Qtouch - How to bridge a 3-5 mm gap between PCB and touch surface - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: May 08, 2020, 06:05:28 pm
content: Hi,I have a PCB with a number of Qtouch pads but mechanically there is a gap between the PCB top surface (with the pads copper is) and the bottom of the 3mm-thick plexiglass front panel.Such air gap is fixed once the design is finalised but at this stage can be anything from 3 to 5 mm.Any suggestions on how I can electrically/capacitively "couple" the plexiglass touch area to the PCB copper pad? One solution would be to have a metal spring (which being conductive would transfer the charges). But cannot find any spring that is soft enough and which is only few mm long while being also 10-15mm in diameter.Any other solutions?Thank you

author: Gyro
author_rank: Super Contributor
timestamp: May 08, 2020, 06:22:37 pm
content: Pads of soft (Black) antistatic foam, cut to size / shape? You should be able to get resistance down to a few k.

author: ataradov
author_rank: Super Contributor
timestamp: May 08, 2020, 06:27:05 pm
content: They sell springs that widen into a bigger "pad" just for that purpose. I'm not sure if any of them are 3-5 mm though.Here is one examplehttps://www.aliexpress.com/item/32655220502.html. This is just a first link I found. They should be available from regular parts suppliers as well.

author: ricko_uk
author_rank: Super Contributor
timestamp: May 08, 2020, 10:18:41 pm
content: Thank you Gyro and Ataradov!Gyro, have you tried it or do you know someone who has?Many thanks

author: Gyro
author_rank: Super Contributor
timestamp: May 09, 2020, 09:47:51 am
content: Not personally, but I doubt an extra few k on a capacitive touch switch would have a significant effect (as I say, use thesoftblack conductive stuff, not the pink dissipative one). It sounds as if it would be fairly quick to test (you must have an old IC box kicking around somewhere).

author: ricko_uk
author_rank: Super Contributor
timestamp: May 09, 2020, 06:11:51 pm
content: Thank you Gyro!

author: jbb
author_rank: Super Contributor
timestamp: May 09, 2020, 09:57:35 pm
content: There are surface mount spring contacts which might do the trick... but end in basically a point. Digikeycarries some.For prototyping, how about sticking some squares of copper tape to the inside of the plexiglass? If going to volume production, it may be practical to get a conductive film or ink or something applied to the plastics for you.Alternatively, a different technology approach is to stick a flex PCB to the plexiglass and put the Touch pads on that. Might also allow fun stuff like using a curved panel, or bending around a corner to put 'buttons' on the sides. You can also get quite interesting stuff done on an FPC, like soldering the QTouch chip or LEDs onto it, adding stiffeners for support or simply adding some double sided tape.Yes, adding an FPC is more parts and money, but it might be a win overall. For example:maybe you could shrink your main board if it doesn't need big Touch contactschange the main board mounting location and get some internal volume backuse the FPC to reach some other stuff in the box (e.g. external connectors)leave a little wiggle room on the FPC to soak up mechanical tolerances in your design

author: ricko_uk
author_rank: Super Contributor
timestamp: May 10, 2020, 12:15:51 am
content: Thank you JBB,all valid and interesting points!