begingroup\$ @Mike - Please use [Ctrl-G] or the equivalent image upload button in the Markdown editor to have your images sent to our server, not Twitpic. If you don't want to have the image inlined for formatting reasons (Note: This wouldn't be a valid reason in this case), that's OK, but please use the generated URL. Thanks! \$\endgroup\$ – Kevin Vermeer Aug 10 '11 at 16:23
  • \$\begingroup\$ Also, some further explanation would probably be warranted. What is this a drawing of? A PCB? \$\endgroup\$ – Kevin Vermeer Aug 10 '11 at 16:43
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I ended up going with the two layer diamond texture, as the Microchip appnote says. I wrote up my details here: http://electronoblog.allanw.org/2011/08/yet-another-pcb-business-card-with-touchpad/

This should close the question.

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